The Effects of Potassium Fertilization on Water-use Efficiency in Crop

8/10/2019 The Effects of Potassium Fertilization on Water-use Efficiency in Crop

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2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.plant-soil.com

J. Plant Nutr. Soil Sci.2013,176,355374 DOI:10.1002/jpln.201200287 355

Review Article

The effects of potassium fertilization on water-use efficiency in cropplants$

Witold Grzebisz1*, Andreas Gransee2, Witold Szczepaniak1,andJean Diatta1

1 Department of Agricultural Chemistry and Environmental Biogeochemistry, Poznan University of Life Sciences, Wojska Polskiego 71F,60-637 Pozna, Poland

2 K + S Kali GmbH, 34111 Kassel, Germany

AbstractThe supplies of water and nitrogen to a plant during its critical stages of growth are the main fac-tors that define crop yield. A crop experiences irregular water deficits during its life cycle in rain-fed agriculture. An effective anti-stress-oriented approach therefore ought to focus on increasingthe units of water productivity. The main objective of this conceptual review is to confirm thatadequate K management can be used as an important tool to alleviate the negative effects ofwater deficit on plant growth, yield-component formation, and yield. The French and Schultzapproach of using the water-limited yield (WLY) was modified in this review into a graphical formand was used to discriminate between yield fractions that depended on the volume of transpiredwater from those that were induced by K fertilizer. By using this method, it was possible todemonstrate the extent of several crop (winter wheat, spring triticale, maize, sugar beet)

responses to the K supply. Yield increases resulting from K application mostly appeared underconditions of mild water deficit. As described for sugar beet, finding the critical period of crop Ksensitivity is a decisive step in understanding its impact on water-use efficiency. It has beenshown that an insufficient supply of K during crucial stages in the yield formation of cereals(wheat, spring triticale), maize, and sugar beet coincides with a depressed development in theyield components. The application of K fertilizer to plants is a simple agronomic practice used toincrease crop tolerance to a temporary water shortage. It may be that the improvement of aplants access to K during mild water-deficiency stress will increase water uptake by the rootcells, which in turn increases their osmotic potential and thereby allows extension growth. Thisgrowth in turn promotes access to other mineral elements (including nitrogen) and water, whichfavor plant growth and yield.

Accepted April 5, 2013

Key words:drought stress / water-limited yield / critical stage / root system

1 Introduction

The yield potential of a particular crop may be defined as themaximum plant productivity in the presence of an ample sup-ply of water and nutrients (Evans and Fisher, 1999). How-ever, rain-fed crops are subject to intermittent water stress intemperate climate zones, which decreases the yield potential,a trait that is primarily controlled by the genome (Debaekeand Aboudrare, 2004; Rabbingeand Diepen, 2000). Actualharvested yields result from the interactions between differentgrowth factors, which are responsible for fulfilling the yieldpotential of cultivated varieties. As a rule, actual yields are

much lower than the attainable maximums and are definedby soil and climatic conditions (Supit et al., 2010). On thebasis of the degree of yield potential realization, Rabbinge(1993) has divided all growth factors into three main groupsin descending order of importance: (1) defining (light, CO2,crop plant), (2) limiting (water, nutrients), and (3) reducing(weeds, diseases, external toxins). Furthermore, Lansigan

(1998) has distinguished four levels of production systems:(1) potential, (2) water-limited, (3) nitrogen-limited, (4) othernutrientlimited. The criteria of the production factors hierar-chy have become the basis of the yield gap concept. Theyield gap is defined as the lost part of a potentially harvestedyield that was achievable at each production level (Dober-mannandCassman, 2002;EvansandFisher, 1999;WallaceandWallace, 2003).

The availability of water together with the nitrogen supply is a

key factor in plant growth. It is well recognized that only withample water supply can crop plants effectively exploit soiland fertilizer nitrogen resources. Therefore, the nitrogen sup-ply must be considered as a prime factor of crop-plant growththat is a decisive aspect of productivity (Gonzalez-Dugoet al.,2010). Even mild water stress can be detrimental to the har-vested yield. Any water deficit significantly disturbs plant

* Correspondence: Dr. Witold Grzebisz; e-mail: [email protected] This article is based on a talk at the IPI-ISSAS 12th InternationalSymposium on Management of Potassium in Plant and Soil Systemsin China, Chengdu, Sichuan, China, July 2527, 2012.

$ This manuscript is partly based on original data from a PhD thesisas presented in Tabs. 1 and 2 and Fig. 1, 2, and 6.


