POSTHARVEST QUALITY OF ‘NEW’ POTATOES: EFFECTS OF IMPROVED WATER AND FERTILIZER USE EFFICIENCY DURING PRODUCTION AND APPLICATION OF ‘RAPID CURING’ DURING STORAGE By MILDRED N. MAKANI A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY UNIVERSITY OF FLORIDA 2014
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2014. Mildred Makani. Postharvest quality of 'new' potatoes
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POSTHARVEST QUALITY OF ‘NEW’ POTATOES: EFFECTS OF IMPROVED WATER AND FERTILIZER USE EFFICIENCY DURING PRODUCTION AND APPLICATION OF
‘RAPID CURING’ DURING STORAGE
By
MILDRED N. MAKANI
A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT
OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY
2 LITERATURE REVIEW .......................................................................................... 20
The Irish Potato ...................................................................................................... 20 U.S. Production and Consumption .......................................................................... 22
Florida’s ‘New’ Potatoes ......................................................................................... 23 Production and Consumption ........................................................................... 23 Irrigation in the Tri-County Agricultural Area ..................................................... 25
Nitrogen Fertilizer Application in the Tri-County Agricultural Area .................... 27 Potato Growth, Development and Tuber Harvest Maturity...................................... 28
Plant Growth and Development ........................................................................ 28
Tuber Maturity Indices ...................................................................................... 29
Plant Vine Killing and Tuber Harvesting ........................................................... 31 Tuber Yield and Harvest Quality Response to Crop Production Practices ............. 32
Response to Irrigation Method ......................................................................... 32
Response to Nitrogen Fertilizer Application Method and Rate ......................... 34 Postharvest Handling and Storage of New Potatoes .............................................. 36
Tuber Skin and Wound Injury ........................................................................... 36 ‘Rapid Curing’ As a Means of Prolonging Shelf-life In New Potatoes ............... 37 Storage of New Potatoes.................................................................................. 38
Effect of Preharvest Factors on Tuber Postharvest Quality .................................... 40 Research Objectives ............................................................................................... 41
3 EFFECT OF DRIP IRRIGATION AND HARVEST TIME ON YIELD AND TUBER STORAGE QUALITY OF TWO TABLESTOCK POTATO CULTIVARS. ................ 43
Introduction ............................................................................................................. 43 Materials and Methods............................................................................................ 45
Plant Material ................................................................................................... 45 Experimental Site and Field Trials .................................................................... 46 Plant Vine Kill and Tuber Harvest Times .......................................................... 47
Tuber Yield and At Harvest Quality .................................................................. 47 Postharvest Storage Analysis ........................................................................... 48 Statistical Analysis ............................................................................................ 49
Total Rainfall Received .................................................................................... 50 Effect of Irrigation Method and Harvest Time On ‘Fabula’ Tuber Yield and At
Effect of Irrigation Method And Harvest Time on ‘Fabula’ Tuber Storage Quality ........................................................................................................... 51
Results – ‘Red LaSoda’ .......................................................................................... 59 Effect of Irrigation Method and Harvest Time On ‘Red LaSoda’ Tuber Yield
and At Harvest Quality .................................................................................. 59
Effect of Irrigation Method And Harvest Time on ‘Red LaSoda’ Tuber Storage Quality ............................................................................................. 59
4 EFFECT OF NITROGEN FERTILIZER RATE, METHOD OF APPLICATION AND HARVEST TIME ON YIELD AND STORAGE QUALITY OF SURFACE-DRIP IRRIGATED POTATOES. I. ‘FABULA’ .......................................................... 75
Materials and Methods............................................................................................ 76 Experimental Site and Field Layout .................................................................. 76
Tuber Yield and At Harvest Quality Analysis .................................................... 78 Postharvest Analysis ........................................................................................ 80 Statistical Analysis ............................................................................................ 81
Effect of Nitrogen Rate and Application Method on Yield and At Harvest Quality ........................................................................................................... 83
Nitrogen Assimilation and Biomass Accumulation ..................................... 83
Tuber Yield and At Harvest Quality ............................................................ 84
Effect of Nitrogen Application Method and Rate on Tuber Storage Quality ...... 86 Initial Storage Quality ................................................................................. 86 Storage Quality .......................................................................................... 87
5 EFFECT OF NITROGEN FERTILIZER RATE, METHOD OF APPLICATION AND HARVEST TIME ON YIELD AND STORAGE QUALITY OF SURFACE-DRIP IRRIGATED POTATOES. II. RED LASODA’. ............................................. 113
Introduction ........................................................................................................... 113 Materials and Methods.......................................................................................... 114
Plant Material ................................................................................................. 114
Effect of Nitrogen Rate and Application Method on Yield and At Harvest Quality ......................................................................................................... 114
Nitrogen Uptake and Biomass Accumulation ........................................... 114 Tuber Yield and At Harvest Quality .......................................................... 116
Effect of Nitrogen Application Method and Rate on Tuber Storage Quality .... 118
6 EVALUATION OF SKIN ADHESION STRENGTH AND ‘RAPID CURING’ AS A MEANS OF MINIMIZING QUALITY LOSSES DURING STORAGE OF ‘NEW’ POTATOES. ......................................................................................................... 140
Materials and Methods.......................................................................................... 143 Experiment 1: Determination Of Tuber Resistance To Skinning Injury In
Relation To Nitrogen Treatment and Harvest Time ..................................... 143
Plant Material ........................................................................................... 143 Vine Kill and Tuber Harvesting ................................................................. 143
Weather Conditions ........................................................................................ 148 Tuber Decay Incidence .................................................................................. 149 Experiment 1: Determination Of Tuber Resistance To Skinning Injury In
Relation To Nitrogen Treatment and Harvest Time ..................................... 149
Experiment 2: Evaluation Of Tuber Wound Periderm Suberization ................ 151 Determination of ‘Rapid Curing’ Conditions ............................................. 151 Effect of Storage Condition On Wound Periderm Suberization ................ 151
Discussion ............................................................................................................ 152 Experiment 1: Determination Of Tuber Resistance To Skinning Injury In
Relation To Nitrogen Treatment and Harvest Time ..................................... 152 Experiment 2: Evaluation Of Tuber Wound Periderm Suberization ................ 155
LIST OF TABLES Table page 3-1 Effect of irrigation method on yield and incidence of physiological disorders in
‘Fabula’ tubers harvested 3 weeks after vine kill ................................................ 65
3-2 Analysis of variance for irrigation method and harvest time on selected quality parameters for ‘Fabula’ tubers stored for 14 d at 10°C, 80-85% RH ....... 66
3-3 Effect of harvest time on peel and pulp dry matter contents of ‘Fabula’ tubers’ initial storage quality (season 2) ......................................................................... 67
3-4 Effect of harvest time after vine kill on tuber peel and pulp ascorbic acid contents of ‘Fabula’ tubers’ initial storage quality (season 2) ............................. 67
3-5 Effect of irrigation method and harvest time on firmness of ‘Fabula’ tubers, stored for 14 d at 10°C, 80-85% RH (season 2) ................................................. 67
3-6 Effect of irrigation method and harvest time on tuber peel and pulp ascorbic acid content of ‘Fabula’ tubers stored for 14 d at 10°C, 80-85% RH (season 2) ........................................................................................................................ 68
3-7 Effect of irrigation method on yield and incidence of physiological disorders in ‘Red LaSoda’ tubers harvested 3 weeks after vine kill ....................................... 69
3-8 Analysis of variance for irrigation method and harvest time for selected quality parameters for ‘Red LaSoda’ tubers stored for 14 d at 10°C, 80-85% RH ...................................................................................................................... 70
3-9 Effect of harvest time on cumulative fresh weight loss of ‘Red LaSoda’ tubers stored for 14 d at 10ºC, 80-85% RH (season 2) ................................................. 71
3-10 Effect of irrigation method and harvest time on tuber firmness of ‘Red LaSoda’ tubers stored for 14 d at 10ºC, 80-85% RH (season 2) ........................ 71
4-1 N fertilizer rates and application methods for ‘Fabula’ potato fertigation trial in 2013 (season 1) and 2014 (season 2) ................................................................ 99
4-2 Plant aboveground (AG) and tuber biomass, on a dry weight basis, at full flower of ‘Fabula’ potatoes grown under five nitrogen treatments in 2013 (season 1) and 2014 (season 2). ........................................................................ 99
4-3 Plant aboveground and tuber nitrogen uptake of ‘Fabula’ potatoes grown under five nitrogen treatments, and harvested at full flowering in 2013 (season 1) and 2014 (season 2). ........................................................................ 99
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4-4 Total Kjeldahl nitrogen of ‘Fabula’ potatoes grown under five nitrogen treatments, and harvested at full flowering in 2013 (season 1) and 2014 (season 2)......................................................................................................... 100
4-5 Tuber yield, size distribution and external physiological disorders in ‘Fabula’ potatoes grown under five fertilizer treatments (season 1) ............................... 101
4-6 Tuber yield, size distribution and external physiological disorders in ‘Fabula’ potatoes grown under five fertilizer treatments (season 2) ............................... 101
4-7 Tuber external disorders in ‘Fabula’ potatoes grown under five fertilizer treatments, and harvested 3 weeks after vine kill (season 1 and 2) ................. 102
4-8 Analysis of variance of season, fertilizer treatment and harvest time effect on tuber initial storage quality in ‘Fabula’ ............................................................... 102
4-9 Effect of fertilizer treatment and harvest time on pulp dry matter content of ‘Fabula’ tubers’ initial storage quality (season 2) .............................................. 103
4-10 Effect of fertilizer treatment on soluble solids content (SSC), total titratable acidity (TTA), and pH of ‘Fabula’ initial storage quality (season 1 and 2) ......... 103
4-11 Analysis of variance of season, fertilizer treatment, and harvest time effect on tuber quality in ‘Fabula’ stored at 10°C, 80-85% RH for 14 d ........................... 104
4-12 Effect of fertilizer treatment and harvest time on tuber peel dry matter content of ‘Fabula’ stored at 10°C, 80-85% RH for 14 d ............................................... 105
5-1 Plant aboveground (AG) and tuber biomass, at full flower of ‘Red LaSoda’ potatoes grown under five nitrogen treatments (season 1 and 2). .................... 129
5-2 Plant aboveground and tuber nitrogen uptake of ‘Red LaSoda’ potatoes grown under five nitrogen treatments, and harvested at full flowering in 2013 (season 1) and 2014 (season 2). ...................................................................... 129
5-3 Total Kjeldahl nitrogen of ‘Red LaSoda’ potatoes grown under five nitrogen treatments, and harvested at full flowering in 2013 (season 1) and 2014 (season 2)......................................................................................................... 130
5-4 Tuber yield, size distribution and external physiological disorders in ‘Red LaSoda’ potatoes grown under five fertilizer treatments (season 1) ................. 131
5-5 Tuber yield, size distribution and external physiological disorders in ‘Red LaSoda’ potatoes grown under five fertilizer treatments (season 2) ................. 131
5-6 Tuber external disorders in ‘Red LaSoda’ potatoes grown under five fertilizer treatments, and harvested 3 weeks after vine kill in 2013 (season 1) and 2014 (season 2) ................................................................................................ 132
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5-7 Analysis of variance of season, fertilizer treatment, and harvest time on tubers’ initial storage quality in ‘Red LaSoda’ ................................................... 132
5-8 Effect of fertilizer treatment and harvest time after vine kill on tissue soluble solids content of ‘Red LaSoda’ tuber initial storage quality (season 1). ............ 133
5-9 Effect of fertilizer treatment and harvest time after vine kill on tissue soluble solids content of ‘Red LaSoda’ tuber initial storage quality (season 2). ............ 133
5-10 Analysis of variance of fertilizer treatment, harvest time on tuber quality in ‘Red LaSoda’ tubers stored at 10°C, 80-85% RH for 14 d ................................ 134
5-11 Effect of harvest time on tuber peel dry matter content of ‘Red LaSoda’ stored for 14 d at 10°C, 80-85% RH ................................................................. 135
6-1 Skin-set readings of non-vine killed ‘Fabula’ tubers grown under five nitrogen treatments and harvested over two weeks (91 to 105 days after planting), during season 1. ............................................................................................... 160
6-2 Effect of vine kill on skin-set readings of ‘Fabula’ tubers harvested over three weeks during season 1 (91 to 112 days after planting) and season 2 (98 to 119 days after planting). ................................................................................... 160
6-3 Effect of vine kill on skin-set readings of ‘Red LaSoda’ tubers harvested over three weeks during season 1 (91 to 112 days after planting) and season 2 (98 to 119 days after planting). ......................................................................... 161
6-4 Average thickness of suberized wound periderm layer of ‘Red LaSoda’ stored for 7 days at 15, 20, and 25ºC, 90-95% RH. .......................................... 161
A-1 Tuber classification used to evaluate marketable and unmarketable yield after harvest, 3 weeks after vine kill. ................................................................. 171
A-2 Effect of interaction of fertilizer treatment and harvest time on tuber pulp dry matter content of ‘Fabula’ stored at 10°C, 80-85% RH for 14 d ........................ 171
A-3 Effect of an interaction of fertilizer treatment, harvest time and storage time on soluble solids content (SSC), and total titratable acidity (TTA) and pH of ‘Fabula’ tubers stored at 10°C, 80-85% RH for 14 d. ........................................ 171
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LIST OF FIGURES Figure page 3-1 Total and cumulative rainfall during growing A) season 1, and B) season 2 in
3-2 Effect of harvest time on firmness of ‘Fabula’ tubers’ initial storage quality (season 2)........................................................................................................... 73
3-3 Fresh weight loss in ‘Fabula’ tubers stored for 14 d at 10ºC, 80-85% RH. ......... 73
3-4 Effect of harvest time on firmness of ‘Red LaSoda’ tubers’ initial storage quality (season 2).. ............................................................................................. 74
4-1 Rainfall and cumulative evapotranspiration in Hastings, FL, during growing A) season 1, and B) season 2 ............................................................................... 106
4-2 Mean air and soil temperatures experienced during growing A) season 1, and B) season 2 ...................................................................................................... 107
4-3 Tuber specific gravity of ‘Fabula’ potatoes grown under five fertilizer treatments......................................................................................................... 108
4-4 Effect of harvest time on the peel dry matter content of ‘Fabula’ tubers harvested 1-3 weeks after vine kill. ................................................................... 108
4-5 Effect of harvest time on the peel and pulp ascorbic acid content of ‘Fabula’ tubers harvested 1-3 weeks after vine kill. ........................................................ 109
4-6 Fresh weight loss of ‘Fabula’ tubers as affected by fertilization, harvest time after vine kill, and storage for 14 d at 10ºC and 80-85% RH (season 1). ......... 110
4-7 Fresh weight loss of ‘Fabula’ tubers as affected by fertilization, harvest time after vine kill, and storage for 14 d at 10ºC and 80-85% RH (season 2). ......... 111
4-8 Firmness of ‘Fabula’ tubers stored for 14 d at 10ºC and 80-85% RH ............... 112
5-1 Tuber specific gravity of ‘Red LaSoda’ potatoes grown under five fertilizer treatments during A) season 1, and B) season 2.............................................. 136
5-3 Pulp dry matter content of ‘Red LaSoda’ tubers’ initial storage quality grown under five fertilizer treatments (season 1). ....................................................... 137
5-4 Firmness of ‘Red LaSoda’ tubers’ initial storage quality harvested 1-3 weeks after vine kill (H1-3) (season 1) ......................................................................... 138
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5-5 Tissue pH of ‘Red LaSoda’ tubers’ initial storage quality, grown under five fertilizer treatments during two seasons ........................................................... 138
5-6 Fresh weight loss of ‘Red LaSoda’ tubers stored for 14 d at 10ºC, 80-85% RH during A) season 1, and B) season 2. ........................................................ 139
6-1 Modified skin-set tester used to measure resistance to periderm injury. .......... 162
6-2 Layout of trays with tubers in racks for ‘rapid curing’ at 20ºC, 90-95% RH.. ..... 162
6-3 Total rainfall experienced during tuber harvest period in season 1 and 2. ........ 163
6-4 Tuber decay in A) ‘Fabula and B) ‘Red LaSoda’ tubers harvested 1 to 3 weeks after vine kill (H1-H3) during two seasons. ............................................ 164
6-5 Fresh weight loss in wounded ‘Red LaSoda’ tubers stored for 7 days at 15, 20, and 25ºC, 90-95% RH. ............................................................................... 165
6-6 Wound periderm suberization of ‘Fabula’ tubers cured for 5 d 20ºC, 90-95% RH, before transfer to 10ºC, 80-85% (T-20) or stored at 10ºC, 80-85% for 14 d (T-10) ............................................................................................................. 165
6-7 Wound periderm suberization of ‘Red LaSoda’ tubers cured for 5 d 20ºC, 90-95% RH, before transfer to 10ºC, 80-85% (T-20) or stored at 10ºC, 80-85% for 14 d (T-10) ................................................................................................... 166
6-8 Fresh weight loss of ‘Fabula’ tubers cured for 5 d 20ºC, 90-95% RH, before transfer to 10ºC, 80-85% (T-20) or stored at 10ºC, 80-85% for 14 d (T-10) ..... 166
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LIST OF ABBREVIATIONS
°C Degree Celsius
d Days
DAP Days after planting
kg.ha-1 Kilograms per hectare
mg 100-1 Milligrams per 100 grams
µm Micrometer
N·m Newton meter
FAO Food and Agricultural Organization
FAWN
USDA
Florida Automated Weather Station
United States Department of Agriculture
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Abstract of Dissertation Presented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy
POSTHARVEST QUALITY OF ‘NEW’ POTATOES: EFFECTS OF IMPROVED WATER AND FERTILIZER USE EFFICIENCY DURING PRODUCTION AND APPLICATION OF
‘RAPID CURING’ DURING STORAGE
By
Mildred N Makani
December 2014
Chair: Steven A. Sargent Cochair: Lincoln Zotarelli Major: Horticultural Sciences
Two tablestock potato cultivars were evaluated for yield and tuber storage quality
response to irrigation method, N-fertilizer rate, application method, and harvest time. In
spring 2011 and 2012, ‘Fabula’ and ‘Red LaSoda’ were irrigated using seepage (SP),
surface drip (SD), or sub-surface drip (Sub-SD) irrigation. To increase tuber skin-set,
plant vines were killed before tuber harvest, 1 to 3 weeks later (H1, H2 and H3) and
stored for 14 days at 10ºC, 80-85% relative humidity. In ‘Fabula’, SD and SP produced
comparable yields, averaging 26,544 kg ha-1. However, soil moisture fluctuations under
SP resulted in higher tuber physiological disorders. ‘Red LaSoda’ was not well adapted
to the soil wetting patterns of drip irrigation, resulting in significantly lower yields and
quality compared to SP. In both cultivars, generally H2 and H3 tubers maintained better
storage quality.