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function, which negatively impacts plant life processes, bothat the molecular and whole-plant levels. Drought stressinduces stomatal closure, reducing evapotranspiration (Eta),and the mass flow of soil solution to the root surface. As aconsequence, the rate of mineral-element deliverydecreases, which limits plant nutrition and growth (Flexasand Medrano, 2002; Farooq et al., 2009; Gonzalez-Dugoet al., 2010;Kuchenbuchet al., 1986; Luan, 2002;Shaoet al.,

2009). The physiological consequence of reduced crop watersupply is a decrease in the rate of growth, which in turndecreases the production of biomass and its partition amongplant organs (grenandFranklin, 2003;DebaekeandAbou-drare, 2004; Shaoet al., 2009). However, other growth fac-tors must also be taken into account because both the waterand the units of N productivity depend on the uptake and utili-zation of other plant nutrients (Cakmak, 2005;Fageria, 2001;Janssen, 1998;Wallace, 1990).

In modern agriculture, the economically and environmentallysound practice of production requires some insight into pro-cesses that increase productivity per water unit. The deple-

tion of soil nutrient reserves is a logical consequence of soilminingoriented agriculture, which results in soil-fertilitydecline. As reported by Woodet al. (2000), K depletion is amajor reason for the degradation of arable land in southeastAsia, Latin America, and the Caribbean. Agriculture in Cen-tral Europe is orientated towards K mining and is the principalreason for low water-use efficiency, resulting in considerableseasonal variability in yields (Grzebisz and Diatta, 2012).Thus, the primary objective for farmers is to decrease theyear-to-year variability of harvested yields. However, fulfillingthis objective depends on the answers to two other urgentquestions: (1) How can the water productivity per unit be in-creased under rain-fed conditions? (2) How can the produc-tivity per unit of nitrogen be increased under limited water

conditions? In terms of agricultural policy, the answers toboth these questions relate to methods and measures, whichcould bring the limiting productivity factors of water and nitro-gen to the attention of farmers.

In light of this information, it can be proposed that the size ofthe crop yield gap is a consequence of insufficient nitrogenproductivity, which in turn results from inadequate water-useefficiency (WUE). The objectives of this conceptual revieware: (1) to describe a quantitative assessment method for Kimpact on WUE, (2) to define critical stages of crop-plant re-sponse to K, and (3) to illustrate that adequate potassiummanagement can be used as an important tool for alleviating

the negative effects of water deficits on plant growth andyield.

2 Potassium fertilizationversuswater-useefficiency (WUE)

2.1 A framework for the WUE concept

The physiological functions of water are characterized atthree levels: (1) cellular (as a component and medium for bio-chemical reactions), (2) tissue (as a link to adjacent cells),and (3) whole plant (as a means of mineral nutrient and hor-

mone transport; Shaoet al., 2009; Monneveuxand Belhas-sen, 1996). In agronomy, a fourth operational level of waterseffect on a crop plant has been developed that summarizesall basic processes at the canopy level. This index is knownas the water-use-efficiency (WUE) index, and in a broadsense it describes the quantity of biomass produced by a par-ticular crop plant in relation to the volume of water that is eva-porated and transpired during its life cycle (Liuet al., 2007).

The WUE index is calculated as the quantity of the main cropproduct (Ya) per unit of water (ETa) used (kg per mm or m3):

WUE YaETa

(1)

Yield (Ya) is defined as the quantity of actual harvestable ormarketable crop parts (seeds, grain, roots, tubers, etc.) pergiven area during a fixed period of time. Evapotranspiration(ETa) is a measure of the water that is used (transpired andevaporated water in mm or m3) by a cultivated crop during itsgrowth period. The ETa is calculated on the basis of thevapor-pressure deficit and harvest index (Debaeke andAboudrare, 2004) as follows:

Ya=k(ETaEs) (HI/VPD), (2)

where:Ya, the yield of a particular crop; k, biomass-to-trans-formation ratio; ETa, total water use (ETa= Tp+ Es); Tp, planttranspiration, Es, soil evaporation; HI, harvest index; VPD,vapor-pressure deficit.

As a rule, it is assumed that any increase in crop yield in aparticular environment is accompanied by a simultaneousincrease in the WUE (Ritcheand Basso, 2008). Therefore,ETa is frequently considered to be a constant part of theoperational index (Liuet al., 2007). However,ZwartandBas-

tiaanssen (2004) showed that ETa is not a constant value.Based on an extended collection of empirical data, theseinvestigators concluded that the coefficient of determinationorr2 for the relationship between the harvested yield (Ya) andETawas low, amounting to 0.35 for wheat and 0.33 for maize.In addition, our findings also show that the r2-coefficientvariability adequately reflects differences in both the climaticconditions and nutrient supply. Therefore, the WUE is a use-ful index for comparing different crops and regions (Liuet al.,2007). Nevertheless, its applicability to discriminating be-tween the growth factors that contribute to yield increasesseems to be limited.