In spring 2013 and 2014, effects of N-fertilizer rate and application method on
method on yield, storage quality and tuber skin-set (in non- and vine killed tubers) was
evaluated. N was applied in standard granular form (224 kg N ha-1) or through SD
irrigation (fertigation rates: 0, 112, 224, 336 kg N ha-1). In both cultivars, N fertigation
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rate of 224 kg N ha-1 had comparable yields, and similar or better storage quality than
the other N treatments. In non-vine killed ‘Fabula’, N stress at 0 and 336 kg N ha-1 likely
delayed tuber development, resulting in lower skin-set. High rainfalls during the harvest
period delayed skin maturation in both cultivars, resulting in similar skin-set in non- and
vine killed tubers. A drier harvest period resulted in higher skin-set, as early one week
after vine kill, in vine killed tubers.
Therefore, irrigating with SD, using 224 kg N ha-1 has the potential to produce
comparable yields to higher N rates, while maintaining tuber postharvest quality in
‘Fabula’. In ‘Red LaSoda’, although drip irrigation produced tubers of comparable
storage quality, SP resulted in higher yields. In both cultivars, tubers harvested two to
three weeks after vine kill generally had higher skin-set and better storability. Storage at
10ºC, 80-85% RH also promoted wound-healing while minimizing weight loss.
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CHAPTER 1 INTRODUCTION
The internal and external quality of potato tubers is very important as it affects
marketability and processing quality. Quality encompasses a combination of
characteristics that give a commodity its degree of excellence and value as food
(Abbott, 1998; Kader 2002). The relative importance of quality parameters depends on
the intended end use (Feltran et al., 2004; Singh and Kumar, 2004).
A study conducted by Jemison et al. (2008) showed that consumers select
tablestock potatoes based on visual characteristics such as shape and color. Therefore,
tubers that are free from any type of blemishes tend to attain a higher market price
(Jemison et al., 2008; Singh et al. 2004). On the other hand, while tuber external
appearance is important, consumers are also concerned with the processing, sensory
and storage quality of tubers. The degree of maturity at harvest is generally recognized
as an important determinant of storability and processing quality (Kumar et al, 2004).
Potato tuber maturity can be viewed as three main components: compositional,
physiological, and physical (Bussan, 2003). Compositional maturity refers to the sugar
concentration of tubers, with mature tubers having minimum levels of sucrose sugars
(Iritani and Weller, 1980; Sowokinos, 1978). Physiologically mature tubers have
reached maximum dry matter content, which usually coincides with maximum starch
accumulation (Sabba et al., 2007; Sowokinos, 1978). The sensory and processing
quality of tubers is greatly influenced by the degree of compositional and physiological
maturity at harvest (Rosenthal, 1999; Sowokinos et al., 2000; Thompson et al., 2008).
Meanwhile, tuber external appearance is greatly influenced by physical maturity, which
18
is the resistance of the periderm (skin) to excoriation (Halderson and Henning, 1993;
Lulai and Orr, 1994; Wilcoxon et al., 1985).
Any form of stress during growth, such as non-uniform soil moisture or nutrient
imbalances, affects various physiological processes. This, in turn, affects leaf growth
and carbon partitioning, thereby negatively affecting tuber maturity and harvest quality
(Tyner, 1997; Wigginton 1974). Storability of tubers is largely determined by the maturity
and condition of the tubers going into storage (Iritani and Weller, 1980). A higher degree
of quantitative and qualitative losses are associated with storage of immature tubers
(Burton, 1989; Iritani and Weller, 1980; Makani, 2010; Wichrowska et al., 2009).
Greater awareness of how preharvest and postharvest factors affect tuber quality
has resulted in increased research on ways of maximizing yields while maintaining tuber
quality. However, most of the research has been devoted to medium and late season
cultivars, commonly known as the fall crop. The fall crop is grown in the fall season and
takes a minimum of four months to mature. Due to longer growing seasons, the crop is
harvested when the tuber skin is set and the vines have commenced natural
senescence. Curing the tubers briefly at high temperatures and relative humidity further
increases physical maturity (skin-set) and heals existing wounds, minimizing storage
losses.
The crop grown in Florida has a shorter growing season of between 90 to 110
days (Mossler and Hutchinson, 2008). These early to medium season cultivars reach
compositional and physiological maturity before the inclement weather conditions of
high rainfall and temperatures set in (Park, et al., 2006). However, the tubers are
harvested before reaching physical maturity, and are therefore characterized by a thin
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periderm which is easily damaged during harvesting and handling (Brecht, 2012;
Chiputula, 2009; Suslow and Voss, 1996). Due to the relatively rapid loss of quality,
these ‘new’ or immature potatoes are stored for very short periods, without curing,
before being distributed to their respective U.S. and Canadian markets (Emekandoko et
al., 2006).
Until recently, production and postharvest handling of new potatoes has been
based on knowledge gained from the behavior of the fall crop. However, applying
findings from studies of the fall crop may cause yield and quality losses in new potatoes.
Therefore, the overall objective of this study was to determine how preharvest factors
affect tuber yield, harvest quality and storability of two tablestock potato cultivars,
commonly grown in Florida.
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CHAPTER 2 LITERATURE REVIEW
The Irish Potato
A native of the Andean highlands of Peru, the commercial potato (Solanum
tuberosum L.) is an annual, herbaceous, dicotyledonous plant belonging to the family
Solanaceae, section Tuberarium. It was derived from a hybrid cross between S.
tuberosum and S. andigenum, and is the most frequently cultivated potato belonging to
the genus Solanum (Salaman et al., 1954).
The aerial parts of the potato plant range from 30 to 80 cm in length, with some
cultivars reaching two meters. White, yellow, purple, blue or variegated inflorescences
are borne on the stems; with many cultivars producing fruit (Linsinka and Leszczynki,
1989). The seed found in the fruit can be used for propagation in breeding; while
vegetative propagation using tubers is the commonly used method in commercial
production (Loria, 2001). Extensive breeding programs have produced many
commercial cultivars which vary in time of maturity (early to late season cultivars), yield
potential, appearance, tuber quality and disease resistance.
The tuber, which is the edible portion, is an enlarged underground stem that
forms at the end of stolons. It maintains the characteristics of the above ground stem,
such as nodes, internodes, scale leaves, and lenticel pores. There are two ends to a
tuber – the bud end and stem end, the latter of which is attached to the stolon. The
tuber consists of four primary zones of tissue (Dean, 1994).The first zone, the periderm
or skin, is the outermost layer and cultivars range in color from brown russet, white, red,
pink, purple or yellow. The skin texture ranges from smooth to netted. The cortex, which
lies between the periderm and the vascular tissue, is the second tissue zone. It contains
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the highest starch concentration. Beneath the cortex lies the perimedullary zone, which
comprises the largest amount of storage tissue. The pith is the central part of the tuber,
and has the lowest starch concentration. The tuber flesh, comprising of the cortex,
perimedullary zone and pith can be white, cream, yellow, purple or striated in color
(Burton, 1989). Tuber shapes are round, oval, oblate, or a combination.
The internal and external quality of potato tubers is very important as it influences
the marketability, processing and sensory quality. Generally, blemish-free, well-shaped
tubers help growers obtain the best market price, while tubers with a good cooking
quality are a favorite among consumers and processors. Nutritionally, the tuber is a low-
fat (0.1%) vegetable; with the digestible dry matter consisting mainly of carbohydrates
(Brown, 2005; Mackay et al., 1987). Raw potatoes have an average energy content of
about 80 kcal per 100 g fresh weight basis (Li, 1985).
Small amounts of antioxidant carotenoids are present in the flesh of all tubers,
ranging from 50 to100 µg per 100 g fresh weight, in white-fleshed tubers. Yellow to
orange-fleshed tubers have a considerably higher amount, averaging 2000 µg per 100 g
fresh weight (Brown, 2005). Significant amounts of iron, thiamine, nicotinic acid and
riboflavin are also found in the vegetable crop. Ascorbic acid (vitamin C) is one of the
most important nutrients in potato. Despite the modest contents (10-30 mg per 100 g
FW), potato tubers are a major source of ascorbic acid because of the large quantities
consumed annually (Tudela et al., 2002). The nutrient, as with other antioxidants, is
found in greater quantities in the potato skin. The potato is a major food source in many
parts of the world largely because of its ability to produce large yields per unit land area
(Dean, 1994).
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U.S. Production and Consumption
Potatoes are grown in over 130 countries across the globe, with the USA
currently being the fourth largest producer – growing the crop commercially in 36 states
(FAO, 2010). In 2013, the U.S. potato industry generated $4.3 billion on over 400,000
ha of harvested land, making potato the top vegetable crop grown (USDA, 2014).
Location of potato production in the U.S. is based on consumer need, new markets and
technological developments (Dean, 1994). The crop is grown year round, with the fall
crop, which is harvested from August through November, contributing about 90% of the
total national production. The primary state producers of the fall crop (also known as fall
crop) include Idaho, Washington, and Wisconsin, producing potatoes worth over $1.9
billion, 56% of the national production in 2012 (USDA, 2012).
However, the spring crop, harvested between April and June, realizes a higher
market price compared to the fall crop. This is because, unlike the fall crop which is
normally stored for months, the spring crop is freshly harvested and marketed, and
therefore meets specific market requirements. Florida, together with Arizona, California,
North Carolina and Texas, are the main producers of the high-value spring crop.
The U.S. potato industry has two main markets: the tablestock and processing
market. Attributes of the cultivar which help determine final use of the tubers include
skin type, set, color, shape and specific gravity. Other desirable characteristics include
yield, adaptability and disease resistance (Pack et al., 2006). Since the late 1980s, the
consumption of potato as food has shifted from tablestock potatoes, to primarily
processed products. The national average per capita utilization of potatoes in 2012 was
36 kg per person in processed products, and 16 kg per person for tablestock potatoes
(USDA, 2013). Processors used 12.8 million metric tons (MT) of potatoes to produce a
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variety of products. Frozen french fries and other frozen products utilized close to 60%
of the processing potatoes, with the rest of the share going to chips, shoestrings
dehydrating, canning, starch and flour production (USDA, 2013). Though a 15%
increase from 2011, only 5.4 million MT were used as tablestock potatoes, with the
common cooking methods being boiling, baking, roasting, frying, microwaving and
steaming.
Irrespective of the lower demand for tablestock potatoes, the industry still
accounts for a big portion, with 30% of the national produce of 2012 being utilized as
tablestock potatoes (National Potato Council, 2013). Less than 6 % of the national
produce was used in the seed industry and for livestock feed (USDA, 2013).
Florida’s ‘New’ Potatoes
Production and Consumption
When a potato cultivar is released, it is classified either as ‘early’, ‘medium’, or
‘late’ season. This is based on vine growth and tuber bulking observations over many
years. Florida potatoes are early to medium season cultivars with an average growing
season of between 90 – 110 days (Mossler and Hutchinson, 2008). They are commonly
known as ‘new’ or ‘early’ potatoes because of their characteristic immature, thin skin
which easily detaches from the underlying cortical tissue during harvest and handling.
‘New’ potatoes can be defined as potatoes harvested before they are completely
mature, marketed immediately after harvesting and whose skin can be easily removed
without peeling (UNECE of Geneva, Standard FFV-30/2001).
With a $135 million market value, the Florida potato industry is relatively small
when compared to other state producers such as Idaho and Washington. However,
Florida contributes a third of the high-value crop produced in the winter or spring season
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(Mossler and Hutchinson, 2008). In the 2013 growing season, Florida’s new potatoes
attained the highest market price of $0.38/kg, while the national average was $0.20/kg
(USDA, 2014).
Florida’s new potatoes are grown all over the state, with an average of 14,933 ha
across the southern and northern counties being used for production. Eighty percent of
potatoes grown in Florida are harvested between May and June (spring growing
season), with a smaller proportion being harvested between January and April (winter
growing season) in the southern parts of state. The lead state producer is the Tri-
County Agricultural Area (TCAA) of Putnam, St. Johns, and Flagler counties in the
northeast, accounting for approximately 85% of the nation’s spring potato production,
and an average yield of 32,630 kg ha-1 (Hochmuth and Hanlon, 2011; USDA, 2012)
Florida is one of the top shipping states in the nation, with most of the growers
establishing pre-season contracts mainly for the processing industry (USDA, 2010).
Cultivars with a high specific gravity, such as ‘Atlantic’ and ‘Harley Blackwell’ are
produced commercially for use in the chipping industry. Tablestock potatoes are also
high value crops, with grower production marketing costs at an average of $4,140 per
acre (Van Sickle et al., 2009). The leading tablestock cultivars grown in Florida include
‘Red LaSoda’, ‘LaRouge’ and ‘LaChipper’, which are all early to medium season
cultivars (Hutchinson et al., 1999).
With potatoes being grown in sandy soils with low water holding capacities and
nutrient content, growers in the TCAA typically irrigate the plants and apply inorganic
fertilizers to maximize yields.
25
Irrigation in the Tri-County Agricultural Area
Water is essential for crop production, accounting for approximately 34% of the
nation’s surface and groundwater usage in the 21st century (USDA, 2012). Florida uses
about 45% of the total freshwater allocated to Agriculture in the country; ranking 9th
nationally and 2nd in the eastern US total irrigated acreage (Marella, 1999; Smajstrla et
al., 1997). Potatoes are the major irrigated crop in the TCAA, with a daily average water
withdrawal of approximately 782 million m3 (Tripensee, 1995; SJRWMD, 1995). An
increase in resident population and erratic rainfall distribution, with 60% of the rain
falling between June and September, has led to increased water demand in the area
(Fereres et al., 2003). Extended periods of dry weather, with some areas experiencing
rainfall deficits of 30 cm, have additionally resulted in a significant decrease in ground
and surface water levels (SJRWMD, 2012).
To supplement the rainfall, majority of the potato crop in the TCAA is irrigated,
with seepage irrigation being the most common method used (Smajstrla et al., 1991;
SJRWMD, 1995). The irrigation season is concentrated between February and May due
to rising daily temperatures and erratic rainfalls. There are two main types of seepage
systems: semi-closed pipeline and open ditch, with the former system being the
common type used by growers in the TCAA. Seepage irrigation is a sub-surface
irrigation system where potatoes are planted on high row beds and shallow lateral
ditches supply irrigation water and facilitate drainage. Water is pumped from deep wells,
raising the natural water table to just below the plant root zone with water moving up by
capillary action to the root zone. The height of the perched water table is maintained by
a loamy fine sand restricting layer, which lies 1 to 1.6 m below the soil surface in the
area (Munoz-Arboleda et al., 2008).
26
Seepage irrigation is a popular method in Florida because it is less expensive to
install and operate (Locascio, 2005). However, the system demands high natural water
tables, or else large quantities of water are required to raise the water table. In addition,
to ensure uniform soil wetting, fields must be relatively level and soils uniformly porous
(Smajstrla et al., 2002). Semi-closed seepage irrigated systems have been reported to
be only 30-70% efficient (Smajstrla et al., 1991). These shortfalls, coupled with
increased competition for water resources have resulted in a growing need to evaluate
drip irrigation as an alternative method for potato production in the TCAA.
Micro irrigation systems such as drip have already proved to be more water use
efficient, using 30-50% less water compared to seepage systems (Niebling and Brooks,
1995; Reyes-Cabrera et al., 2014). With drip irrigation, drippers are placed on the soil
surface (surface drip), or buried (sub-surface drip), with the water being discharged
close to the plant root zone, at a controlled rate. The soil surface is only wetted within
30-60 cm of the water source, thereby maximizing the amount of moisture in the crop
root zone, and minimizing evaporation (Haman et al., 1996). Drip irrigation also reduces
nutrient leaching, erosion, deep percolation, runoff and pumping costs, saving an
average of $110 ha-1 when compared to seepage irrigation (Dukes et al., 2004;
Smajstrla et al., 1991).
Originally, the drip irrigation systems were not popular due to their high
installation and maintenance costs when compared to other conventional methods like
seepage and sprinkler irrigation. However, improvements in the system, such as better
tubing material that can be used over many growing seasons, has renewed interest in
the irrigation method (Camp, 1998). Burying the drip tape below the soil surface, as
27
practiced in sub-surface drip irrigation, further protects the tape from damage and
further reduces evaporation or runoff (Shock, 2006).
Nitrogen Fertilizer Application in the Tri-County Agricultural Area
Carbon, hydrogen, and oxygen, which are assimilated from carbon dioxide and
water uptake, constitute about 95% of the potato biomass. The remaining comes from a
range of macro- and micro-elements which are absorbed from the soil solution (Bucher
and Kossman, 2007). Nitrogen (N), phosphorus (P), and potassium (K) are important as
they greatly affect tuber growth, development and quality. In nutrient deficient soils,
application of organic or inorganic fertilizers improves plant growth and increases tuber
yield (Allison et al., 2001; Trehan and Sharma, 2003; White et al., 2007).
N is a constituent of amino acids, proteins, and nucleic acids, making it the fourth
most abundant element found in tubers. It is absorbed by the plant primarily in the form
of ammonium (NH4+) or nitrate (NO3-) and is the most limiting nutrient in sandy soils (Li
et al., 1999). The nutrient influences biomass accumulation and partitioning, dry matter
content, specific gravity, sugar content, tuber defects and maturity (Marschner, 1995;
Shock et al., 1990). In the past years, excessive N fertilizer application was justified as a
means of ensuring high yields, and rates as high as 300 kg ha-1 were applied in the
1980s (Hochmuth at al., 1993; Weingartner et al., 1999). This liberal application of N
and irrigation water increased nitrate leaching and runoff, in addition to other losses
through gaseous exchange and fixation, thereby reducing the amount of N available to
plants. Further research concluded that 150-250 kg N ha-1 was adequate for most soils
(White et al., 2007). The current University of Florida-IFAS recommended N fertilizer
rate in the TCAA is 224 kg ha-1 (Hochmuth and Hanlon, 2000; Hutchinson et al., 2008).
28
Policymakers and producers are continuously developing new N management
strategies for potato that will further improve nutrient use efficiency and conserve the
environment. Nutrient use efficiency in soils is greatly influenced by fertilizer application
timing and placement (Baligar and Bennett, 1986; Joern and Vitosh, 1995; White et al.,
2007). The current grower practice in the TCAA is band-application of granular N
fertilizer at crop emergence, followed by a side dress at tuber initiation. There is a
period of about 45 days between planting and rapid N uptake by the plant, during which
the risk of leaching is high (Zebarth and Milburn, 2003). More precise application timing
and placement of fertilizer is therefore critical to achieve maximum nutrient use
efficiency.