AgnusandHerwaarden(2001) have proposed another meth-od for a simple and quick assessment of water productivity atthe field level, which is known as the French and Schultzapproach (FAS). This method considers the problem ofimbalanced water use by crop plants. Its major thesisassumes that a plants water supply is not the dominant factorin determining the quantity of the harvested yield, butaccounts for environmental and management factors as well,which limit water-use productivity. These assumptions areseemingly contradictory to the multiple-limitation hypothesis,(MLH), which treats water as the main growth factor (Rubioet al., 2003). In fact, the FAS concept raises the problem ofthe efficient crop use of water, which in turn depends on fac-


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tors that affect its uptake, from soil resources to biophysicalprocesses (Cakmak,2005;RengelandDamon, 2008).

The FAS method of water productivity assessment was ori-ginally based on wheat research in Australia (French andSchultz, 1984; Passioura, 2006). Operationally speaking,FAS relies on the attainable (water-limited) yield determina-tion. This procedure requires data on the maximum unit water

productivity (TE) and amount of water available for transpira-tion by a crop canopy during the growth season. The algo-rithms for the water-limited-yield (WLY) calculation are as fol-lows:

WLY TER RES (3)

WLY TER RES WR (4)

where: TE, the transpiration efficiency (TE = k/ VPD);R, thesum of rainfall during the growth period; Es, the seasonal soilevaporation, equal to 110 mm; and WR, water reserves in therooted soil volume at the beginning of growth for a particularcrop.

The original FAS algorithm was composed of two units, thefirst of which was the TE index, which was originally esti-mated to be 20 kg grain ha1 mm1. This level of water-unitproductivity can be achieved only under optimum growth con-ditions, i.e., without disturbances from low soil fertility orattacks by diseases and pathogens (Turner,2004; Passioura,2006). However, even in Australia the standard value is underdiscussion, with wider values ranging from 20 to 30 kg grainha1 mm1 (AgnusandHerwaarden, 2001). The second com-ponent of the equation is the water balance R, which is thetotal rainfall from the beginning to the end of the vegetativeseason. Therefore, this method of water-productivity calcula-

tion relies on the maximally available water supply for theseason. In the temperate regions of the world, especially incontinental zones, an important source of water is soil waterreserves, or WR, which originate from winter precipitation.For the purposes of this study, Eq. 3 has been extended intoEq. 4 to include a unit that quantitatively describes internalsoil water resources.

However, the FAS approach has another advantage. It canbe applied to evaluate the fractional effect of any particularfactor that limits water-unit productivity, that is to say, any fac-tor that impacts its use efficiency. It has been assumed thateach growth factor affects water uptake by the crop from thesoil and that its biophysical processes are critical for increas-ing water-unit productivity. The graphical interpretation of thismethod that is proposed in this review allows for the discrimi-nation between effects that result from the action of tran-spired water and the tested factor. Operationally, the har-vested yield is divided into two parts. The first part, i.e., theWLY results from the amount of transpired water, is consid-ered to be the most important. The second part of the graphshows the quantitative effect of a factor that is responsible forthe efficient or nonefficient use of water. This is not as impor-tant as the WLY. Experimental factor results including K canbe presented in the classical FAS form as shown in Fig. 1, orgraphically represented as a fractional yield (Figs. 2, 3, 4).

2.2 Potassium effects on WUE: case studies

2.2.1 Winter wheat

The applicability of the FAS concept has been validated forfour groups of crops that were cultivated in temperateregions, including our own experiments in Poland wherethere is a high continental climate impact, resulting in an ele-


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Figure 1: Graphical interpretation of winter wheat yield response to available water and K against a background with two standards oftranspiration efficiency (basic data adapted fromMusolf, 2003).

J. Plant Nutr. Soil Sci.2013,176,355374 Potassium fertilization under drought stress 357


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vated frequency of spring and/or summer droughts (FalloonandBetts, 2010;Olesenet al., 2011). Cereals dominate theagricultural production of this region (Rabbingeandvan Die-pen, 2000). Winter wheat yields depend significantly on bothsoil fertility and soil management. The level of grain yields inPoland is approximately 4 t ha1, which constitutes 66% ofthe attainable yield as defined by the water supply. Asreported byGrzebiszand Diatta(2012), the main reason for

low yields is not only the water supply but also the limitedsupply of nutrients, mainly potassium. This has been fully cor-roborated by data from field experiments. As shown in Fig. 1,winter wheat yields are irregularly scattered without any sig-nificant response to the quantities of supplied water. It is nec-essary to mention that the establishment of optimum growthconditions for winter wheat is difficult. This crop is sensitiveboth to water-deficiency stress and excess during the wholegrowing season (Passioura, 2006). The water-limited yield inCentral Europe is fixed for a majority of countries at 6.5 t ha 1

(MARS, 1995;Rabbingeandvan Diepen,2000). As implicitlyindicated by the WLY20 and WLY20+WR threshold lines,grain yields for most treatments under study showed compli-cated response patterns to the water supply (Fig. 1). Underlow-precipitation conditions, the yields depended on soilwater resources to a great extent, i.e.,the amount of water atthe beginning of the growth season. These reserves are animportant source of water for crops in the temperate regions,especially for those that are dominated by continental watersupplies (Debaekeand Aboudrare, 2004;Supitet al., 2010).As indicated by some of the treatments, the wheat respondedto irrigation and K-fertilizer application. In two of the threepoints on and above the WLY20+WR line, the optimal supplyof K during the dry year of 1999 was the source of yieldincrease (Fig.1). However, other previously unrecognized

growth limitations have appeared under high-water condi-tions (current precipitation + irrigation), as indicated by theyield-points below the WLY20 line.