Applying fertilizers through fertigation has been proposed as a means to meet
production and environmental goals of both growers and policymakers. Fertigation is
the delivery of dissolved fertilizer through drip tapes. This method allows growers to
better synchronize fertilizer application with plant demands, by splitting N applications
throughout the growing period (Shock et al., 2006; Westermann et al. 1988). Fertilizer
applications through drip tapes also allow application where roots are concentrated,
further improving efficiency (Schwankl and Prichard, 2001) and therefore possibly
reducing the amount of fertilizer required to achieve good yields (Shock, 2006).
Potato Growth, Development and Tuber Harvest Maturity
Plant Growth and Development
The life cycle of the potato plant can be divided into five main growth stages, with
early season cultivars reaching maturity in less than 4 months, while late cultivars can
take up to 7 months, depending on the prevailing weather conditions (Kay, 1973; Rowe,
1993; Mossler and Hutchinson, 2008).
29
The first plant growth stage, sprout development (I), begins after planting, when
eyes break dormancy and produce sprouts. This is followed by a 15 to 30 day long
vegetative stage (II), during which leaves, roots and stems develop, and photosynthesis
commences. Tuber initiation stage (III), with duration of 10 to 14 days, begins when
tubers develop at the stolon tips but are not appreciably enlarging.
The tuber bulking stage (IV) is the longest stage, with duration of 60 to over 120
days. During this stage, tuber cells expand with the accumulation of water, nutrients and
carbohydrates, increasing tuber size and weight. The final development phase, the
maturation stage (V), begins with canopy senescence. Leaves become chlorotic and
then necrotic, falling off as the stage progresses. Photosynthesis decreases, tuber
growth slows, with any further increases in tuber dry matter resulting from translocation
of photosynthates from the vegetative tops into the tubers. Tuber maturity is reached
when dry matter content reaches a maximum, reducing sugars content is at a minimum,
and the tuber skin has set.
Tuber Maturity Indices
Tuber maturity at harvest is generally recognized as an important determinant of
tuber storage ability and processing quality (Kumar et al, 2004). Maturity can be viewed
as composed of three components: compositional, physiological, and physical maturity
(Bussan, 2003). Compositional maturity refers to the sugar concentration of tubers, with
mature tubers having minimum levels of sucrose sugars (Iritani and Weller, 1980;
Sowokinos, 1978). Tuber starch content is a measure of physiological maturity.
Physiologically mature tubers have reached maximum dry matter content, which usually
coincides with maximum starch accumulation (Sabba et al., 2007; Sowokinos, 1973).
30
The sensory and processing quality of tubers is greatly influenced by the degree of
compositional and physiological maturity of the tubers at harvest.
Physical maturity refers to resistance to excoriation of the periderm, which is a
measure of skin maturation (Halderson and Henning, 1993; Lulai and Orr, 1994;
Wilcoxon et al., 1985). The periderm is a protective tissue comprising of three parts: the
phellogen (cork cambium), the phellem (cork), and the phelloderm. The phellogen,
which is comprised of actively dividing meristematic cambium cells originates from the
epidermis, and produces phellem cells outwardly and phelloderm cells towards the
cortex (Fahn, 1990). As phellem cells develop, they become suberized and die, to form
the protective layer known as the ‘skin’ (Ginzberg et al., 2009).
Suberin, a complex biopolyester, is deposited on the cell walls when plants are
exposed to biotic or abiotic environmental stress stimuli (Schreiber et al., 2005). It is
comprised of two major domains, a phenolic domain and an aliphatic domain, in
addition to soluble waxes found within the matrix (Lulai and Corsini, 1998). Suberization
of the tuber skin provides a barrier to desiccation of internal tissues and impedes
microbial invasion.
An immature periderm contains a phellogen layer with thin radial cell walls that
fracture easily during harvesting and handling, resulting in the slipping of the phellem
layer (Lulai and Freeman, 2001; Suslow and Voss, 2000). As the periderm matures,
meristematic activity slows, phellogen cell walls thicken with the physiological bonding
process known as ‘skin-set’ occurring during the final stages of periderm maturation.
Tuber physical maturity is of primary importance in Florida’s new potatoes
because the tubers reach horticultural maturity before the periderm has fully matured.
31
The tubers have a poorly developed skin that rubs off easily during harvest and
packaging (Sabba and Lulai, 2002). This can potentially lower the market value of fresh
market potatoes as appearance is one of the key factors influencing consumer
preference. In addition, exposure of the internal tissue increases the occurrence of
water loss and disease pathogen entry.
Plant Vine Killing and Tuber Harvesting
In order to promote skin-set and develop resistance to excoriation, potato plant
vines are killed 7 to 21 days before harvest, a process termed ‘vine kill’ (Bohl, 2003;
Season 1 SP 26,104 az 17,696 a 67.8 306 a 1.2 6,038 a 23.1 SD 26,983 a 17,947 a 66.5 214 a 0.8 6,123 a 22.7 Sub-SD
18,198 b 13,429 b 73.8 122 a 0.7 2,788 b 15.3
Season 2 SP 26,104 a 19,704 a 75.5 245 a 0.9 2,442 a 9.4 SD 25,728 a 20,959 a 81.5 92 a 0.4 1,303 b 5.1 Sub-SD 12,299 b 8,785 b 71.4 92 a 0.8 1,483 b 12.1 z Means within a column (irrigation method) followed by the same small letter, within a particular year, do not differ significantly according to
Tukey’s test at 5% level. yPercent of total yield
Adapted from “Drip as alternative irrigation method for potato in Florida sandy soils” by Reyes-Cabrera et.al., 2014, American Journal of Potato Research.
66
Table 3-2. Analysis of variance for irrigation method and harvest time on selected quality parameters for ‘Fabula’ tubers stored for 14 d at 10°C, 80-85% RH
z Peel or Pulp means within a column (irrigation method) followed by the same small letter, or by the same
capital letter within a row (harvest time) do not differ significantly according to Tukey’s test at 5% level yIrrigation methods, where SP – seepage, SD – surface drip, and Sub-SD – sub-surface drip
x Harvest time: H1, H2, H3 is 1, 2,or 3 wks. after vine kill, respectively
Table 3-4. Effect of harvest time after vine kill on tuber peel and pulp ascorbic acid contents of ‘Fabula’ tubers’ initial storage quality (season 2)
Peel Ascorbic Acid Content (mg 100 g-1)
Pulp Ascorbic Acid Content (mg 100 g-1)
Harvest Timex SPy SD Sub-SD SP SD Sub-SD
H1 6.8 aAz 7.7 aA 8.1 aA 19.0 aB 23.8 aA 25.0 aA
H2 7.0 aA 6.1 aA 6.2 aA 12.2 bA 12.5 bA 13.3 bA
H3 7.8 aA 8.6 aA 7.6 aA 8.1 cA 10.5 bA 10.4 bA
z Peel or Pulp means within a column (irrigation method) followed by the same small letter, or by the same
capital letter within a row (harvest time) do not differ significantly according to Tukey’s test at 5% level. yIrrigation methods, where SP – seepage, SD – surface drip, and Sub-SD – sub-surface drip
x Harvest time: H1, H2, H3 is 1, 2,or 3 wks. after vine kill, respectively
Table 3-5. Effect of irrigation method and harvest time on firmness of ‘Fabula’ tubers, stored for 14 d at 10°C, 80-85% RH (season 2)
Storage Harvest Timex
time (d)
Bioyield Force (N) SPy SD Sub-SD
H1 0 7 14
17.7 aBz 17.1 aB 23.1 aA 13.6 bA 14.4 bA 16.6 bA 14.7 bA 13.9 bA 14.5 bA
H2 0
7 14
12.5 aA 13.2 aA 13.2 aA 12.8 aA 13.0 aA 13.7 aA 13.2 aA 12.5 aA 14.4 aA
H3 0
7 14
14.3 aA 14.4 aA 14.1 aA 14.5 aA 13.5 aA 12.8 aA 14.6 aA 13.3 aA 13.5 aA
z Means within a column followed by the same small letter for each irrigation method at each harvest time,
or by the same capital letter within a row at the same level of storage time, do not differ significantly according to Tukey’s Test (p<0.05). yIrrigation methods, where SP – seepage, SD – surface drip, and Sub-SD – sub-surface drip
x Harvest time: H1, H2, H3 is 1, 2,or 3 wks. after vine kill, respectively.
68
Table 3-6. Effect of irrigation method and harvest time on tuber peel and pulp ascorbic acid content of ‘Fabula’ tubers stored for 14 d at 10°C, 80-85% RH (season 2)
Storage
Harvest Timex
Time (d)
Peel Ascorbic Acid Content (mg 100 g-1)
SPy SD Sub-SD
Pulp Ascorbic Acid Content (mg 100 g-1)
SP SD Sub-SD
H1 0 7 14
6.8 aAz 7.7 aA 8.1 aA 6.8 aA 7.5 aA 7.9 aA 7.1 aA 7.4 aA 7.8 aA
19.0 aBz 23.8 aA 25.0 aA 12.7 bB 18.8 bA 18.2 bA 11.8 bB 17.9 bA 16.3 bA
H2 0
7 14
7.0 aA 6.1 aA 6.2 aA 6.6 aA 6.7 aA 6.3 aA 6.5 aA 7.6 aA 6.0 aA
12.2 aA 12.5 aA 13.3 aA 13.8 aA 12.3 aA 11.9 aA 12.4 aA 13.4 aA 13.9 aA
H3 0
7 14
7.8 aA 8.2 aA 7.6 aA 7.0 aA 8.6 aA 7.7 aA 7.1 aA 7.4 aA 7.1 aA
8.1 aA 10.5 aA 10.4 aA 10.6 aA 10.9 aA 11.6 aA 10.4 aA 12.3 aA 12.4 aA
z Means within a column followed by the same small letter for each irrigation method, at each harvest time, or by the same capital letter within a
row at the same level of storage time, do not differ significantly according to Tukey’s Test (p<0.05). yIrrigation methods, where SP – seepage, SD – surface drip, and Sub-SD – sub-surface drip
x Harvest time: H1, H2, H3 is 1, 2,or 3 wks. after vine kill, respectively
69
Table 3-7. Effect of irrigation method on yield and incidence of physiological disorders in ‘Red LaSoda’ tubers harvested 3 weeks after vine kill
Season 1 SP 40,788 az 29,242 a 71.7 826 a 2.0 7,484 a 18.3 SD 23,092 b 14,182 b 61.4 0 c 0.0 6,296 a 27.3 Sub-SD
27,736 b 18,323 b 66.1 214 b 0.8 6,133 a 22.1
Season 2 SP 31,501 a 23,343 a 74.1 1,254 a 3.9 5,350 a 17.0 SD 21,837 b 15,813 b 72.4 61 c 0.3 3,989 a 18.3 Sub-SD 15,813 c 10,291 c 65.1 245 b 1.6 3,718 a 23.5 z Means within a column (irrigation method) followed by the same small letter, within a particular year, do not differ significantly according to
Tukey’s test at 5% level. yPercent of total yield
Adapted from “Drip as alternative irrigation method for potato in Florida sandy soils” by Reyes-Cabrera et.al., 2014, American Journal of Potato Research.
70
Table 3-8. Analysis of variance for irrigation method and harvest time for selected quality parameters for ‘Red LaSoda’ tubers stored for 14 d at 10°C, 80-85% RH
I x H ns ns ns ns ns ns ns * I x S ns ns ns ns ns ns ns * H x S ns ns ns * ns ns ns * I x Y - - - ns - - - ns H x Y - - - * - - - ns S x Y - - - * - - - ns
I x H x S ns ns ns ns ns ns ns * I x H x S x Y - - - * ns - - ns
* Significant at p<0.05; ns: not significant; - data collected in one season
71
Table 3-9. Effect of harvest time on cumulative fresh weight loss of ‘Red LaSoda’ tubers stored for 14 d at 10ºC, 80-85% RH (season 2)
Storage Harvest Timex
Time (d)
Fresh Weight Loss (%) SPy SD Sub-SD
H1 7 14
2.4 bBz 1.6 bC 3.0 bA 3.3 aB 2.2 aC 4.1 aA
H2 7
14
1.5 bB 1.1 bB 1.1 bB 3.8 aA 1.9 aB 2.0 aB
H3 7
14 1.0 aA 0.4 aB 0.7 aAB
1.0 aA 0.8 aA 1.0 aA zMeans within a column followed by the same small letter for each irrigation method, or by the same
capital letter within a row at the same harvest level do not differ significantly according to Tukey’s Multiple Range (p<0.05). yIrrigation methods, where SP – seepage, SD – surface drip, and Sub-SD – sub-surface drip
x Harvest time: H1, H2, H3 is 1, 2,or 3 wks. after vine kill, respectively
Table 3-10. Effect of irrigation method and harvest time on tuber firmness of ‘Red LaSoda’ tubers stored for 14 d at 10ºC, 80-85% RH (season 2)
Storage Bioyield Force (N) Harvest Timex
Time (d)
SP SD Sub-SD
H1 0 7
14
21.4 aAz 22.9 aA 22.5 aA 15.5 bA 16.1 bA 16.8 bA 14.8 bA 13.7 bA 15.6 bA
H2 0
7 14
14.3 aA 14.1 aA 13.8 aA 15.1 aA 13.2 aA 13.9 aA 15.9 aA 14.9 aA 14.5 aA
H3 0
7 14
13.9 aA 15.4 aA 15.1 aA 13.9 aA 13.4 aA 14.6 aA 15.5 aA 14.9 aA 15.1 aA
z Means within a column followed by the same small letter for each irrigation method at each harvest time,
or by the same capital letter within a row at the same level of storage time, do not differ significantly according to Tukey’s Test (p<0.05). yIrrigation methods, where SP – seepage, SD – surface drip, and Sub-SD – sub-surface drip
x Harvest time: H1, H2, H3 is 1, 2,or 3 wks. after vine kill, respectively
72
A)
B) Figure 3-1. Total and cumulative rainfall during growing A) season 1, and B) season 2 in
Hastings, FL.
0
20
40
60
80
100
120
140
0
20
40
60
80
Vin
e k
ill
Ha
rvest 1
Ha
rvest 2
Ha
rvest 3
Ra
infa
ll (m
m)
Days after planting
Rain Cumu. Rain
0
20
40
60
80
100
120
140
160
180
200
0
20
40
60
80
Vin
e-k
ill
Ha
rvest 1
Ha
rvest 2
Ha
rvest 3
Ra
infa
ll (m
m)
Days after planting
Rain Cumu. Rain
73
Figure 3-2. Effect of harvest time on firmness of ‘Fabula’ tubers’ initial storage quality
(season 2). Harvest times: H1, H2, H3 is 1, 2, or 3 wks. after vine kill, respectively
Figure 3-3. Fresh weight loss in ‘Fabula’ tubers stored for 14 d at 10ºC, 80-85% RH. Harvest times: H1, H2, H3 is 1, 2, or 3 wks. after vine kill, respectively
0
5
10
15
20
25
H1 H2 H3
Bio
yie
ld F
orc
e (
N)
Harvest time after vine kill (weeks)
Seepage
Surface Drip
Sub-SD
0
1
2
3
4
5
6
7
0d 7d 14d
Fers
h w
eig
ht lo
ss (
%)
Storage time (days)
H1
H2
H3
74
Figure 3-4. Effect of harvest time on firmness of ‘Red LaSoda’ tubers’ initial storage
quality (season 2). Harvest times: H1, H2, H3 is 1, 2, or 3 wks. after vine kill, respectively.
0
5
10
15
20
25
30
H1 H2 H3
Bio
yie
ld F
orc
e (
N)
Harvest time after vine kill (weeks)
Seepage
Surface Drip
Sub-SD
75
CHAPTER 4 EFFECT OF NITROGEN FERTILIZER RATE, METHOD OF APPLICATION AND
HARVEST TIME ON YIELD AND STORAGE QUALITY OF SURFACE-DRIP IRRIGATED POTATOES. I. ‘FABULA’
Introduction
Optimizing water and nitrogen (N) management is critical to the potato yield and
tuber quality (Joern and Vitosh, 1995). Drip irrigation has already proved to be a water
use efficient method, producing high yields and good tuber harvest quality (Sammis,
1980; Smajstrla et al., 1995; De Tar et al, 1996). In addition to water, drip irrigation is
capable of supplying dissolved or liquid fertilizer close to the area where plant roots are
concentrated, a method commonly known as fertigation (Phene et al., 1994; Wadell et
al., 1999). Through fertigation, both the timing and rate of nutrient application can be
better controlled to match plant growth needs (Shock et al., 2006; Papadopoulos, 1988).
Nitrogen (N) is a very important macronutrient for potatoes because it influences
biomass partitioning between vines and tubers, tuber bulking rates and subsequent
yields and tuber quality. Excess N favors vine growth over tuber growth and maturation,
thereby decreasing tuber quality, delaying senescence and tuber maturity (Zvomuya et
al., 2002; Love et al., 2005). On the other hand, low N fertilizer limits tuber size, reduces
marketable yield and triggers early plant senescence (Belanger et al., 2002). The IFAS
guidelines for N fertilizer in potatoes is 224 kg ha-1, which most growers apply in
granular form at pre-plant (30 to 40 d before planting), at emergence, followed by two
to three side-dressings after plant emergence. The common application methods of
banding or broadcasting the fertilizer across the beds, increase leaching occurrences,
especially in years with high rainfall, thereby decreasing N availability to the plant
(Hochmuth and Hanlon, 2000; Hutchinson et al., 2008).
76
Fertigation has the potential to minimize N leaching, together with reducing other
losses through volatization, erosion and deep percolation (Shock et al., 2006; Wadell et
al., 1999). Another advantage of fertigation is the ability to split N applications over the
growing season to better match plant growth requirements. This improves N utilization,
minimizes ground water contamination and possibly reduces the amount of fertilizer
required to achieve good yields and tuber quality (Westermann et al., 1988;
Papadopoulos, 1990; Wadell et al., 1999; Shock et al., 2006). Applying N fertilizer
through fertigation could therefore possibly help meet production and environmental
goals of both growers and policymakers. It is therefore important to understand how
interactions of fertilizer application methods, rates and other critical preharvest factors
affect tuber yield, harvest quality, and storability.
The purpose of this experiment was to determine how N fertilizer application
method, rate and harvest time affect tuber yield harvest quality and storability of
surface-drip irrigated potatoes. The first hypothesis was that improving N fertilizer
placement and timing in fertigation application method produces higher yields and better
tuber quality compared to band-application of granular fertilizer. The second hypothesis
was that more precise application of N fertilizer to root zone, and matching crop growth
demands, in fertigation results in higher nutrient use efficiency and optimum yields and
tuber quality.