2.2.2 Spring triticale

In temperate regions with strong influence from a continental

climate, spring cereals are much more vulnerable to water-deficiency stress than winter crops (Hlavinka et al., 2009;Martyniuk, 2008). As shown in Fig. 2, the assumed high de-pendence of WUE on other growth factors has been docu-mented. Spring triticale was grown under a strongly diversi-fied water supply via irrigation as well as an artificiallyimposed water deficit (Wyrwa, 1997) through the use of ashelter. The plant response to water-deficiency stress wasstudied in two chronologically separated stages called BBCH30-49 and BBCH 60-80. The principal goal of the first periodwas to evaluate the changes in total biomass and the numberof ears that resulted from the induced water deficit. The mainobjective of the second period was to assess the impact ofartificially imposed drought on the yield components. As

expected, all targets were fully achieved. The effect of thesecond experimental factor,i.e.,K application, was evaluatedby using the graphical formula from the FAS. The differencethat resulted from the growth factor application, i.e.,betweenthe actual harvested yield and the calculated WLY, wasexpressed as a yield gain (Y-G) or loss (Y-L).

The data reported in Fig. 2 implicitly underline the reliability ofthe graphical FAS concept. Firstly, it was possible to calculatethe maximum attainable yield resulting from the maximumunit water productivity. This conclusion is corroborated by the


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K- K+ K- K+

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yieldofgrain/kgha-1

WLY20 Y-G Y-LYield fraction

c b a a c c bc b bc b a a c c b b

Figure 2:Structural components of grain yield from spring triticale as affected by water and K supply (base data adapted from Wyrwa, 1997).WLY = water-limited yield for TE = 20 kg grain ha1, mm1; Y-G = yield gain, Y-L = yield loss fractional yields; IR = irrigated at BBCH 30 and 60for 3 weeks; D30, D60 = drought imposed at BBCH 30 and 60, for 3 weeks, respectively, RE = rain-fed conditions. Statistical analysis concernsreal yields; columns marked with the same letters are not significantly different.



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almost 100% equivalence for the real and calculated yields inirrigated treatments. It can therefore be concluded that underan ample water supply, the effect of K fertilizer was insignifi-

cant and indirectly indicative of high soil K fertility. Secondly,the FAS method enabled the elucidation of the yield-formingeffect for the growth factor of interest, i.e., K fertilizer. Thestress-alleviation effect was best corroborated in the D30 andD60 treatments. Plants grown without a K supply (K mainplot) had yields that were below the calculated WLYs, whichwas quite a different response pattern to artificially imposeddrought than in the plots with freshly applied potassium (K+main plot). For all cases with K-fertilizer applications, exclud-ing the irrigated treatments, a significant yield response wasobserved. This finding implicitly illustrates the importance ofK fertilizer as a measure to at least partially alleviate mildwater-deficiency stress.

A detailed explanation of this phenomenon cannot be con-ducted without insight into the formation of a given yield com-ponent. The harvest index, or HI, can be considered to becrucial at the canopy level as shown in Eq. 2. It is simplydefined as the relative mass of harvested grains per mass ofthe whole plant or for the biomass of the canopy. In addition,the index indirectly shows how growing conditions affect thenumber of grains and their individual weight. The first compo-nent reached its potential expression starting at the shootingperiod and going until the watery stage of grain development,with elevation at heading. The number of ears per unit areaand grains per ear are used to indicate a cereals physiologi-cal development (Shearmanet al., 2005;Zercheand Hecht,1999). The second component, which refers to the grainmass and in practice is counted as the thousand-grain weight(TGW), depends on the factors that impact cereal growth dur-ing the grain-filling period (YangandZhang, 2006). As pre-sented in Fig. 2, the yield gain in the D30 treatments, i.e., withdrought imposed during the shooting and booting of spring tri-ticale, illustrates the importance of potassium managementto plants during the primary stage of grain yield developmentand is responsible for the number of grains per unit area.

The chosen WUE-calculation method affects the evaluationof applied factors to some extent in the case of potassium. To

validate this conclusion, calculations were performed on theWUE indices for spring triticale based on both methods dis-cussed above. The ETa in 1994 was 4710 m3 ha1 and

4850 m3 ha1 in 1995, as calculated for the Brody SynopticStation and based on the FAO methodology (Brouwer andHeibloem, 1986). Both methods allow the evaluation ofapplied potassiums effect on WUE (Tab. 1). As shown inFig. 2, the grain yield of spring triticale decreased in concertwith increasing water shortage. However, the degree of yielddrop was much lower in treatments with K fertilizer. Thiseffect can be explained fairly well by using both indices. Onthe basis of the ETa, these indices underline a quantitative dif-ference in evaluations of K yield effects. In the present case,the calculated WUE-ETa indices were significantly higher forK-fertilized plants. The WUE-FAS indices allow for a view ofthe contrary course of K management and its impact on waterproductivity, which in turn indicates the alleviating effect ofapplying K fertilizer.

2.2.3 Maize

Maize is a crop with extremely high potential yields, but it pre-sents economic risk to growers because of its high sensitivityto external growth factors, which can induce year-to-yeargrain-yield variability (TollenaarandLee, 2002). Maize yieldsin Poland are far below yield potentials for current cultivatedvarieties. The main reasons for these variations are low soil-fertility levels and imbalanced fertilization (Potarzycki, 2010).The impact of soil fertility has been reported bySzczepaniak

et al. (2010). As presented in Fig. 3, maize that was cultivatedon light soil produced 3.2 and 9.4 t ha1, but 8.1 and 10.1 tha1 grain were produced on medium soil during 2006 and2007, respectively. However, the general course of the ETaover both growing seasons showed small variations. The cal-culated sum of evapotranspired water amounted to 6220 and5720 m3 ha1 in 2006 and 2007, respectively. Two main rea-sons for the yield differences were noted. The primary reasonrelates to the distribution of precipitation, which was unfavor-able for maize in 2006. The negative water balance, whichoccurred at the onset of anthesis, exceeded 150 mm. How-ever, the decisive factor was not the quantity of suppliedwater, but its use by plants, which depended on both the soil


Table 1:WUE indices calculated for spring triticale by two different methods1.

Potassium Water Available water1

/ mmWUE-ETa/ kg ha1 mm1

WUE-FAS

/ kg ha1 mm1

treatments mean SD mean SD mean SD

Withoutpotassium(K)

IR 428.6 16.2 13.9 0.07 20.2 3.22

RE 338.7 2.3 11.4 0.14 22.8 0.34

D60 269.9 36.8 7.4 0.01 21.4 24.22D30 284.1 31.3 5.4 0.07 14.1 14.56

Withpotassium(K+)

IR 428.6 16.2 15.4 0.07 22.3 3.58

RE 338.7 2.3 15.0 0.64 30.3 1.44

D60 269.9 36.8 13.7 0.64 39.2 19.04

D30 284.1 31.3 11.8 0.71 31.7 22.24

1 based onWyrwa(1997)



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fertility and the K management. On sandy loam, the yields ofmaize fertilized with K were higher than the calculated WLYs,regardless of the weather conditions. This finding implicitlyindicates that K significantly increased water-use efficiency.Maize yields in loamy sand also exceeded the WLYs andresponded to applied K. However, this pattern prevailed onlyduring years without water stress. There was a long-lastingwater shortage in 2006, and the calculated WLYs were muchhigher than the actual yields and indirectly indicated the

dominant role of the water supply in maize that was grown onlight soil, which is naturally poor in K internal resources(Fotyma,2007).

2.2.4 Sugar beet

The third group of plants that are cultivated in temperateregions, especially in Europe, are potatoes and sugar beets(OlesenandBindi, 2002). One key factor in the expected re-sponse of these two crops to potassium fertilization is the fre-quency and severity of droughts during the growth season. Inthe case of sugar beet, there is no fixed value for the maxi-mum unit of water productivity (TE). However, it could be

assumed that the highest productivity of this crop reaches50kg ha1 mm1 (Jones, 2010), on the basis of dry-matteryield. On the basis of this value, the FAS critical standardwas fixed at 150kg of fresh beets ha1 mm1 of transpiredwater (storage roots + respective amount of leaves). On thebasis of a simulation study with a limited K and water supply,it has been documented that K is a factor that significantlyaffects the WUE (Musolfet al., 2004a). As shown in studiesthroughout Europe, the response of both crops to K fertiliza-tion is generally not consistent, both in humid and semiconti-nental European environmental zones, as reported by Allis-sonet al. (2001) andMilfordet al. (2000) in Great Britain andBargand Grzebisz(2004) in Poland. Sugar beets are typi-

cally fertilized with K, often heavily so (Milfordet al., 2000).An extended study that was conducted in the 1990s by Woj-ciechowskiet al. (2002) in Poland showed that K applicationaffects the WUE. However, the response depends on thecourse of weather during beet-vegetation development. Theapplied graphical form of the FAS method allowed for thedetermination of WLYs with high accuracy with respect toattainable sugar beet yields (Fig.4). During three annual ser-ies of experiments in two distinct localities on soils originating