Materials and Methods
Experimental Site and Field Layout
The study was carried out at the UF/IFAS Florida Partnership for Water
Agriculture and Community Sustainability, Cowpen Branch in Hastings, FL, during the
spring seasons of 2013 (season 1) and 2014 (season 2). Tablestock potato cultivars
77
‘Fabula’ and ‘Red LaSoda’ were evaluated for response to five nitrogen fertilizer
treatments, three harvest times, and 14 d storage duration. The field experiment was a
completely randomized design with treatments in a split plot design and four
replications. The main plot was fertilizer treatment and the sub-plot was cultivar.
The field (79-m long, 18-m wide) was prepared by disking and plowing the soil,
after which rows (78-m long, 0.35-m high) were established, and three cross-cuts
formed to lay the polypipes for drip irrigation. The rows were hilled to improve drainage
and ease during harvesting. The land was divided into 40 plots, each comprising of 4
rows, with the cultivars and fertilizer treatments being randomly assigned. Potato seed
pieces were planted on January 28, 2013 and January 29, 2014, using 20-cm in-row
spacing and 15-cm depth. Phosphorus (P) fertilizer at 112 kg ha-1 rate and Potassium
(K) at 168 kg ha-1 rate was applied pre-plant; an additional side-dress of 140 kg ha-1 of
K was applied at plant emergence. Drip lines (16-mm inner diameter, 8-mm thickness,
30-cm emitter spacing) with a flow rate of 0.5 L h-1 100 m-1 (RO-DRIP, John Deere
Water, Moline, IL, USA) were buried 5 cm above the seed piece after planting. Four
irrigation events, each 25 mins long, were applied each day; with irrigation commencing
38 and 30 days after planting (DAP) in season 1 and 2, respectively.
There were a total of four fertigation treatments with N rates of 0, 112, 224, and
336 kg ha-1 (F-0, F-112, F-224, F-336) and one granular treatment of 224 kg N ha1 (G-
224) (Table 4-1). The fertigation rates were selected to determine if the fertilizer
application method would result in a lower (0 and 112 kg ha-1) or higher (336 kg ha-1)
rate than the current IFAS recommended rate of 224 kg N ha-1. The granular treatment,
78
at 224 kg N ha-1, was used to compare current grower fertilizer application methods with
the fertigation application method.
The fertilizer amount in each fertigation treatment was split over five weeks
matching the plant growth pattern and projected demands. During each application,
granular ammonium nitrate fertilizer (34-0-0) was completely dissolved in water before
being pumped into the main irrigation pipe. Five polypipes were connected to the main
irrigation line, each delivering a level of fertilizer treatment to the respective plots.
Valves were used to close the other polypipes while applying a particular treatment. To
minimize residual fertilizer build-up in the main irrigation line, water was flushed through
in between treatments. To determine effect of application method, the recommended
rate of 224 kg N ha-1 in the form of granular ammonium nitrate was also applied as a
fifth fertilizer treatment. The fertilizer was banded and incorporated into the soil at plant
emergence and when plants were 15-20 cm long.
All pesticide and herbicide applications followed the Florida potato production
recommendations (Hutchinson et al., 2004). Weather data (temperature, wind speed,
relative humidity, and solar radiation) was obtained from a weather station located at the
site. Daily reference evapotranspiration (ETo) was obtained from the Florida Automated
Weather Network (FAWN; www.fawn.ifas.ufl.edu).
Tuber Yield and At Harvest Quality Analysis
The two inner rows were divided into four sections, each 3 m long, where plant
establishment, biomass, and tuber sampling was carried out. A stand count was done
when the plants were well established, with average number of plants per plot used to
calculate plant density. Plant vines and tuber biomass accumulation was evaluated at
full flower (84 and 92 DAP in season 1 and 2, respectively), by randomly selecting two
were affected by storage. Many studies have reported a reduction in specific gravity at
high N rates, supporting observations in this study, where the zero treatment had a
significantly higher ratio than the highest fertigation treatment of 336 kg N ha-1 (Figure
4-3) (Dahlenburg et al., 1990; MacLean, 1984; Porter and Sisson, 1993; White and
Sanderson, 1983). This decline in specific gravity at high N is attributed to excess N
being available particularly during late tuber bulking, possibly delaying tuber maturity
and/or the higher N uptake resulting in higher protein content and lower total soluble
solids (Kunkel and Dow, 1961; Vakis, 1978; Papadopoulos, 1988). Meanwhile, some
authors have not detected significant effects of N rate on specific gravity, as seen during
season 2 of this study (Westermann and Kleinkopf, 1985; Millard and Marshall, 1986).
The results in this study showed that potatoes grown with the granular treatment had a
significantly lower specific gravity than all fertigation treatments, except 336 kg N ha-1.
This further indicates the inadequacies of applying high N rates through drip irrigation.
Evaluation of tuber dry matter content, which is closely related to specific gravity,
over three harvest times showed a greater effect of harvest time on the quality
parameter both before and during storage. The peel content in this study consisted of
primarily the periderm and a few directly underlying layers of cortical cells. Meanwhile
the pulp was comprised of all the inner tissue, that is, the cortex, xylem, perimedullary
and pith. The peel and the pulp are comprised of very different types of cells, and
therefore, it is expected for them to react differently to the same treatment.
94
In season 1, only the peel dry matter content (initial storage quality) of H1 tubers
was affected, with no differences among fertilizer treatments (Figure 4-4). The opposite
trend happened in season 2, where pulp tissue had a higher content in the zero N
compared to fertigation 224 kg N ha-1 at H1 (Table 4-9). This temporal variation in tuber
tissue affected indicates that response of dry matter content to both N fertilizer and
harvest time largely depends on seasonal weather conditions, particularly rainfall. The
rainfall occurrences between vine kill and H1 likely caused increased water content in
the peel tissue, in order to maintain cell turgor pressure against increased negative
solute potential, resulting in a lower peel dry matter content (Westermann et al., 1994).
On the other hand, fertilizer treatment had a greater effect on the pulp dry matter
content during season 2. Previous studies, where the peel and pulp have not been
separated, have shown increased dry matter content with increased N rate, up to the
point of ‘over-fertilization’, with no significant increase thereafter (Westermann et al,
1994; Joern and Vitosh, 1995) or a marked decline (Wszelaczynska and Poberezy,
2011). Findings from this study suggest that 224 kg ha-1 might have been the point of
over-fertilization, resulting in a decreased rate of dry matter translocation to the tubers
during growth. However, weather conditions could have a played a greater role in this,
as this was observed only during the drier season 2. In addition, this was only observed
at the first harvest time, with no significant differences in pulp dry matter among the
fertilizer treatments at the later harvests.
In storage, the significant increase in peel dry matter content at 7 d of H1 tubers,
with no significant differences between fertilizer treatments, was likely due to immaturity
of the tubers (Table 4-12). Water loss may have resulted in increased starch
95
concentration in cells, which constitutes about 70 - 75% of the dry matter content
(Lisinska and Leszczynski, 1989; Kaaber et al., 2001; Nipa et al., 2013). Increased cell
suberization and wound periderm formation has also been reported to account for
increased dry matter (Blekinsop et al., 2002). Meanwhile, storage time had less effect
on the pulp dry matter content, with no significant change throughout for all fertilizer
treatments (Table A-2).
Similar to tuber dry matter content, harvest time had an effect on ascorbic acid
content of tubers’ initial storage quality (Figure 4-5). The decrease in content with
increased delay in tuber harvesting was likely due to a reduction in biosynthesis as the
vines begin to die (Kelly and Somers; 1949; Namek and Moustafa, 1953; Gonella et al.,
2008). According to Mozafar (1993), the response of ascorbic acid to increased N rates
is to reach a maximum biosynthesis point, after which level the content starts to decline.
High N rates cause a delay in maturity and subsequent stage at which tubers exhibit
maximum ascorbic acid content (Shekhar et al., 1978). Our results therefore indicate
that all fertilizer treatments were within the ideal range for adequate nutrient
biosynthesis, similar to findings by Baker et al. (1950). The decline in ascorbic acid
content with increased N rate observed in many other studies, contradicting our studies,
may be due to a shorter growing season for ‘Fabula’, compared to late season cultivars
such ‘Russet Burbank’. In storage, losses of ascorbic acid through oxidation and
formation of dehydroascorbic acid have been reported (Augustin 1975; Watada, 1987;
Perkins 1993; Dale et al., 2003; Phillips at al. 2010). However, unlike in most of these
studied where tubers were stored for months, the short storage conditions and duration
carried out in this study may have prevented an accelerated breakdown.
96
Fresh tuber weight loss during storage was affected by both harvest time and
fertilizer treatment during season 1. The weight loss with increased storage time for all
H1 tubers, irrespective of the fertilizer treatment (Figure 4-6 A), was likely due to an
immature skin not being an effective vapor barrier (Sabba and Lulai, 2002). Skin
maturity increased with a delay in harvest time, resulting in lower weight loss at later
harvests (Adams, 1975; Tyner, 1997; Blenkinsop et al. 2002, Nipa et al., 2013). The
higher weight loss observed in the zero N treatment, in H2 and H3, suggests a delay in
increased tuber maturation after vine kill for tubers produced at this fertilizer level
(Figures 4-6 B and C). This short term effect may have been over-looked in the
previously published studies where weight loss increased with increased N rate, during
six to nine months storage of cured potatoes (Jablonski, 2006; Wszelaczynska and
Poberezny, 2011). However, rate of N fertilizer did not significantly affect tuber weight
loss the following season, confirming work by Dahlenburg et al. (2000). This was likely
due to the drier conditions during vine kill and harvest time promoting uniform suberin
deposition in all fertilizer treatments.
The changes in weight loss seemed to affect tuber turgidity, and subsequently
firmness during storage. The increase in firmness at 7 d storage supports findings by
Gamea et al. (2009), who accounted it to high elasticity of the tubers due to higher
percentage weight loss in the first 25 d of storage (Figure 4-8). A slower rate of weight
loss in the more mature tubers from the later harvests likely accounted for changes in
firmness being observed only towards the end of storage.
Titratable acidity, pH and soluble solids content affect the biochemical quality of
potatoes, which ultimately affect the shelf-life and cooking quality. The pH determines
97
deterioration potential by fermentation and enzyme activity (Cecchi, 1999). The
maximum activity of phosphorylase (starch breakdown) is reached at pH 5.5, while
invertase (sucrose breakdown) is pH 4.7 (Pressey, 1966; Iritani and Weller, 1973). The
pH increase with increased fertilizer rate, before storage (Figure 4-10) and maintaining it
throughout storage, is indicative of a higher tendency of deterioration in the higher
fertilizer treatments. However, for ‘Fabula’, all fertilizer treatments maintained a more
acidic environment, minimizing enzyme activity throughout storage, indicating a lower
fermentation potential. Meanwhile, although the zero N treatment had significantly lower
organic acids and soluble solids at harvest (Figure 4-10), there were no changes in all
treatments throughout storage. This indicates a low respiration rate and/or minimum
build-up of sugars in all treatments, during storage (Chitarra and Chitarra, 1990).
Conclusions
This study revealed the potential of drip irrigation as a means of maximizing
yields, tuber quality and storability in Florida potatoes. Generally, tuber yields were
greatly influenced by the N fertilizer rate and/or application method, while harvest time
had a greater effect on tuber quality before, and during storage.
The fertigation rates of 112 and 224 kg N ha-1 had similar N uptake, biomass
accumulation and N content, to that of the granular treatment. In addition, 112 kg N ha-1
fertigation rate was enough to ensure that nitrogen was not the limiting factor, producing
total yields similar to the higher rates. However, N leaching due to heavy early season
rainfall in season 2 reduced both total and marketable yields for this lower N rate. This
indicates that 112 kg N ha-1 fertigation might be too risky, in the event of seasonal
weather differences, particularly heavy early season rainfall. On the other hand, the
fertigation rate of 224 kg N ha-1 was not affected by the varying weather conditions,
98
maintaining similar yields and a higher specific gravity than the granular method
throughout this study.
The initial and storage quality for most parameters of the tubers was highly
dependent on the harvest time. Any differences in quality among the fertilizer treatments
were generally observed in tubers harvested one week after vine kill. Tubers harvested
at this stage are physiologically and compositionally less mature than the later harvests,
resulting in greater storage losses. However, the storage conditions and duration used
in this study proved to be highly effective in minimizing quality loss, especially when
tubers are harvested two to three weeks after vine kill. Generally, the fertigation rates of
112 and 224 kg N ha-1 proved to maintain a similar or better storage quality than the
granular treatment for most of the quality parameters.
This study therefore concludes that fertigation using surface drip in ‘Fabula’
potato has the potential to produce high yields and tuber quality. There were indications
that nitrogen fertigation rates as low as 112 kg ha-1 could produce good yields and tuber
quality. However, given the varying seasonal weather conditions experienced in Florida,
particularly of low or high rainfall events, a fertigation rate of 224 kg N ha-1 is
recommended as it proved to produce more consistent yields and tuber quality. In
addition, the ideal harvest time recommended is two to three weeks after vine kill,
because the tubers are more mature and therefore maintain acceptable quality when
stored at 10°C, 80-85% RH, irrespective of the fertilizer rate applied.
99
Table 4-1. N fertilizer rates and application methods for ‘Fabula’ potato fertigation trial in 2013 (season 1) and 2014 (season 2)
Treatment Application Method N Rate (kg.ha-1)
F-0 Fertigation 0
F-112 Fertigation 112
F-224 Fertigation 224
F-336 Fertigation 336 G-224 Granular 224
Table 4-2. Plant aboveground (AG) and tuber biomass, on a dry weight basis, at full flower of ‘Fabula’ potatoes grown under five nitrogen treatments in 2013 (season 1) and 2014 (season 2).
Dry Biomass Weight (g plant-1) Season 1 Season 2 Treatmentz AG Tuber Total H.I.x AG Tuber Total H.I.
F-0 14.2 by 37.2 b 51.4 b 0.72 a 14.5 b 19.1 b 33.6 b 0.76 a F-112 28.3 ab 64.0 a 92.3 ab 0.69 a 38.1 a 61.3 a 99.4 a 0.62 c F-224 39.1 a 60.9 a 100.0 a 0.60 b 42.2 a 64.2 a 106.4 a 0.66 b F-336 35.5 a 64.8 a 100.3 a 0.63 b 39.7 a 57.1 a 96.8 a 0.69 b G-224 30.5 a 54.9 a 85.4 ab 0.64 b 38.5 a 58.8 a 97.3 a 0.65 bc zFertilizer treatments of F-0, F-112, F-224, and F-336 represent fertigation rates of 0-336 kg ha
-1, and G-
224 is granular application at 224 kg ha-1
.
y Means within a column followed by the same small letter do not differ significantly according to Tukey’s
Test at 5% level. xHarvest index: Tuber biomass/Total biomass
Table 4-3. Plant aboveground and tuber nitrogen uptake of ‘Fabula’ potatoes grown under five nitrogen treatments, and harvested at full flowering in 2013 (season 1) and 2014 (season 2).
Nitrogen Uptake (gN plant-1) Season 1 Season 2 Treatmentz AG Tuber Total AG Tuber Total
F-0 0.24 cy 0.29 c 0.53 c 0.20 c 0.37 c 0.57 c F-112 0.79 b 0.93 b 1.72 b 0.85 b 1.06 b 1.92 b F-224 1.48 a 1.05 ab 2.53 a 1.59 a 1.81 ab 2.34 a F-336 1.33 a 1.38 a 2.71 a 1.71 a 1.16 b 2.40 a G-224 1.29 a 1.05 ab 2.34 a 0.95 b 2.09 a 2.40 a zFertilizer treatments of R-0, R-112, R-224, and R-336 represent fertigation rates of 0-336 kg ha
-1, and G-
224 is granular application at 224 kg ha-1
. y Means within a column followed by the same small letter do not differ significantly according to Tukey’s
Test at 5% level
100
Table 4-4. Total Kjeldahl nitrogen of ‘Fabula’ potatoes grown under five nitrogen treatments, and harvested at full flowering in 2013 (season 1) and 2014 (season 2).
Treatmentz
Total Kjeldahl Nitrogen (%)
Season 1 Season 2
AG Tuber AG Tuber
F-0 2.1 by 2.0 b 2.0 b 1.3 b F-112 3.1 ab 2.5 ab 2.5 ab 1.8 ab F-224 4.1 a 3.1 a 3.1 a 2.0 a F-336 4.1 a 4.7 a 4.4 a 2.1 a G-224 4.4 a 2.8 ab 2.8 ab 1.9 ab zFertilizer treatments of R-0, R-112, R-224, and r-336 represent fertigation rates of 0-336 kg ha
-1, and G-
224 is granular application at 224 kg ha-1
. y Means within a column followed by the same small letter do not differ significantly according to Tukey’s
Test at 5% level
101
Table 4-5. Tuber yield, size distribution and external physiological disorders in ‘Fabula’ potatoes grown under five fertilizer treatments (season 1)
Total Yield Marketable Yieldw Unmarketable Yieldv Physiological Disordersu
F-0 12,069 bz 6,422 b 53.2 4,301 aa 35.6 1,347 c 11.2 F-112 22,171 ab 13,698 ab 61.8 3,185 b 14.5 5,288 a 23.9 F-224 27,697 a 21,382 a 77.2 2,848 b 10.3 3,468 ab 12.5 F-336 27,624 a 19,812 a 71.7 2,928 b 10.6 4,884 ab 17.7 G-224 21,684 ab 15,715 ab 72.5 2,715 b 12.5 3,254 b 15.0 zMeans within a column followed by the same small letter do not differ significantly according to Tukey’s Test at 5% level.
yFertilizer rates of F-0, F-112, F-224, and F-336 represent fertigation rates of 0-336 kg ha
-1, and G-224 is granular application at 224 kg ha
-1.
xPercent of total yield
wMarketable yield: tuber grades A1, A2, A3.
vUnmarketable grades A4, B, C.
uGreening, growth cracks, misshapen, decay
Table 4-6. Tuber yield, size distribution and external physiological disorders in ‘Fabula’ potatoes grown under five fertilizer treatments (season 2)
Total Yield Marketable Yieldw Unmarketable Yieldv Physiological Disordersu
F-0 5,394 c 2,033 c 37.7 3,229 b 59.9 132 b 2.4 F-112 13,254 b 8,473 b 63.9 3,926 ab 29.6 855 a 6.5 F-224 17,144 ab 12,939 ab 75.5 3,596 b 21.0 609 a 3.6 F-336 19,337 ab 15,015 ab 77.6 3,578 b 18.5 745 a 3.9 G-224 26,562 a 21,029 a 79.2 4,466 a 16.8 1,068 a 4.0 zMeans within a column followed by the same small letter do not differ significantly according to Tukey’s Test at 5% level.
yFertilizer rates of F-0, F-112, F-224, and F-336 represent fertigation rates of 0-336 kg ha
-1, and G-224 is granular application at 224 kg ha
-1.
xPercent of total yield
wMarketable yield: tuber grades A1, A2, A3.
vUnmarketable grades A4, B, C.
uGreening, growth cracks, misshapen, decay
102
Table 4-7. Tuber external disorders in ‘Fabula’ potatoes grown under five fertilizer treatments, and harvested 3 weeks after vine kill (season 1 and 2)
Fertilizer treatmenty
External Physiological Disorders (kg ha-1)
Season 1 Season 2
GRx GC Misshapen Decay GR GC Misshapen Decay
F-0 198 bz 0 b 78 a 1,072 c 0 a 0 a 0 a 132 a F-112 734 a 62 b 125 a 4,367 a 0 a 151 a 0 a 705 a F-224 477 ab 0 b 81 a 2,909 b 0 a 0 a 151 a 459 a F-336 617 a 763 a 308 a 3,196 b 286 a 0 a 0 a 459 a G-224 242 b 0 b 0 a 2,737 b 738 a 88 a 0 a 242 a
z Means within a column followed by the same small letter do not differ significantly according to Tukey’s Test at 5% level.
yFertilizer rates of F-0, F-112, F-224, and F-336 represent fertigation rates of 0-336 kg ha
-1, and G-224 is granular application at 224 kg ha
Year (Y) * * ns ns ns ns ns ns Fertilizer (F) ns * ns ns ns * * * Harvest (H) * * * * ns ns ns ns Interactions: Y x F ns * ns ns ns ns ns ns Y x H * * ns ns ns ns ns ns H x F ns * ns ns ns ns ns ns Y x H x F ns * ns ns ns ns ns ns
* Significant at p<0.05; ns: not significant
103
Table 4-9. Effect of fertilizer treatment and harvest time on pulp dry matter content of ‘Fabula’ tubers’ initial storage quality (season 2)
Harvest Timex
Pulp Dry Matter Content (%)
Fertilizer Treatmenty
F-0 F-112 F-224 F-336 G-224
H1 16.8 aAz 14.7 aAB 13.8 aB 13.9 aAB 14.6 aAB H2 13.3 bA 14.3 aA 14.1 aA 14.1 aA 14.6 aA H3 13.1 bB 14.3 aA 14.7 aA 14.5 aA 15.0 aA z Means within a column (fertilizer treatment) followed by the same small letter, or by the same capital
letter within a row (harvest time) do not differ significantly according to Tukey’s test at 5% level. yFertilizer rates of F-0, F-112, F-224, and F-336 represent fertigation rates of 0-336 kg ha
-1, and G-224 is
granular application at 224 kg ha-1
. x Harvest time: h1 – 1 wk. after vine kill; h2 – 2 wks. after vine kill; h3 – 3 wks. after vine kill.