from loamy sand, the sugar beet yields reached their maxi-mum productivity within two years (1996, 1997). Storage rootyields were averaged over locations and treatments andamounted to 70.5 t ha1, whereas the WLY was fixed at 77.5 tha1. The latter value is close to a potential sugar beet yieldof 80.2 t ha1 in Poland, as reported by Supitet al. (2010).Therefore, the fixed unit value of TE at 150 kg ha1 mm1 canbe used to evaluate the effects of K on the sugar beet WUE.In the third year (1998), there was a mild drought in the sum-mer months, and the WLY was calculated at a much lowerlevel. The effect of applied K on the WUE was significant atboth locations. In the first area its optimum rate amounted to66 kg K ha1, whereas in the second 198kg K ha1 were suffi-cient to reach the maximum attainable yield of 80 t ha 1.

3 Potassium as a water-stress-alleviatingagent

3.1 Determining the critical stage in crop-plantsensitivity to K supply

The physiological roles of K in plants have been extensivelydescribed in recent reviews (Maathuis, 2009; Marschner,2012). Potassium is the main osmotic solute in plants and itaffects water homeostasis both at the cellular and the whole-


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yieldofgrain/tha-1

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d de b bc d de bc bc a a d d

Figure 3:Structural components of maize-grain yields as affected by weather for two soil textural classes and potassium treatments (base dataadapted fromSzczepaniaket al., 2010). WLY = water-limited yield for TE = 20 kg grain ha1 mm1; Y-G = yield gain; Y-L = yield loss fractionalyields. Statistical analysis concerns real yields; columns marked with the same letters are not significantly different.



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plant level. Therefore, its accumulation in the cell is a drivingforce for water uptake, which in turn impacts the water statusof its reaction milieu, which is important for the rate of meta-bolic and growth processes, respectively (Marschner, 2012).Potassium is a cofactor for numerous key enzymes that con-trol basic plant metabolic reactions. Therefore, K deficiencydepresses the rate of photosynthesis, and in turn affects theamounts of assimilate production and partitioning to roots

and shoots (Amtmann and Armengaud, 2009; Cakmak,2005). Potassium accumulation is the key force that governsthe rate of cell expansion and consequently influences therate of a particular plant parts growth, such as the leaves orroots (Hawkesfordet al., 2012; Walkerat al., 1998). Potas-sium is a very mobile ion in the plant body and a large portionof it cycles between the shoots and roots. As a result, pres-sure-driven solute transport in plant tissues is the key mech-anism for controlling water status, such as the closing andopening of stomata. The cotransport of K+ ions with nitrateions (NO3 ) in the xylem and its circulation with malate in thephloem are classical examples of K-transportation functionsduring its rootshoot turnover. The transportation of assimi-lates in the phloem is also K concentration-dependent

(Engelset al., 2012;White, 2012;Szczerbaet al., 2009).

It can therefore be supposed that any disturbance in theplants potassium supply during its life cycle depresses thedevelopment of plant biomass and the formation of structuralyield components. This assumption explains the prioritizationof K nutrition in the strategy of a particular crop plants perfor-mance during the growth season. In response, some ques-tions concerning the impact of potassium on both nitrogenand WUE or both are needed: does K uptake depend, tosome extent, on water supply, which would in turn disturbnitrogen delivery to aerial plant parts? This question can be

formulated in a way that stresses the agronomic point ofview: is it possible to decrease the negative impact of watershortage on the development of structural yield componentsthrough an adequate supply of potassium?

The general pattern of K accumulation in a crop plant overthe course of its vegetative development is different in com-parison to the sigmoid-like curves of dry-matter accumulation

(Yin et al., 2003). A specific course of K accumulation hasbeen found for wheat (Barraclough, 1986,Cannell, 1984), oil-seed rape (Barg et al., 2005; Orlovius, 2000), and sugarbeet (Grzebiszet al., 1998). The main difference is related toa well-defined maximum, which is elevated during the wholeseason. One of the most important characteristics of K-accu-mulation patterns during the growth season is its significantdecrease following the maximum. This nonspecific drop maybe substantial,i.e.,approaching and even13of the K maxi-mum at harvest (Barraclough, 1986; Beaton and Sekhon,1985; Grzebisz et al., 1998). Therefore, the K demand ofhigh-yielding arable crops is often underestimated in agro-nomic practice, and critical growth stages are not always welldefined.