Table 4-10. Effect of fertilizer treatment on soluble solids content (SSC), total titratable acidity (TTA), and pH of ‘Fabula’ initial storage quality (season 1 and 2)
Fertilizer treatmenty
SSC TTA pH
% %
F-0 2.73 b 0.12 b 2.70 cz
F-112 3.35 b 0.13 ab 3.47 b F-224 3.85 ab 0.15 a 3.88 ab F-336 4.13 a 0.14 a 4.02 a G-224 4.15 a 0.15 a 4.13 a z Means within each column followed by the same small letter do not differ significantly according to
Tukey’s test at 5% level. yFertilizer rates of F-0, F-112, F-224, and F-336 represent fertigation rates of 0-336 kg ha
-1, and G-224 is
granular application at 224 kg ha-1
.
104
Table 4-11. Analysis of variance of season, fertilizer treatment, and harvest time effect on tuber quality in ‘Fabula’ stored at 10°C, 80-85% RH for 14 d
Y x F ns * ns ns * ns ns ns ns Y x H * * ns ns * ns ns ns ns Y x S ns ns ns ns * ns ns ns ns F x H * * ns ns * ns ns ns ns F x S * ns ns ns * ns ns ns ns H x S * ns ns * * * ns ns ns
Y x F x H ns ns ns ns * ns ns ns ns Y x F x S ns ns ns ns * ns ns ns ns Y x H x S ns ns ns ns * ns ns ns ns F x H x S * ns ns ns * * ns ns ns
Y x F x H x S ns ns ns ns * ns ns ns ns * Significant at p<0.05; ns: not significant
105
Table 4-12. Effect of fertilizer treatment and harvest time on tuber peel dry matter content of ‘Fabula’ stored at 10°C, 80-85% RH for 14 d
Harvest Timey
Storage time (d)
Peel Dry Matter Content (%)
Fertilizer Treatmentx
F-0 F-112 F-224 F-336 G-224
H1 0 10.9 bAz 11.1 bA 10.4 aA 10.8 bA 11.3 bA 7 13.6 aA 13.6 aA 13.9 aA 13.9 aA 14.7 aA 14 14.4 aA
14.3 aA 11.4 bB 14.1 aA 11.1 bB
H2 0 10.3 aA 10.8 aA 11.3 aA 10.1 aA 10.2 aA 7 10.0 aA 11.6 aA 11.9 aA 11.5 aA 10.6 aA 14 10.7 aA
11.4 aA 11.7 aA 10.6 aA 10.1 aA
H3 0 10.7 aA 11.4 aA 11.3 aA 11.7 aA 11.5 aA 7 11.1 aA 10.5 aA 11.3 aA 12.1 aA 11.8 aA 14 11.7 aA 11.0 aA 11.5 aA 12.4 aA 11.7 aA z Means within a column followed by the same small letter for each irrigation method, at each harvest
time, or by the same capital letter within a row at the same level of storage time, do not differ significantly according to Tukey’s Test (p<0.05).
y Harvest time: h1 – 1 wk. after vine kill; h2 – 2 wks. after vine kill; h3 – 3 wks. after vine kill.
xFertilizer rates of F-0, F-112, F-224, and F-336 represent fertigation rates of 0-336 kg ha
-1, and G-224 is
granular application at 224 kg ha-1
106
A)
B) Figure 4-1. Rainfall and cumulative evapotranspiration in Hastings, FL, during growing
A) season 1, and B) season 2
0
50
100
150
200
250
300
350
400
450
500
0
20
40
60
80
Vin
ekill H1
H2
H3
Ra
infa
ll (m
m)
Days after planting
Rain Cumu. Rain Cumu. Etc
0
50
100
150
200
250
300
350
400
450
500
0
20
40
60
80
Vin
ekill H1
H2
H3
Ra
infa
ll (m
m)
Days after planting
Rain Cumu. Rain Cumu. Etc
107
A)
B) Figure 4-2. Mean air and soil temperatures experienced during growing A) season 1,
and B) season 2
0
5
10
15
20
25
30
0
20
40
60
80
Vin
e k
ill
H1
H2
H3
Te
mp
era
ture
°C
Days after planting
Air-2m Soil-10cm
0
5
10
15
20
25
30
0
20
40
60
80
Vin
e k
ill
H1
H2
H3
Tem
pera
ture
°C
Days after planting
Air-2m Soil-10cm
108
Figure 4-3. Tuber specific gravity of ‘Fabula’ potatoes grown under five fertilizer treatments. Means separation: same letters do not differ significantly according to Tukey’s Test (p<0.05)
Figure 4-4. Effect of harvest time on the peel dry matter content of ‘Fabula’ tubers harvested 1-3 weeks after vine kill (H1-3). Means separation: same letters do not differ significantly according to Tukey’s Test (p<0.05)
a
ab
ab
bc
c
1.038
1.04
1.042
1.044
1.046
1.048
1.05
1.052
F-0 F-112 F-224 F-336 G-224
Specific
gra
vity
Nitrogen Fertilizer Treatment
b
a
a
0
2
4
6
8
10
12
14
16
H1 H2 H3
Peel dry
matt
er
conte
nt (%
)
Harvest time after vine kill (weeks)
109
Figure 4-5. Effect of harvest time on the peel and pulp ascorbic acid content of ‘Fabula’ tubers harvested 1-3 weeks after vine kill (H1-3). Means separation: same small letter (peel ascorbic acid content), or same capital letter (pulp ascorbic acid content) do not differ significantly according to Tukey’s Test (p<0.05)
a
b
b
A
B B
0
5
10
15
20
25
30
H1 H2 H3
Ascorb
ic a
cid
conte
nt
(mg/1
00g F
W)
Harvest time after vine kill (weeks)
Peel
Pulp
110
A)
B)
C) Figure 4-6. Fresh weight loss of ‘Fabula’ tubers as affected by fertilization, harvest time
after vine kill, and storage for 14 d at 10ºC and 80-85% RH (season 1), where A) harvest 1, B) harvest 2; C) harvest 3
0
1
2
3
4
5
6
0d 7d 14d
Fre
sh w
eig
ht
loss (
%)
Storage time (days)
Season 1: H1
F-0
F-112
F-224
F-336
G-224
0
1
2
3
4
5
0d 7d 14d
Fre
sh w
eig
ht
loss (
%)
Storage time (days)
Season 1: H2
F-0
F-112
F-224
F-336
G-224
0
1
2
3
4
5
0d 7d 14d
Fre
sh w
eig
ht
loss (
%)
Storage time (days)
Season 1: H3
F-0
F-112
F-224
F-336
G-224
111
A)
B)
C) Figure 4-7. Fresh weight loss of ‘Fabula’ tubers as affected by fertilization, harvest time
after vine kill, and storage for 14 d at 10ºC and 80-85% RH (season 2), where A) harvest 1, B) harvest 2, C) harvest 3
0
1
2
3
4
5
6
0d 7d 14d
Fre
sh w
eig
ht
loss (
%)
Storage time (days)
Season 2: H1
F-0
F-112
F-224
F-336
G-224
0
1
2
3
4
5
6
0d 7d 14d
Fre
sh w
eig
ht
loss (
%)
Storage time (days)
Season 2: H2
F-0
F-112
F-224
F-336
G-224
0
1
2
3
4
5
6
0d 7d 14d
Fre
sh w
eig
ht
loss (
%)
Storage time (days)
Season 2: H3
F-0
F-112
F-224
F-336
G-224
112
Figure 4-8. Firmness of ‘Fabula’ tubers stored for 14 d at 10ºC and 80-85% RH
10.5
11
11.5
12
12.5
13
13.5
14
0d 7d 14d
Bio
yie
ld forc
e (
N)
Storage time (days)
H1
H2
H3
113
CHAPTER 5 EFFECT OF NITROGEN FERTILIZER RATE, METHOD OF APPLICATION AND
HARVEST TIME ON YIELD AND STORAGE QUALITY OF SURFACE-DRIP IRRIGATED POTATOES. II. RED LASODA’.
Introduction
Differences in response to water and nitrogen management among potato
cultivars has been reported by several authors (Westermann et al, 1994; Arsenault et
al., 2001; Belanger et al., 2001; Long et al., 2004). Florida grows a wide range of early
to medium season potato cultivars, due to the short growing season, while majority of
growers in other states favor medium to late season cultivars (Pack et al., 2003).
‘Red LaSoda’ is one of the leading tablestock cultivars grown in Florida. It is a
mutant of the red variety ‘La Soda’ and was released by the USDA and Louisiana
Agricultural Experiment Station in 1953 (The Potato Association of America, 2010). ‘Red
LaSoda is considered an early to medium season cultivar, reaching maturity in 85 to 95
days (Hutchinson et al., 2008). The tubers are round or oblong shaped, with a deep red
skin color which fades with maturity. The flesh of the tuber is white, and the specific
gravity relatively low compared to other red-skinned cultivars. Although slightly
susceptible to hollow heart and growth cracks, it is reported to have good storability. It is
primarily used for boiling, baking, chipping and french frying (www.potatoes.wsu.edu).
The objective of this study was to determine the effect of N fertilizer application
method, rate, harvest time and storage on tuber yield and quality of surface-drip
irrigated ‘Red LaSoda’ potatoes. The underlying hypothesis was that by improving
fertilizer placement and timing, fertigation would produce higher yields and better tuber
quality compared to band-application with granular fertilizer. The second hypothesis
(average17.4 mg 100 g-1) or tissue titratable acidity (average 0.14%).
Discussion
There were no significant differences in plant populations between the two
seasons, indicating similar seed germination and plant establishment in both years.
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N uptake was significantly affected by fertilizer rate and application method. N
was removed by potato plants at approximately the same rate in all fertigation
treatments, except for the zero N treatment (R-0), which was significantly. A decline in
tuber N recovery was observed in fertigation treatments during season 2, with no
significant seasonal changes in uptake for the granular treatment lower (Table 5-2). This
can be accounted for by heavy early season rainfalls coinciding with fertigation
applications, causing fertilizer to move beyond the root zone, thereby depleting
available soil N and reducing uptake (Fan and Mylavarapu, 2010, Kelling et al. 1997). A
sidedress in the granular treatment likely maintained fertilizer in the root zone, resulting
in higher N uptake.
Plant vines and tuber N content and biomass at full flower were also influenced
by fertilizer rate. Generally aboveground and tuber N content increased at higher N
rates, compared to the zero N treatment (Table 5-3), as observed in other studies
(Hutchinson et al., 2003). Zero N application also markedly reduced aboveground and
tuber biomass accumulation (Table 5-1). This is due to N deficiency which reduced leaf
area and canopy development, photosynthates production and translocation to the
tubers (Ojala et al., 1990). An increase in N rate generally increased both above ground
and tuber biomass, as observed in previous studies (Belanger et al., 2001; Ojala, et al.,
1990; Sharma and Dubey, 2000).
Availability of N to the growing plant also influenced tuber yields and size
distribution. A higher N fertilization had a positive effect on tuber total yield, with all N
treatments producing significantly higher yields than plants in the zero N treatment
(Tables 5-4 and 5-5), similar to previous studies (Belanger et al., 2002; Hutchinson et al,
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2003; Sharma and Dubey, 2000). Higher crop growth at higher N rates resulted in more
synthesis and translocation of photosynthates to the tubers, and hence increased yields
(Kumar et al, 2007). However, increasing N rates beyond R-112 (112 kg ha-1) did not
significantly increase yield output. According to Rosen et al. (1993), this can be
attributed to a greater yield response by plants at the first fertilizer increment, followed
by a more gradual increase with succeeding increments. N uptake results agreed with
effects of N on total yields, where N unavailability (due to early season leaching rains in
season 2) during tuber initiation and bulking affected fresh tuber growth rate (Ojala et
al., 1990), which resulted in yield decline for all fertigation treatments during season 2
(Tables 5-2 and 5-5). Meanwhile, delayed N applications in the granular treatment
maintained a high N uptake and total yields in both seasons. Similar results of increased
yields with delayed N application (due to reduced leaching) were reported by Kelling
and Speth (1997). A lower harvest index for 112 kg N ha-1 treatment in season 2,
possibly affected yield, resulting in lower total yields than the granular treatment (Tables
5-1 and 5-5). This was likely due to nutrient stress during the vegetative growth stage,
particularly tuber bulking, leading to a reduction in dry matter accumulation (Yungen et
al., 1958; Hughes, 1974; Mackerron and Davies, 1986; Fabeiro at al., 2000).
Availability of N to the growing plant also influenced tuber size distribution,
incidence of physiological disorders and specific gravity. N stress at the zero N
treatment reduced tuber bulking rate, increased proportion of undersized tubers, and
decreased marketable yields (Tables 5-5 and 5-6) , which is agreement with other
studies at the same N rate (Dahlenburg et al., 1990; Hutchinson et al., 2003). Reduced
N uptake during season 2 significantly increased undersized tuber yield for N fertigation
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rates of 112 kg ha-1 and higher. On the other hand, higher levels of N were associated
with increased external defects during season 1 (Table 5-4) similar to findings by Long
et al. (2004). This was likely due to decreased deposition of suberin waxes with
fungicidal properties (Hochmuth and Hanlon, 2011) and/or decreased membrane
permeability (McGuire and Kelman, 1984) at the higher N rates, which increased
susceptibility to soft rot bacteria. Shekhawat et al. (1982) and Kumar et al. (1991) also
reported increased tuber decay with increased N rate.
In addition, as observed at higher N rates in other studies, excess N availability
during tuber bulking in the 224 and 336 kg ha-1 treatments likely delayed tuber maturity
(Porter and Sisson, 1993; Dahlenburg et al., 1990; Westermann et al., 1994;
Mohammed et al., 1999). On the other hand, a significantly higher N uptake rate in the
fertigation and granular N treatments of 336 kg ha-1 possibly resulted in higher specific
gravity (Figure 5-1), as reported in previous studies (Kunkel and Dow, 1961; Vakis,
1978; Papadopoulos, 1988).
N and harvest time were indirectly linked to potato storability through their effect
on tuber relative maturity and quality at harvest. Fresh weight loss in storage was
affected by harvest time, with tubers harvested one week after vine kill losing a
significantly higher percent in both seasons (Figure 5-6). In season 2, tubers harvested
two weeks after vine kill also lost more weight during storage than the final harvest time
(Figure 5-6 B).
Burton (1989) indicated that about 98% of tuber weight loss during storage was
due to water loss through immature periderms, which are not effective vapor barriers.
Delaying harvest time probably increased deposition of suberin waxes in the periderm,
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improving skin maturity as reported in other studies (Blenkinsop et al. 2002, Tyner,
1997; Makani, 2010; Nipa et al., 2013). On the other hand, N rate had no effect on
weight loss, contradicting some reports which indicated increased weight loss for higher
N rates (Jablonski, 2006; Wszelaczynska and Poberezny, 2011). This was likely due to
a cultivar effect, with an earlier tuber development associated with ‘Red LaSoda’
(Hutchinson et al., 2008), resulting in early skin maturation, irrespective of the N rate.
This is further supported by work done by Dahlenburg et al. (1990) who saw no
significant difference in weight loss for tubers fertilized with N rates ranging between 0
to 320 kg ha-1. Love and Pavek (1989) also reported lower weight loss in smooth-
skinned cultivars due to lower permeability, compared to net-skinned cultivars. ‘Red
LaSoda’ is classified as smooth-skinned (Hutchinson et al., 2008).
Weather conditions seemed to have an effect on dry matter content of tubers’
initial storage quality, with treatment effects being observed during the wetter season 1
only. The peel dry matter content depended on the harvest time, with a significantly
higher content being measured in tuber initial storage quality during the first two
harvests (Figure 5-2). High rainfall amounts just prior to the first harvest likely caused an
influx of water into the peel tissue, in order to maintain cell turgor pressure against
increased negative solute potential, resulting in a lower peel dry matter content
(Westermann et al., 1994). This water was later lost through transpiration during storage
(Lisinska and Leszczynski, 1989; Kaaber et al., 2001; Nipa et al., 2013), resulting in an
increase in peel dry matter content with increased storage time for H1 and H2 tubers
(Table 5-11).
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On the other hand, fertilizer treatment had a greater effect on the pulp dry matter
content of tubers’ initial storage quality; a significantly higher pulp dry matter content
was observed in the zero N treatment (Figure 5-3). Pulp DMC decreased significantly at
112 kg ha-1, with no differences at higher rates. These results are in partial agreement
with other researchers who reported no significant increases in dry matter content after
a particular N rate (Lauer 1986; Westermann et al, 1994; Joern and Vitosh, 1995;
Kumar et al., 2007). There are no clear indications why pulp dry matter content in tuber
initials was higher in the zero N treatment, contradicting findings by Kumar et al. (2007)
and many other authors, who saw an increase in dry matter content with increased N
rate. The likely explanation could be less water uptake in the zero N treatment during
the wet harvest period in season 1, resulting in significantly higher DMC. A greater
weather effect is supported by the fact that this was only observed during one season.
Pulp dry matter content was maintained throughout storage, indicating that storage
conditions were ideal for limiting weight loss due to transpiration, as stated by
Blenkinsop et al. (2002).
Similar to peel dry matter content, firmness in tuber initials was significantly
affected by harvest time. The influx of water into tubers due to high rainfall at the
beginning of the harvest period in season 1 likely increased cell turgor, resulting in
significantly higher firmness at the earlier harvest times (Figure 5-4). According to Afek
et. al. (200), water loss in storage breaks down cell turgor, decreasing tuber firmness.
Cargill (1976) noted tuber loss in firmness is only realized at 5% or higher weight loss.
In this study, weight loss during storage did not exceed 3%, resulting in no change in
tuber firmness during storage, in both seasons.
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N rate had no effect on ascorbic acid content (AAC) in tuber initial storage
quality, consistent with other findings (Baker et al., 1950). Similar AAC was also
observed at all harvest times, indicating early tuber development and maturity in ‘Red
LaSoda’ tubers, as indicated by Olson et al. (2010). In the present study, storage of
physiologically and compositionally mature tubers at 10°C, 85-85% RH for 14 d
minimized the oxidative and enzymatic degradation of AAC as has been reported in
long-term storage tests (Augustin 1975; Watada, 1987; Dale et al., 2003; Phillips at al.
2010).
An interaction of N rate and harvest time had an effect on soluble solids content
(SSC) in tubers’ initial storage quality. Although N fertigation rates of 0 and 336 kg ha-1
generally had significantly lower soluble solids at all harvest times, particularly in season
2 (Table 5-9), no further changes were observed during storage, indicating minimum
conversion to sugars in all treatments, during storage.
Conclusions
N fertilizer application method and rate affected tuber yield and harvest quality of
‘Red LaSoda’ tubers. Generally, insufficient N during critical growth periods in the zero
N fertigation treatment reduced tuber yields and size due to lower bulking rates.
Increasing N fertigation rate to 112 kg ha-1 fertigation was sufficient to promote higher
yields due to more synthesis and translocation of photosynthates to tubers. However, a
lower harvest index for this lower rate during season 2 reduced total yields. This was
more likely due to seasonal weather differences, particularly rainfall patterns, since this
was observed during season 2 only, which experienced heavy early seasonal rainfall.
Reduced N uptake during season 2 significantly increased undersized tuber yield for N
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fertigation rates between 112 kg ha-1 and 336 kg ha-1. On the other hand, higher levels
of N were associated with increased external defects during season 1.
Results from this study also showed that the availability of N to the plant was
greatly determined by the application method and timing of application. Scheduling
fertigation applications, to meet projected crop N demands, produced comparable yields
at lower fertigation rates of 112 and 224 kg N ha-1 in season 1. However, heavy early
season rainfall in season 2 depleted soil N, resulting in significantly lower yields in the
fertigation treatments. This indicates that, in the event of high rainfalls at the beginning
of the season, delayed or supplemental N applications might still be required to optimize
yield in fertigation.
The initial and storage quality for most parameters of the tubers was highly
dependent on the harvest time. Any differences in quality of tuber initial storage quality
was generally observed in the less mature tubers harvested one week after vine kill.
Significant differences between fertilizer treatments in tuber initial storage quality were
generally accounted for by N stress at the lowest N rate of 0 kg ha-1. In storage, the only
parameters affected by increased storage time were tuber fresh weight and peel dry
matter content, with harvest time having a greater effect on storage quality losses.
Tuber quality was maintained throughout in all the other parameters, indicating good
storability for all N rates and application methods. This was likely due to a cultivar
effect, with an earlier tuber development associated with ‘Red LaSoda’ (Olson et al.,
2010), resulting in early maturation, irrespective of the N rate.
This study therefore concludes that fertigation with surface drip irrigation, using
224 kg N ha-1, is capable of producing comparable yields and tuber storage quality in
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‘Red LaSoda’. There were indications of granular application being more advantageous
in the event of heavy early season. However, the rainfall distribution experienced in
season 1 was more representative of the Florida growing season, where higher rainfall
typically occurs towards the end of the season. In addition, ‘Red LaSoda’ tubers can be
harvested as early as one week after vine kill, because the cultivar matures earlier. It is
therefore, capable of maintaining acceptable quality when stored at 10°C, 80-85% RH,
irrespective of the fertilizer rate applied.
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Table 5-1. Plant aboveground (AG) and tuber biomass, at full flower of ‘Red LaSoda’ potatoes grown under five nitrogen treatments (season 1 and 2).
Treatmentz
Dry Biomass Weight (g plant-1)
AG Tuber Total Harvest indexx
R-0 19.2 by 38.5 c 57.7 b 0.67 a R-112 38.1 a 61.8 b 99.9 a 0.62 b R-224 32.5 a 75.5 a 108.0 a 0.70 a R-336 29.4 a 70.5 a 99.9 a 0.71 a G-224 31.2 a 67.2 ab 98.4 a 0.68 a
zFertilizer treatments of R-0, R-112, R-224, and R-336 represent fertigation rates of 0-336 kg ha
-1, and G-
224 is granular application at 224 kg ha-1
. y Means within a column followed by the same small letter do not differ significantly according to Tukey’s
Test at 5% level. xHarvest index: Tuber biomass/Total biomass
Table 5-2. Plant aboveground and tuber nitrogen uptake of ‘Red LaSoda’ potatoes grown under five nitrogen treatments, and harvested at full flowering in 2013 (season 1) and 2014 (season 2).
Nitrogen Uptake (gN plant-1)
Season 1 Season 2 Treatmentz AG Tuber Total AG Tuber Total
R-0 0.25 by 0.51 c 0.76 c 0.42 b 0.27 c 0.69 c R-112 0.96 a 1.67 ab 2.63 ab 0.81 a 0.71 b 1.52 b R-224 0.97 a 1.59 ab 2.56 ab 1.03 a 0.85 b 1.88 b R-336 0.95 a 1.93 a 2.88 a 1.20 a 1.18 a 2.38 a G-224 0.87 a 1.47 b 2.34 b 0.81 a 1.61 a 2.42 a
zFertilizer treatments of R-0, R-112, R-224, and R-336 represent fertigation rates of 0-336 kg ha
-1, and G-
224 is granular application at 224 kg ha-1
. y Means within a column followed by the same small letter do not differ significantly according to Tukey’s
Test at 5% level
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Table 5-3. Total Kjeldahl nitrogen of ‘Red LaSoda’ potatoes grown under five nitrogen treatments, and harvested at full flowering in 2013 (season 1) and 2014 (season 2).
Treatmentz
Total Kjeldahl Nitrogen (%)
Season 1 Season 2
Above ground Tuber Above ground Tuber
R-0 1.70 c 0.95 c 2.45 b 1.13 b R-112 2.91 b 1.66 b 3.30 a 1.50 ab R-224 3.16 ab 1.88 ab 2.71 ab 1.85 a R-336 3.62 a 2.11 a 3.52 a 1.73 a G-224 3.58 ab 2.01 ab 3.32 a 1.61 a
zFertilizer treatments of R-0, R-112, R-224, and r-336 represent fertigation rates of 0-336 kg ha
-1, and G-
224 is granular application at 224 kg ha-1
. y Means within a column followed by the same small letter do not differ significantly according to Tukey’s
Test at 5% level
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Table 5-4. Tuber yield, size distribution and external physiological disorders in ‘Red LaSoda’ potatoes grown under five fertilizer treatments (season 1)
Total Yield Marketable Yieldw Unmarketable Yieldv Physiological Disordersu
F-0 12,939 b 7,438 b 57.5 3,919 a 30.3 1,582 b 12.2 F-112 25,459 a 19,639 a 77.1 2,191 b 6.6 3,629 a 14.3 F-224 28,497 a 23,973 a 84.1 2,499 ab 8.8 2,026 ab 7.1 F-336 29,048 a 23,881 a 82.2 2,569 ab 8.8 2,598 ab 8.9 G-224 20,439 ab 15,599 ab 76.3 2,616 ab 12.8 2,224 ab 10.9 zMeans within a column followed by the same small letter do not differ significantly according to Tukey’s Test at 5% level.
yFertilizer rates of F-0, F-112, F-224, and F-336 represent fertigation rates of 0-336 kg ha
-1, and G-224 is granular application at 224 kg ha
-1.
xPercent of total yield
wMarketable yield: tuber grades A1, A2, A3.
vUnmarketable grades A4, B, C.
uGreening, growth cracks, misshapen, decay
Table 5-5. Tuber yield, size distribution and external physiological disorders in ‘Red LaSoda’ potatoes grown under five fertilizer treatments (season 2)
Total Yield Marketable Yieldw Unmarketable Yieldv Physiological Disordersu
F-0 6,833 b 3,024 b 44.3 3,427 b 50.2 382 a 5.6 F-112 11,878 b 7,710 ab 64.9 3,750 ab 31.6 418 a 3.5 F-224 14,931 ab 9,445 ab 63.3 4,814 a 32.2 672 a 4.5 F-336 15,937 ab 11,053 ab 69.4 4,491 a 28.2 393 a 2.5 G-224 21,445 a 16,124 a 75.2 4,319 a 20.1 1,002 a 4.7 zMeans within a column followed by the same small letter do not differ significantly according to Tukey’s Test at 5% level.
yFertilizer rates of F-0, F-112, F-224, and F-336 represent fertigation rates of 0-336 kg ha
-1, and G-224 is granular application at 224 kg ha
-1.
xPercent of total yield
wMarketable yield: tuber grades A1, A2, A3.
vUnmarketable grades A4, B, C.
uGreening, growth cracks, misshapen, decay
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Table 5-6. Tuber external disorders in ‘Red LaSoda’ potatoes grown under five fertilizer treatments, and harvested 3 weeks after vine kill in 2013 (season 1) and 2014 (season 2)
Treatmenty
External Physiological Disorders (kg ha-1)
Season 1 Season 2
Green GC Misshap Decay Green GC Misshap Decay
R-0 360 az 51 b 99 b 1,071 b 92 b 0 a 26 b 264 a R-112 444 a 99 b 169 ab 2,917 a 297 a 0 a 290 a 84 a R-224 44 b 0 b 88 b 1,893 ab 59 b 0 a 139 ab 220 a R-336 539 a 521 a 228 ab 1,310 b 154 ab 0 a 0 b 239 a G-224 173 ab 0 b 356 a 1,695 ab 532 a 0 a 334 a 136 a
z Means within a column followed by the same small letter do not differ significantly according to Tukey’s Test at 5% level.
yFertilizer rates of R-0, R-112, R-224, and R-336 represent fertigation rates of 0-336 kg ha
-1, and G-224 is granular application at 224 kg ha
-1.
Table 5-7. Analysis of variance of season, fertilizer treatment, and harvest time on tubers’ initial storage quality in ‘Red LaSoda’
Y x F ns * ns ns ns * ns ns Y x H * ns ns ns * * ns ns H x F ns ns ns ns ns * ns ns
Y x H x F ns ns ns ns ns * ns ns * Significant at p<0.05; ns: not significant
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Table 5-8. Effect of fertilizer treatment and harvest time after vine kill on tissue soluble solids content of ‘Red LaSoda’ tuber initial storage quality (season 1).
Harvest Timex
Soluble Solids Content (%)
Fertilizer Treatmenty
R-0 R-112 R-224 R-336 G-224
H1 2.98 bBz 3.93 aA 3.65 aA 3.88 aA 3.90 aA H2 3.30 aB 3.33 bB 3.45 abAB 3.75 aA 3.55 aAB H3 3.14 abB 3.18 bB 3.20 bB 3.43 bAB 3.60 aA
z Means within a column (fertilizer treatment) followed by the same small letter, or by the same capital
letter within a row (harvest time) do not differ significantly according to Tukey’s test at 5% level. yFertilizer rates of R-0, R-112, R-224, and R-336 represent fertigation rates of 0-336 kg ha
-1, and G-224
is granular application at 224 kg ha-1
.x Harvest time: h1 – 1 wk. after vine kill; h2 – 2 wks. after vine kill; h3 – 3 wks. after vine kill.
Table 5-9. Effect of fertilizer treatment and harvest time after vine kill on tissue soluble solids content of ‘Red LaSoda’ tuber initial storage quality (season 2).
Harvest Timex
Soluble Solids Content (%)
Fertilizer Treatmenty
R-0 R-112 R-224 R-336 G-224
H1 3.40 bCz 4.01 aB 4.07 aB 3.32 aC 4.55 aA H2 3.75 aC 4.18 aB 4.33 aAB 3.43 aC 4.62 aA H3 3.58 abC 4.09 aB 4.20 aAB 3.37 aC 4.56 aA
z Means within a column (fertilizer treatment) followed by the same small letter, or by the same capital
letter within a row (harvest time) do not differ significantly according to Tukey’s test at 5% level. yFertilizer rates of R-0, R-112, R-224, and R-336 represent fertigation rates of 0-336 kg ha
-1, and G-224 is
granular application at 224 kg ha-1
. x Harvest time: h1 – 1 wk. after vine kill; h2 – 2 wks. after vine kill; h3 – 3 wks. after vine kill.
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Table 5-10. Analysis of variance of fertilizer treatment, harvest time on tuber quality in ‘Red LaSoda’ tubers stored at 10°C, 80-85% RH for 14 d
Y x F * * ns ns ns ns ns ns ns Y x H ns ns ns ns * ns ns ns ns Y x S ns ns ns ns * ns ns ns ns F x H ns ns ns ns ns ns ns ns ns F x S ns ns ns ns ns ns ns ns ns H x S ns ns ns ns * ns ns ns ns
Y x F x H ns ns ns ns ns ns ns ns ns Y x F x S ns ns ns ns ns ns ns ns ns Y x H x S ns ns ns ns * ns ns ns ns F x H x S ns ns ns ns ns ns ns ns ns
Y x F x H x S ns ns ns ns ns ns ns ns ns * Significant at p<0.05; ns: not significant
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Table 5-11. Effect of harvest time on tuber peel dry matter content of ‘Red LaSoda’ stored for 14 d at 10°C, 80-85% RH
Storage timey
Peel dry matter content (%)
Harvest Timex
H1 H2 H3
0d 14.93 bBz 14.89 cB 17.54 aA 7d 17.49 aA 17.05 bA 16.39 aA
14d 16.59 aB 19.64 aA 16.43 aB z Means within a column followed by the same small letter do not differ significantly according to Tukey’s
Test at 5% level. yStorage time at 10ºC, 80-85% RH
xHarvest time – 1 week (H1), 2 weeks (H2), 3 weeks (H3) after vine kill.
136
A)
B) Figure 5-1. Tuber specific gravity of ‘Red LaSoda’ potatoes grown under five fertilizer
spacing) with a flow rate of 0.5 L h-1100 m-1 (RO-DRIP, John Deere Water, Moline, IL,
USA) buried 5 cm above the seed piece.
Vine Kill and Tuber Harvesting
The two inner rows in each plot were divided into four sections, each 3-m long,
where tuber sampling was carried out. At maturity (91 and 98 DAP in seasons 1 and 2,
144
respectively) one section in each plot was randomly selected to be the non-vine kill
treatment. Polythene mulch was used to completely cover the plants, just prior to the
desiccant spray, to avoid vine killing these sections. A single application of the chemical
desiccant glufosinate ammonium (Rely® 280 Herbicide; Bayer Crop Science, Research
Triangle Park, NC, USA), applied at a rate of 1535 ml ha-1, was sprayed throughout the
entire field trial. The plastic mulch covers were removed after the minimum re-entry time
(4 hours) using personal protective equipment had expired.
Harvest times were selected on the basis of number of weeks after vine kill. An
initial harvest (H0) was carried out by hand-harvesting tubers from two randomly
selected plants in each plot, prior to vine kill spray. Tubers were carefully placed in
paper bags, to avoid skinning injury while transporting to the laboratory for tests. The
same procedures were used for subsequent weekly harvests from the non-vine killed
(NVK) and vine killed (VK) sections. These were carried out 1-3 weeks after the vine
killing time point (H1 to H3). To quantify tuber decay after vine killing, a 3-m vine-killed
section in each plot was harvested at the same time points of 1-3 weeks after vine,
using a mechanical harvester.
Tuber Skin-set Tests
Upon arrival at the laboratory, loose sand was gently brushed off the tubers,
before sorting and randomization from each fertilizer treatment (n=4). Total resistance to
skinning was determined by recreating the skin-set testing device developed by J.L.
Halderson and R.C. Henning (1993), as described by Lulai and Orr (1993). The skin-set
testing device consisted of a Snap-on® “Torquometer”, model TQSO50FUA (Kenosha,
WI). The torquometer had a ‘main pointer’, which provided real-time readings, and a
‘follow-up pointer’, which showed the maximum torque reading reached during the skin-
145
set tests. A #2 rubber stopper (20-mm diameter) was mounted on the end of a spring
loaded shaft which was connected to the torquometer. To minimize slippage during
tests, water resistant grit paper (#100, 20-mm diameter) was glued to the end of the
rubber tip (Weir, 2001), and the rubber stopper was replaced at each harvest time. To
take a torque reading, the rubber stopper was pressed against the skin on the
equatorial portion of each tuber, ensuring the main pointer remained at the zero point.
The torquometer was then twisted gradually until the skin sheared and a complete disk
of skin broke loose. The maximum torque reading (0 to 0.67 N•m range) was recorded
as the measured skin-set value and the follow-up pointer was re-set to zero for the
subsequent reading.
In season 1, the skin strength increased with succeeding harvests and by H3 the
device became difficult to use. The upper part of the device tended to detach; to avoid
this source of error, the following modifications were made to the device prior to season
2. The torquometer was rotated 90º in relation to the handle and fitted onto a drill press
stand. This permitted the torquometer to be pressed and secured against the tuber
surface with uniform force for each reading (Figure 6-1). Next, the handle was twisted
as, in the previous version, and shear strength was measured. The modified device
considerably reduced user fatigue and improved consistency in season 2.