Based on the course of K accumulation during the growthseason, three main characteristics of K uptake by a particularcrop can be determined. They are as follows: (1) maximum Kaccumulation, (2) the absolute rate of K accumulation, and(3) the relative rate of K accumulation. The second para-meter,i.e., the absolute rate of K accumulation (ARK) deter-mines the most sensitive stage of a given crops response toK supply. The basic criterion of its determination is the defin-ed K-accumulation rate. For example, assuming an ARK atthe level of 8kg K ha1 d1, the length of the K-maximum-uptake rate is approximately 14 d (Fig.5). In this case, this


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L1 L2

location, years, K rates / kg K ha-1

yieldofbeet/kg

ha-1

Y-L

Y-G

WLY

b b b b b b b b a b b b b b b b b b b b a a a b

Figure 4: Structural components of sugar beet yields from storage roots as affected by K-fertilizer rates (base data adapted fromWojciechowskiet al., 2002). WLY = water-limited yield for TE = 150kg beets ha1 mm1; Y-G = yield gain; Y-L = yield loss fractional yields.Statistical analysis concerns real yields; columns marked with the same letters are not significantly different.



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period would extend from the 72nd to the 95th day of sugarbeetvegetation development. The period of highest demand

in the sugar beet canopy for K coincides with its main growthparameters. Firstly, the absolute rate of dry-matter and nitro-gen accumulation in both the leaves and roots by a high-yielding sugar beet canopy takes place between 80 and 90days for sugar beet vegetation (Grzebiszet al., 2012,Szcze-paniak et al., 2012). The relative rate of K accumulation(RRAK) is another crop characteristic, which is useful indetermining the earliest stage of its sensitivity to a particularnutrient supply. In the present case, the RRAK reached amaximum around the 7th week after plant emergence. At thisperiod of sugar beet development, the rate of foliage expan-sion depends on the supply of nitrogen, which is a decisivefactor in solar-energy capture (Malnouet al., 2006). Accord-ing toMarschneret al. (1996) the rate of nitrate-ion uptake issignificantly related to a concomitant K supply. Both para-meters,i.e.,the maxima for the absolute rate of K accumula-tion (ARK) and the relative rate of K accumulation (RRAK),define the critical period of a crops response to K supply.

Both K-accumulation parameters are most likely inherentlyrelated to the rate of root-system extension (length,RL) dur-ing the growth season. The key characteristics of root-systemgrowth during the growing season are the transition point(TPRL) and the rate of extension (RLr). Both parameters havebeen calculated for sugar beets (based on Windt, 1995) byusing the first derivative of the polynomial function of the thirddegree (RL):

RL= 0.012d3 + 3.022d2 76.26d 379.3 for R2 = 0.99,

RL= 0.036d2 + 6.044d+ 76.26,

TPRL= 83.94dandRLr= 330 m ha1 d1.

The transition point (TPRL) describes the time point (daysafter sowing) at which the root system reaches its highestrate of growth, as indicated by the maximum RLr value. Thesugar plantation yielded 80 t ha1 of beets, reaching its maxi-mum rate of root-system extension 84 d after sowing, i.e.,during the main period of K uptake (Fig. 5).

A comparison of critical stages of K accumulation by thesugar beet canopy and root-system extension parameters

indicates a significant coherence in both processes. How-ever, there are no scientific data to implicitly corroborate thishypothesis. A question remains as to what extent the patternof root growth in a particular crop, for example, the TP RLoccurrence, is affected by environmental and agronomic con-ditions.Vincentand Gregory(1989) concluded that the gen-eral pattern of root growth in wheat reflects changes in dry-matter partitioning over the course of crop development onthe basis of their own studies and others that were conductedin Great Britain during the 1970s and 1980s. The authors didnot observe any significant effect of nitrogen treatment andseason. The conservative behavior of wheat systemdynamics over the growth season has also been corrobo-rated by Zhang et al. (2009). This study, which was con-ducted from 1990 to 2008, showed that the distribution ofwheat roots along the soil profile at harvest was only slightlyaffected by a difference in water regimes, seasons, and vari-ety selection. As documented byZhanget al. (2009) in wheatandGrzebiszand Kryszak(1992) in winter rye, a mild short-age of water or nutrients increases the depth of crop rooting.These observations show that the K-uptake rate is driven bythe aboveground demand for water and nutrients in equili-brium with the root-system expansion (Kinget al., 2003;VanNoordwijkandvan de Geijn, 1996). This conceptual analysisof the critical period of sugar beet response to K supply canbe applied to other crops. On the basis of key crop character-istics, this procedure can indicate the stages that are poten-

tially sensitive to a reduced K supply. It is worth mentioningthat there are only a few studies on the dynamics of root sys-tems and the simultaneous accumulation of nutrients (Barra-clough, 1986).

3.2 Potassium as a water-stress-alleviating agent:case studies

Wheat, maize, and sugar beets have been selected toexplain the role of externally applied K in alleviating water-deficiency stress.