Experiment 2: Evaluation Of Tuber Wound Periderm Suberization
Determination of ‘Rapid Curing’ Conditions
Freshly-harvested ‘Red LaSoda’ tubers were used to determine the most suitable
rapid curing conditions for new potatoes in spring 2012. Tubers were carefully washed
and fan-dried for 10 mins to remove all surface moisture, before wounding each tuber.
146
A small circular area of periderm and underlying cortical tissue (20-mm diameter,
2-mm deep) was marked and carefully excised with a sharp blade from the equatorial
region of each tuber and discarded. The tubers were placed on trays in racks; to
maintain a high humidity (90-95%), humidification trays were placed in alternating order
with the trays containing the tuber samples (Figure 6-2). Humidification was
accomplished by soaking laboratory bench paper with deionized water. Loosely fitted
plastic covers were placed over the crates to maintain the high humidity, while still
allowing ventilation. Data loggers were placed in the racks to monitor temperature and
relative humidity.
The racks were placed in each of three temperature-controlled rooms. Three
curing temperatures of 15, 20, or 25ºC were selected, based on previous research.
Tubers (n=3) were evaluated for suberin deposition thickness on the wound site, using
histochemical analysis, at 1, 3, 5, and 7 d in storage. Whole tuber (n=4) fresh weight
was also evaluated at the beginning (0 d) and end of storage (7 d). The storage
temperature which promoted maximum suberin deposition in the shortest storage time,
while minimizing fresh weight loss, was then selected for further ‘rapid curing’ tests.
Effect of Storage Condition On Wound Periderm Suberization
In spring 2013 (season 1) and 2014 (season 2), ‘Fabula’ and ‘Red LaSoda’
tubers were grown using surface-drip irrigation and fertilized with granular N at 224 kg
ha-1. Tubers were hand-harvested three weeks after vine kill, and bagged in paper bags
to avoid skinning injury during transportation to the laboratory. Upon arrival, tubers were
washed and wounded using the method described in the tests to determine the rapid
curing conditions. After wounding, half of the tubers were placed in one of two storage
treatments. In the first treatment (T-20), tubers (n = 25) were placed in the humidity
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controlled racks (90-95% RH) and placed in the rapid curing storage room of 20ºC. After
5 d at the curing conditions, the T-20 tubers were transferred to the simulated
commercial storage conditions (10ºC, 80-85% RH) for the rest of the storage period.
The other half of the tubers wounded tubers (n=25) were stored in the second storage
treatment (T-10) of 10ºC, 80-85% RH, simulating commercial storage conditions, for
the entire 14 d storage duration. Evaluations for wound periderm suberization
(histochemical analysis), whole tuber fresh weight loss and dry matter content were
carried out at 0, 5, and 14 d.
Tissue Preparation And Histochemical Analysis
At each sampling time of 5 d and 14 d, tissue blocks (1 cm x 2 cm x 1 cm deep)
from the wounded area of each tuber sample (n=3) were excised and fixed in a solution
of formalin : acetic acid : 95% ethanol : deionized water (3 : 1 : 10 : 7, v/v/v/v). The
tissue blocks were sectioned into 10 µm sections using a sliding hand microtome
(model DK-10, Edmund Scientific, Barrington, N.J.). The sectioned tissues were
dehydrated in a graded ethanol series according to standard procedures (Ruzin, 1999).
Sections were mounted on subbed microscope slides and dried in a 58°C oven for 12 h,
followed by a full day at room temperature to ensure full adhesion to the microscope
slide.
Tissue was washed twice in xylene to remove paraffin, and then dehydrated
once more to 70% ethanol in a graded series. A 1% w/v Safranin O solution (Sigma-
Aldrich, Saint Louis, MO) was dissolved at room temperature. Tissue sections were
stained in the solution for 2 h, before the slides were rinsed in deionized water with
gentle agitation. The sections were counterstained in 0.15% w/v Fast Green FCF
solution (Sigma-Aldrich, Saint Louis, MO), according to standard procedures by
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Johansen (1940). The procedure allows analysis of suberized cells by staining them into
an intense red color.
The stained sections were examined using standard light microscopy with an
Olympus system microscope (model BX51, Olympus America, Inc., Melville, NY) using
10x magnification. The depth (µm) of suberized cell layers on the wound periderm was
measured with a caliber fixed in the microscope lens eye.
Statistical Analysis
The two cultivars, ‘Fabula’ and ‘Red LaSoda’, were analyzed separately using
Analysis of Variance (SAS Institute Inc. Version 9.3, Cary, NC, USA). A general linear
mixed model (PROC GLIMMIX) was performed to determine preharvest and
postharvest main and interaction effects on tuber skin maturity. The analysis was
implemented as a completely randomized design with split plot, where the main plots
were fertilizer treatment, harvest time and vine kill. Treatment means were separated
using the Tukey’s test and confidence limits of 95% calculated for each mean.
Results
Weather Conditions
Seasonal differences in rainfall occurrences just prior to vine kill and over the
following three week harvest period were observed (Figure 6-3). In season 1, a
cumulative rainfall amount of 23 mm occurred just prior to vine kill. An even higher
amount of 268 mm was experienced the following week between vine kill and the first
harvest (H1). Season 2 harvest period was drier, with only 36 mm cumulative rainfall
occurring between vine kill and H3.
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Tuber Decay Incidence
Tuber decay incidence in both potato cultivars was affected by harvest time and
season. Generally, high decay amounts of 2,756 kg ha-1 and 2,273 kg ha-1 for ‘Fabula’
and ‘Red LaSoda’, respectively, were observed in season 1. The following season, the
decay incidence was lower, with an average of only 592 kg ha-1 over the three week
harvest period, in both cultivars.
Decay incidence also increased with increased delay in tuber harvesting. During
season 1, decay in ‘Fabula’ increased from 1,104 kg ha-1 (4% of total yield) at H2 to
1,652 kg ha-1 (7% of total yield) at H3; there was no decay at H1. In season 2, tuber
decay was observed at all harvest times, although H3 had 466 kg ha-1 (11% of total
yield), which was significantly higher than the earlier harvests, which averaged 71 kg ha-
1 (2% of total yield) (Figure 6-4 A). In ‘Red LaSoda’, decay was observed at the final two
harvests, even under the drier conditions in season 2; average decay was 1,137 kg ha-1
(5% of total yield) and 287 kg ha-1(9% of total yield) at H2 and H3, respectively (Figure
6-4 B).
Experiment 1: Determination Of Tuber Resistance To Skinning Injury In Relation To Nitrogen Treatment and Harvest Time
There was a significant interaction between harvest time, fertilizer treatment and
season on the tuber skin strength of non-vine killed (NVK) and vine killed (VK) ‘Fabula’
tubers. In season 1, harvest time x fertilizer treatment had a significant effect on the skin
strength of NVK tubers (Table 6-1). There were no differences in skin strength among
the fertigation treatments just prior to vine kill time (H0), averaging 0.18 N•m. At H2, skin
strength for F-336, which was 0.15 N•m was significantly lower than F-112 and F-224,
which averaged 0.21 N•m. A significant increase in strength was observed in all
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fertigation treatments at H3, where F-0 (0.25 N•m) was significantly lower than F-112
and F-224, which both averaged 0.31 N•m. There were no differences in skin strength
between the granular and fertigation application methods in NVK tubers, at all harvest
times. Meanwhile, VK tubers were affected by harvest time only, with skin strength
increasing from an average of 0.18 N•m at the H0 and H1, to 0.26 at H2 (Table 6-2).
There were no differences in average skin-set readings between NVK and VK tubers at
all harvest points.
In season 2, harvest time had a greater effect on both NVK and VK in ‘Fabula’.
Significantly higher skin strength of 0.29 N•m was only observed at H3 for NVK tubers,
while the first three harvests averaged 0.25 N•m (Table 6-2). On the other hand, VK
tubers significantly increased from 0.24 N•m at H0, to 0.31 N•m at H1, with no further
significant increases. VK tubers harvested 1-3 weeks after vine kill had significantly
higher skin-set, average 0.32 N•m, compared to NVK tubers (average 0.26 N•m), during
the same time period.
In ‘Red LaSoda’, year x harvest time had a significant (p < 0.05) effect on the
tuber skin strength of both NVK and VK tubers (Table 6-3). In season 1, NVK ‘Red
LaSoda’ tubers at H2 had a significantly higher reading of 0.26 N•m, compared to an
average of 0.19 N•m for H0 and H1. A similar trend was observed in the VK tubers, with
no differences in skin-set readings at all harvests between NVK and VK treatments.
In season 2, the greatest skin strength in NVK was observed in H1 and H3
tubers, averaging 0.28 N•m, which was significantly higher than H0 and H2. In VK
tubers, H1 (0.29 N•m) had a higher reading than H0 and H2 (average 0.24 N•m); while
H3 the highest reading of 0.33. Differences in average skin-set between NVK and VK
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tubers was only observed at H3, where the NVK had a significantly lower reading of
0.27 N•m, compared to 0.33 N•m for VK tubers.
Experiment 2: Evaluation Of Tuber Wound Periderm Suberization
Determination of ‘Rapid Curing’ Conditions
To determine the rapid curing condition and duration, suberin deposition on
wounded tuber periderms was evaluated following 7 d storage at 15, 20, or 25ºC. A
significant interaction between storage temperature and storage time affected the
thickness of the suberized layer in the wound periderm. Suberization was observed at 3
d in all storage conditions, increasing with storage time for all temperatures (Table 6-4).
At 5 d, the thickest suberin layer of 26.7 µm was observed at 25ºC; while tubers
stored at 20ºC averaged 17.6 µm and 15ºC had the lowest reading of 13.7 µm. The
greatest thickness by the end of storage (7 d) was observed at 25ºC, with no significant
differences at the other two storage temperatures. On the other hand, the highest
cumulative weight loss during storage of 2.33% was observed at 25ºC, while the other
storage conditions averaged 0.97% (Figure 6-5). Storage temperature had no effect on
whole tuber dry matter content, with all treatments averaging 18.8% throughout storage.
Based on this data, it was therefore concluded that storage at 20ºC for 5 d would
facilitate adequate curing in the shortest time, without compromising tuber quality.
Effect of Storage Condition On Wound Periderm Suberization
Differences were observed in wound periderm suberization and fresh weight loss
between the two storage treatments of 10ºC, 80-85% (T-10) and 20ºC, 90-95% (T-20) in
‘Fabula’ tubers. At the end of the curing period (5 d), tubers stored at 20ºC, 90-95% (T-
20) had significantly higher suberization of 16.5 µm, compared to 12.5 µm in those at
10ºC, 80-85% (T-10) (Figure 6-6). Upon transfer to 10ºC, 80-85%, T-20’s suberization
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further increased to 24.0 µm during the remaining storage time, while no further
increases where seen in T-10 tubers. T-20 tubers also maintained significantly higher
weight loss of 3.55% throughout storage of 3.55%, compared to 2.87% in T-10 (Figure
6-8).
‘Red LaSoda’ followed a similar trend, showing more suberization in T-20 tubers
(18.4 µm) than T-10 (9.3 µm) at 5 d. By the end of storage, T-20 tubers had a
significantly thicker suberin layer of 26.8 µm, while T-10 was 13.6 µm (Figure 6-7).
There were no significant differences in weight loss between the treatments throughout
storage, averaging 1.51% loss at the end of storage. Whole tuber dry matter content
was not affected by storage condition or duration, averaging 13.25% and 14.93% in
‘Fabula’ and ‘Red LaSoda’, respectively.
Discussion
Experiment 1: Determination Of Tuber Resistance To Skinning Injury In Relation To Nitrogen Treatment and Harvest Time
Delaying tuber harvesting after vine kill by three weeks resulted in increased
‘Fabula’ tuber decay in both growing seasons. During season 1, tubers harvested two
weeks after vine kill also recorded a high decay incidence, added to the fact that the
overall decay was generally higher in this season too (Figure 6-4 A). This was attributed
to high rainfalls experienced during the harvest period in season 1. Meanwhile, ‘Red
LaSoda’ tubers harvested 2 and 3 weeks after vine kill had significantly higher decay
incidence, in both seasons (Figure 6-4 B).
During tuber growth, lenticels are the main ports of entry for decay pathogens. As
the tubers mature, suberin, a complex biopolyester, is deposited at the base of the
lenticel, forming a closing layer (Lulai, 2007). Waxes with fungicidal properties found in
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this suberized closing layer are believed to aid in keeping pathogens out (Lulai and
Corsini, 1998; Tyner, et al., 1997). By delaying tuber harvesting, given the high rainfall
amounts in season 1, lenticels cells were allowed to proliferate and rupture through the
protective suberized layer (Makani, 2010), predisposing the tuber to decay (Tyner, et
al., 1997). This resulted in high decay incidence as early as two weeks after vine kill, in
both cultivars.
According to Adams (1975), drier soil conditions promote lenticel suberization,
which would explain the decreased severity of decay during season 2 in both cultivars.
However, ‘Red LaSoda’ maintained a high decay incidence at both H2 and H3,
indicating the cultivar was more susceptible to decay. Genotypic differences in
resistance to bacterial infection have also been reported in previous studies (Bain and
Perombelon, 1988; Kumar et al, 1991).
Tuber periderm resistance generally increased with increased delay in harvest
time, in both cultivars. By affecting plant growth and development, fertilizer treatments
had an effect on periderm maturation in NVK ‘Fabula’ tubers. The highest level of N
fertigation treatment in this study (336 kg N ha-1) likely promoted vegetative growth over
tuber development (Hope et al., 1960; Shock, 2006; Wszelaczynska and Poberezny,
2011), thereby delaying tuber maturation at H1 (Table 6-1). The other extreme N
treatment of 0 kg N ha-1 also delayed skin-set with increased harvest time, resulting in
significantly lower periderm resistance than 112 and 224 kg N ha-1 at the final harvest.
This could possibly indicate that any form of plant stress, be it too high or too low N rate,
can affect phellogen activity and periderm maturation in NVK potatoes. However, these
results were only observed during season 1.
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In season 2, harvest time had a greater effect on the skin-set of NVK ‘Fabula’
tubers, with significantly higher readings at the final harvest (Table 6-2). The condition
of plants vines affects periderm maturation, increasing skinning resistance with
increased vine senescence (Bowen, 1996; Yagamuchi et al., 1966). With irrigation shut
off two weeks before the initial harvest (H0), and lower amounts of rainfall occurring
during the harvest period in season 2, natural vine senescence likely progressed
uniformly in all fertilizer treatments. However, significantly higher skin-sets were
observed at the final harvest only, indicating a delay in progression of plant senescence
for NVK tubers.
A delay in tuber harvesting also increased periderm maturation in VK ‘Fabula’
tubers (Table 6-2). Increased skin-set was observed two weeks after vine kill during
season 1, agreeing with previous studies (Halderson and Henning; 1993; Lulai and Orr
1993). In season 2, the drier weather during harvest period resulted in faster vine
senescence (Figure 6-3). Therefore, increased skin-set was observed as early as one
week after vine kill, agreeing with studies by Pavilsta (2002). Lack of significant rainfall
amounts during the harvest period in season 2 also increased the vine kill efficiency,
resulting in higher skin-set in tubers harvested 1-3 weeks after vine kill, compared to
NVK.
A slightly different pattern was observed in ‘Red LaSoda’ tubers. In season 1,
significantly higher skin-set was only observed at H2, with no differences in skin
resistance readings due to vine kill (Table 6-3). This indicates that high rainfalls
received just prior to vine kill and between H1 maintained growth and lush vines
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(Kempenaar and Struik, 2007; Lulai and Orr, 1994) in this cultivar, delaying senescence
and subsequent skin-set (Braue et al, 1983; Yagamuchi et al, 1966).
The drier conditions in season 2 accelerated vine senescence in both NVK and
VK tubers, resulting in significantly higher skin-sets by H1 (Table 6-3). The decrease in
skin resistance for both treatments at H2 is best explained by a temporary increase in
the fragile nature of the phellogen cell walls, as reported by Lulai (2002).
The similar skin-set readings between NVK and VK tubers until H2 during both
seasons can be explained by early periderm maturation and/or a general delay in
response to vine killing.
Experiment 2: Evaluation Of Tuber Wound Periderm Suberization
The main potato crop (storage potato) is commonly held at 15-18°C and 80-95 %
relative humidity (RH) for an average of 2 weeks (Hide et al., 1994; Suslow and Voss,
2000), before long-term storage. The results in this study show that increasing the
temperature to 20ºC while maintaining relative humidity at 90-95%, shortened the
required curing period in new potatoes (Table 6-4). Wound healing in tubers involves
the formation of closing layers of suberized cells, followed by suberization of newly
developed cells under the closing layer.
A quantifiable amount of wound periderm suberin was detected at 5 d in both the
curing (20ºC, 90-95%) and simulated commercial storage conditions (10ºC, 80-85%)
(Figures 6-6 and 6-7). This indicates increased stress-induced suberin deposition in
both storage treatments (Lulai, 2001). Additional stress at the high curing temperatures
increased suberization rate, while high humidity minimized cell desiccation at the wound
site (Burton, 1989; Kim and Lee, 1993; Lulai, 2007). Transferring tubers at 5 d from the
curing conditions to the lower storage temperature and humidity seemed to trigger more
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stress-induced suberization. Lulai (2001) reported that any type of tuber stress
increased the rate of suberin deposition. Meanwhile, tubers of both cultivars kept at 10
ºC, 80-85% for the entire storage time maintained significantly lower suberization.
However, there were cultivar differences in weight loss response to storage
temperatures. The higher curing temperatures resulted in significantly more weight loss
in ‘Fabula’, compared to storage at the commercial conditions (Figure 6-8). According to
Lulai and Orr (1994), a delay in wound periderm formation and suberization leads to
higher weight loss at the beginning of storage. The 5-day time period for rapid curing
used in this study was based on rate of wound-healing in ‘Red LaSoda’, of which
quantifiable amounts were only detected at 3 days in storage. It is therefore possible
that ‘Fabula’ had a delayed response to wound-induced suberization, resulting in
significantly higher weight loss at the beginning of curing.
Another reason could be water loss through the native periderm. ‘Fabula’
generally has a longer seed dormancy period, thereby shortening the growing season.
This likely resulted in immature tubers, characterized by lower periderm suberization
and higher permeability (Lulai and Orr, 1994). This is further evidenced by the fact that
the earlier germinating ‘Red LaSoda’ had significantly lower weight loss throughout
storage, with no significant differences between storage conditions. No significant
differences were observed in whole tuber dry matter content in both cultivars, indicating
that storage conditions had a lesser effect on the physiological properties of the tubers.
Conclusions
Adequate skin-set reduces wounding and skinning of tubers during harvesting
and handling, thereby minimizing qualitative and quantitative losses during storage. It is
therefore important to use objective methods to measure tuber skin maturity before
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harvesting. The hand-held skin tester used in season 1 during this study proved difficult
to use as tuber skin resistance increased. The modified tester used in season 2 was
more efficient; being able to measure skin-set was at all harvest times. It also minimized
user fatigue. There were generally no differences in the range of skin-set readings
between the two devices.
The weather conditions experienced in season 1 were more representative of
Florida’s spring potato growing season, which is characterized by wet harvest periods
and increasing ambient temperatures, promoting high tuber decay. This study showed
that ‘Fabula’ tubers harvested three weeks after vine kill were more susceptible to
decay in both seasons. By delaying tuber harvesting, lenticels proliferated and ruptured
through the protective suberized layer, predisposing the tuber to decay. In ‘Red
LaSoda’, a higher decay incidence at both H2 and H3 is indicative of a lower resistance
to bacterial infection in the cultivar.
The effect of fertilizer rate, vine killing and harvest time also depended on the
prevailing weather conditions during harvest. High rainfall amounts during the harvest
period delayed skin-set in VK ‘Fabula’ tubers, with increased skin resistance observed
two weeks after vine kill. Drier weather during the harvest period resulted in faster vine
senescence; increased skin-set was observed as early as one week after vine kill. A
similar trend was observed in VK ‘Red LaSoda’ tubers; although a temporary increase
in the fragile nature of the phellogen cell walls likely caused a decrease in skin-set at
harvest 2 in season 2 only. There were also indications that the extreme N rates of 0
and 336 kg ha1 induced ‘Fabula’ plant stress, which affected tuber growth,
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development, and phellogen activity, resulting in a lower skin-set. However, this was
only observed in NVK plants in season 1.
The high rainfall in season 1 delayed vine desiccation after spraying, resulting in
no significant differences in skin-set readings between NVK and VK tubers in ‘Fabula’
tubers. Lower rainfall the following season increased the vine kill efficiency, resulting in
a higher skin-set in VK tubers. On the other hand, vine killing seemed to have less of an
effect on the earlier maturing ‘Red LaSoda’. This may have been due to early tuber
growth and development, resulting in a more mature periderm at vine kill time.
In storage, curing at 20ºC, 90-95% for 5 d increased stress-induced suberin
deposition, resulting in a thicker suberized wound-periderm in both cultivars, compared
to non-cured tubers. However, significantly lower fresh weight loss was observed in
‘Fabula’ tubers stored in simulated commercial storage conditions of 10ºC, 80-85% for
14 d. No differences in weight loss were observed between cured and non-cured ‘Red
LaSoda’ tubers. This indicated that quality of skinned or wounded new potato tubers is
better maintained at 10ºC, 80-85% RH.
In conclusion, this study showed that ‘Fabula’ and ‘Red LaSoda’ tubers
harvested two weeks after vine kill had increased skin-set, which can help minimize skin
injury during harvest and handling. However, in the event of drier harvest periods, with
no high rainfall occurring just prior to vine kill, tubers could be harvested as early as one
week after vine kill. The results also showed that rapid curing of wounded tubers after
harvest increased wound periderm suberization. However, the high curing temperatures
caused quality loss in ‘Fabula’; compared to storage at 10ºC, 80-85% RH. In ‘Red
LaSoda’, both storage treatments maintained tuber quality. This was attributed to a
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more mature native periderm in the cultivar. It can therefore be concluded that, given
the short-term storage of new potatoes, storage at 10ºC, 80-85% RH is sufficient
enough to promote adequate wound healing, while minimizes storage losses. The
benefits of curing reported in for the fall crop, are likely due to the tubers having a more
mature native periderm at harvest, which minimizes weight loss during curing.
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Table 6-1. Skin-set readings of non-vine killed ‘Fabula’ tubers grown under five nitrogen treatments and harvested over two weeks (91 to 105 days after planting), during season 1.
Harvest timez
Skin-Set Readings (Torque, N•m)
Fertilizer treatment
F-0y F-112 F-224 F-336 G-224
H0 0.17 bAx 0.18 bA 0.17 bA 0.19 bA 0.18 bA H1 0.18 bAB 0.21 bA 0.20 bA 0.15 bB 0.18 bAB H2 0.25 aB 0.31 aA 0.30 aA 0.29 aAB 0.28 aAB xMeans within a column (fertilizer treatment) followed by the same small letter or within a row (harvest
time) followed by the same capital letter, do not differ significantly according to Tukey’s Test at 5% level. yFertilizer rates of F-0, F-112, F-224, and F-336 represent fertigation rates of 0-336 kg ha
-1, and G-224 is
granular application at 224 kg ha-1
. zHarvest times at maturity before vine kill (H0), 1 week (H1) and 2 weeks after vine kill (H2).
Table 6-2. Effect of vine kill on skin-set readings of ‘Fabula’ tubers harvested over three weeks during season 1 (91 to 112 days after planting) and season 2 (98 to 119 days after planting).
Harvest timey
Skin-Set Readings (Torque, N•m)
Season 1 Season 2
Non-vine kill Vine kill Non-vine kill Vine kill
H0 0.18 bAx 0.18 bA 0.24 bA 0.24 bA H1 0.18 bA 0.19 bA 0.26 abB 0.31 aA H2 0.28 aA 0.26 aA 0.24 bB 0.32 aA H3 -z - 0.29 aB 0.34 aA xMeans within a column (vine kill treatment) followed by the same small letter or within a row (harvest
time), for each season, followed by the same capital letter, do not differ significantly according to Tukey’s
Test at 5% level. yHarvest times at maturity before vine kill (H0), 1 week (H1), 2 weeks (H2), and 3 weeks (H3) after vine
kill (H2). z- missing data point.
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Table 6-3. Effect of vine kill on skin-set readings of ‘Red LaSoda’ tubers harvested over three weeks during season 1 (91 to 112 days after planting) and season 2 (98 to 119 days after planting).
Harvest timey
Skin-set Readings (Torque, N•m)
Season 1 Season 2
Non-vine kill Vine kill Non-vine kill Vine kill
H0 0.20 bAx 0.20 bA 0.23 bA 0.23 cA H1 0.18 bA 0.19 bA 0.28 aA 0.29 bA H2 0.26 aA 0.28 aA 0.23 bA 0.24 cA H3 -z - 0.27 aB 0.33 aA xMeans within a column (vine kill treatment) followed by the same small letter or within a row (harvest
time), for each season, followed by the same capital letter, do not differ significantly according to Tukey’s Test at 5% level. yHarvest times at maturity before vine kill (H0), 1 week (H1), 2 weeks (H2), and 3 weeks (H3) after vine
kill (H2). z- missing data point.
Table 6-4. Average thickness of suberized wound periderm layer of ‘Red LaSoda’ stored for 7 days at 15, 20, and 25ºC, 90-95% RH.
Storage time (days)
Suberized layer (µm)
Storage Temperature (ºC)
15 C 20 C 25 C
3 d 5.0 cAx 5.7 bA 10.0 bB 5 d 13.7 bC 17.6 aB 26.7 aA 7 d 18.6 aB 20.0 aB 27.5 aA xMeans within a column (fertilizer treatment) followed by the same small letter or within a row (harvest
time) followed by the same capital letter, do not differ significantly according to Tukey’s Test at 5% level.
162
Figure 6-1. Modified skin-set tester used to measure resistance to periderm injury.
Figure 6-2. Layout of trays with tubers in racks for ‘rapid curing’ at 20ºC, 90-95% RH (left rack), and stimulated storage conditions at 10ºC, 80-85% (right rack).
163
Figure 6-3. Total rainfall experienced during tuber harvest period in season 1 and 2, where VK is vine kill and H1-3 is harvest 1-3 after vine kill.
0
20
40
60
80
100
120
VK
H1
H2
H3
2m
Rain
Tota
l (m
m)
Days after planting
Season 1
Season 2
164
A)
B)
Figure 6-4. Tuber decay (kg ha-1) in A) ‘Fabula and B) ‘Red LaSoda’ tubers harvested 1 to 3 weeks after vine kill (H1-H3) during two seasons.
0
200
400
600
800
1000
1200
1400
1600
1800
2000
H1 H2 H3
Tuber
decay (
kg
/ha)
Harvest time after vine kill (weeks)
Season 1
Season 2
0
200
400
600
800
1000
1200
1400
1600
1800
H1 H2 H3
Tuber
decay (
kg
/ha)
Harvest time after vine kill (weeks)
Season 1
Season 2
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Figure 6-5. Fresh weight loss in wounded ‘Red LaSoda’ tubers stored for 7 days at 15, 20, and 25ºC, 90-95% RH.
Figure 6-6. Wound periderm suberization of ‘Fabula’ tubers cured for 5 d 20ºC, 90-95% RH, before transfer to 10ºC, 80-85% (T-20) or stored at 10ºC, 80-85% for 14 d (T-10)
0
0.5
1
1.5
2
2.5
3
15 C 20 C 25 C
Fre
sh w
eig
ht lo
ss (
%)
Storage Temperature (degree C)
0
5
10
15
20
25
30
0d 5d 14d
Suberin layer
(µm
)
Storage time (days)
T-10
T-20
Transfer to 10ºC, 80-85% RH
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Figure 6-7. Wound periderm suberization of ‘Red LaSoda’ tubers cured for 5 d 20ºC, 90-95% RH, before transfer to 10ºC, 80-85% (T-20) or stored at 10ºC, 80-85% for 14 d (T-10)
Figure 6-8. Fresh weight loss of ‘Fabula’ tubers cured for 5 d 20ºC, 90-95% RH, before transfer to 10ºC, 80-85% (T-20) or stored at 10ºC, 80-85% for 14 d (T-10)
0
5
10
15
20
25
30
0d 5d 14d
Suberin layer
(µm
)
Storage time (days)
T-10
T-20
Transfer to 10ºC, 90-95% RH
0
0.5
1
1.5
2
2.5
3
3.5
4
0d 5d 14d
Fre
sh w
eig
ht lo
ss (
%)
Storage time (days)
T-10
T-20
Transfer to 10ºC, 80-85%
167
CHAPTER 7 SUMMARY AND FINAL CONCLUSIONS
Optimizing water and nitrogen (N) management is critical to potato yield and
tuber quality. Soil moisture and N stress during growth has generally been associated
with decreased yields, tuber harvest quality and storage ability. By applying water close
to the plant root zone, drip irrigation has the potential to minimize moisture stress, while
producing high yields and good tuber harvest quality. In addition, nutrient use efficiency
can be improved when fertilizer application timing and rate is better controlled to match
plant growth needs, using drip irrigation.
Many studies have shown comparable or improved yields and tuber quality with
drip irrigation. However, most of these studies been done on the fall crop, which are
generally medium- to late-maturing cultivars. The fall crop is usually cured to improve
skin-set, before the tubers are put into long-term storage at low temperatures. On the
other hand, Florida’s ‘new’ potatoes are early- to medium-season cultivars, which are
harvested with an immature periderm, making them more prone to postharvest quality
losses. The influence of preharvest factors on yield and storage quality of new potatoes
has not been fully explored, and applications of recommendations made for the fall crop
to the new potatoes could potentially cause yield and quality losses. Therefore, the
overall objective of this study was to determine how preharvest factors affect tuber yield,
harvest quality and storability of two tablestock potato cultivars, commonly grown in
Florida.
An interaction of irrigation method and harvest time on tuber yield and storage
quality was evaluated in spring 2011 and 2012. In ‘Fabula’ adequate water supply in
seepage and surface drip irrigation resulted in significantly higher yields than sub-
168
surface drip. However, significantly higher physiological disorders were observed in the
seepage irrigated tubers. In storage, harvest time had a greater effect on tuber quality;
less mature tubers, harvested one week after vine kill, experienced significant losses in
tuber quality with increased storage time. Meanwhile, in ‘Red LaSoda’, higher tuber
yields and at harvest quality was achieved with seepage irrigation. High decay was
observed in the drip irrigation methods, which was attributed to ‘Red LaSoda’ likely not
being well adapted to the soil wetting patterns of drip irrigation. In storage, similar to
‘Fabula’, significant losses were only found in the less mature tubers harvested one
week after vine kill, which also lost more weight and softened faster.
With surface drip irrigation showing good potential, particularly in ‘Fabula’, the
second objective was to determine how N fertilizer application method, rate and harvest
time affected tuber yield and storage quality of surface-drip irrigated potatoes. In
‘Fabula’, fertigation rates as low as 112 kg N ha-1 produced yields comparable to higher
N rates. However, fertigation rate of 224 kg N ha-1 consistently produced yields
comparable to the granular treatment, regardless of the varying seasonal weather. In
storage, differences in quality among the fertilizer treatments were generally observed
in tubers harvested one week after vine kill. The fertigation rate 224 kg N ha-1
maintained similar or better storage quality than the granular treatment, for most of the
quality parameters. Similar yield and tuber harvest quality results were observed in ‘Red
LaSoda’. In storage, the only parameters affected by increased storage time were tuber
fresh weight and peel dry matter content, in tubers harvested one week after vine kill.
This indicated that tuber maturity in ‘Red LaSoda’ was generally achieved as early as
harvest two, likely due to a longer growing season.
169
The third objective was to determine the effect of N rate, plant vine kill and
harvest time on tuber physical maturity. Total resistance to skinning was measured
using a modified skin-set testing device developed by J.L. Halderson and R.C. Henning
(1993). Seasonal variations in weather had a huge effect on the skin-set of both vine
killed and non-vine killed ‘Fabula’. The high rainfall in season 1 delayed vine desiccation
after spraying, resulting in no significant differences in skin-set readings between non-
and vine killed ‘Fabula’ tubers. Higher skin resistance in vine killed tubers was only
observed two weeks after vine kill. Lower rainfall the following season resulted in faster
vine senescence; increased skin-set was observed as early as one week after vine kill.
A similar trend was observed in vine killed ‘Red LaSoda’ tubers
The final objective was to evaluate rapid curing as a means of minimizing storage
losses in skinned tubers by promoting suberization and wound-healing. Rapid curing
However, the high temperatures compromised quality in ‘Fabula’; while no significant
differences were observed in storage quality between cured and non-cured ‘Red
LaSoda’ tubers. This indicated that storage at the simulated commercial conditions of
10ºC, 80-85% RH were ideal in minimizing quality loss in wounded tuber.
These results therefore showed that surface drip, using 224 kg N ha-1 has the
potential to produce high yields while maintaining tuber postharvest quality in ‘Fabula’.
In ‘Red LaSoda’ although drip irrigation produced comparable storage quality, seepage
resulted in higher yields. There is need for more research to determine if further
adjustments in the drip irrigation scheduling could produce yields comparable to
seepage irrigation.
170
The study also showed that tuber resistance to skin injury was highly dependent
on weather conditions. Given the characteristic high rainfalls experienced during the
growing season in Florida, which delay skin-set, the recommended harvest time to
avoid skin injury in both cultivars is two to three weeks after vine kill.
However, since skin-set is highly dependent on weather conditions, as reported
in this study, it therefore critical for growers to have access to objective measuring tools,
to better determine when to harvest. The modified device developed in this study is
portable, accurate, user friendly and is a viable option that can be used to measure
skin-set in new potatoes.
Lastly, based on observations in this study, 10°C, 80-85% RH is the
recommended storage condition for new potatoes, since it generally maintained
acceptable quality of tubers harvested two to three weeks after vine kill. These storage
conditions also proved to be effective in maintaining quality of wounded tubers, by
promoting adequate wound-healing while minimizing weight loss and storage decay.
171
APPENDIX SUPPORTING DATA
Table A-1. Tuber classification used to evaluate marketable and unmarketable yield after harvest, 3 weeks after vine kill.
Size identification
code
Diameter size range (cm) Classification
C 1.27 – 3.81 Unmarketable
B 3.81 – 4.78 Unmarketable
A1 4.78 – 6.35 Marketable
A2 6.35 – 8.26 Marketable
A3 8.26 – 10.16 Marketable
A4 ˃ 10.16 Unmarketable
Table A-2. Effect of interaction of fertilizer treatment and harvest time on tuber pulp dry matter content of ‘Fabula’ stored at 10°C, 80-85% RH for 14 d
Harvest Timex
Pulp Dry Matter Content (%)
Fertilizer Treatmenty
F-0 F-112 F-224 F-336 G-224
H1 16.1 aAz 14.9 aB 15.3 aAB 14.8 aB 14.6 aB
H2 14.6 bA 14.9 aA 14.8 aA 13.8 aB 14.2 aA
H3 13.8 bA 13.9 bA 14.4 aA 14.3 aA 13.8 aA z Means within a column (fertilizer treatment) followed by the same small letter, or by the same capital
letter within a row (harvest time) do not differ significantly according to Tukey’s test at 5% level. yFertilizer rates of F-0, F-112, F-224, and F-336 represent fertigation rates of 0-336 kg ha
-1, and G-224 is
granular application at 224 kg ha-1
. x Harvest time: h1 – 1 wk. after vine kill; h2 – 2 wks. after vine kill; h3 – 3 wks. after vine kill.
Table A-3. Effect of an interaction of fertilizer treatment, harvest time and storage time on soluble solids content (SSC), and total titratable acidity (TTA) and pH of ‘Fabula’ tubers stored at 10°C, 80-85% RH for 14 d.
Fertilizer treatmenty
SSC TTA pH
% %
F-0 2.85 c 0.11 c 3.25 bz
F-112 3.48 b 0.13 b 3.52 b F-224 3.73 ab 0.14 ab 3.99 ab F-336 3.96 a 0.15 a 4.29 a G-224 3.95 a 0.15 a 4.27 a z Means within each column followed by the same small letter do not differ significantly according to
Tukey’s test at 5% level. yFertilizer rates of F-0, F-112, F-224, and F-336 represent fertigation rates of 0-336 kg ha
-1, and G-224 is
granular application at 224 kg ha-1
.
172
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BIOGRAPHICAL SKETCH
Mildred N. Makani was born in Masvingo, Zimbabwe, the last child in a family of
seven. She did her primary school education at Victoria High School, later proceeding to
do her Cambridge Ordinary Levels at St. Dominic’s High School, Chishawasha. She
completed her Advanced Levels at Victoria High School, before enrolling at the
University of Zimbabwe, where she graduated with a Bachelor of Science honors
degree in Plant Science.
Mildred then joined a private company which specialized in providing virus-free
plant material of drought tolerant crops such as sweet potato and cassava. She
specialized mostly in the postharvest area, with her work involving research and
extension work on the handling, storage and value-addition of the root crops after
harvest. In 2009, Mildred decided to pursue her graduate studies, joining Dr. Steven
Sargent’s Lab to pursue a Master of Science degree in Horticultural Sciences. Upon
completion of her master’s degree, she continued with a Doctor of Philosophy in the