-4

-2

0

2

4

6

8

10

12

25 45 65 85 105 125 145 165

days from emergence / d

absoluterateofKaccumulation,

ARK/kgKha-1d

-1

-10

0

10

20

30

40

50

60

70

80

relativerateo

fKaccumulation,

RRAK

/gkg-1d

-1

AR-K RRA-KK characteristics:

Figure 5:Characteristics of K-accumulationdynamics in sugar beet during the growthseason (adapted fromGrzebiszet al., 1998).



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3.2.1 Wheat

The majority of arable lands worldwide are cropped with cer-eals (FAOSTAT, 2012). The sensitivity of this group of cropsto K is generally weakly recognized at the canopy level. Themain reason for this knowledge gap is the high genetic adapt-ability of cereal species to varying soil fertility and water envir-onments (Calderiniand Slafer, 1999;Zhanget al., 2009). In

Europe, wheat is among the most productive cereals, but italso requires the most fertile soils (Rabbinge and Diepen,2000). The yield potential of this crop in Europe is high, ran-ging from 7.7 t ha1 in Bulgaria to 12.4 t ha1 for Ireland, inturn indicating a slight decrease following the increasingimpact of the continental climate. However, actual yieldsshow a much deeper contrast between European countries(Olesenand Bindi, 2002;Supitet al, 2010). The size of theyield gap depends upon both the growth conditions (soil andclimate) and nutrient management, especially K (Grzebiszand Fotyma, 2007; Grzebiszand Diatta, 2012; Olesen andBindi, 2002;Rabbingeandvan Diepen, 2000).

A winter wheat yield of 10 t ha1 exerts the highest demandfor K during the period that extends from the beginning ofshooting (BBCH 30) to the beginning of anthesis (BBCH 60;Cannell, 1984). This period lasts approximately 45 d, and thedaily rate of K uptake amounted to 4.4 kg K ha1. In this case,the maximum rate of K accumulation occurred at heading,and then slightly decreased towards anthesis. At this particu-lar stage of growth, wheat reached maximum K accumulationof 320kg K ha1. In addition, its K uptake always precededthe amount of simultaneously accumulated nitrogen. There-fore, the pattern of K accumulation in wheat during a givenseason can be considered to be a prerequisite of high grainyield. This conclusion is supported by a wheat study fromGreat Britain. As reported by Barraclough (1986), the daily

rate of nitrogen and K uptake of 2 and 3 kg ha1 during the cri-tical period (180260 d after sowing) is required to coverrequirements for the production of 9.5 t ha1 of wheat grain.As reported by King et al. (2003), the shoot-dry-massincrease is about sevenfold during this period, which indir-ectly underlines a high sensitivity to the shortage of bothnutrients. However, the impact of water shortage on wheatperformance in the British Islands is much lower than in Cen-

tral Europe or in semiarid regions (Doddet al., 2011; Rab-bingeandDiepen,2000;Turner, 2004). The main reason forthis yield gap is the frequency and severity of drought duringwheat development (Passioura, 2006).

Figure 6 shows the results of simultaneously imposed waterand K stress, which significantly affected the K-accumulationrate (ARK) of wheat during the growth season. For most treat-ments, the highest ARKwas achieved by wheat at the bootingstage (BBCH 40-49). With an ample water supply, therecorded growth rate of wheat plants at this stage did notdepend on the soil content of available K. Therefore, it can beconcluded that an adequate water supply during the most cri-

tical stages of wheat growth (from shooting to anthesis) can,to some extent, be alleviated by additional water. In manyparts of the world, irrigation is the key to ameliorating water-deficiency stress, and it significantly increases wheat yields,i.e.,WUE (Liuet al., 2007). As shown in Fig. 6, plants experi-encing a water shortage developed different K-accumulationstrategies. Those grown on K-rich soil could maintain a suffi-ciently high rate of K accumulation during most of the season.However, plants experiencing both water and K stresses hada decreasing trend in their K-accumulation rates from thebooting stage onwards.

In light of the known K-accumulation patterns for wheat, aquestion arises about the response of yield structure compo-


0

1

2

3

4

5

6

7

20 40 61 82 103 141days from spring regrowth / d

absoluterateofKaccumulation/kgKha-1d

-1

K- IR K- D K+ IR K+ Dtreatments:

LSD0.05

BBCH 30-49 BBCH 61-80

IR/D IR/D

Figure 6:Potassium-accumulation rate in wheat under different water and K-fertilizer treatments (basic data adapted fromMusolf,2003). KIR,K+IR = irrigated treatments without and with K application; KD, K+D = treatments with double drought imposed for 3 weeks during stemelongation and anthesis, without and with K application.



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nents to the shortage of water. Cereals are sensitive to waterdeficits for a period extending from shooting to ear formationand are under strong stress until maturity (Day, 1981;Grze-biszet al., 2009;Krummet al., 1990). As reported by Baqueet al. (2006), potted wheat plants living under drought duringthe vegetative part of their development first reduced thenumber of effective tillers, i.e., ears. This yield-forming ele-ment was decisive for the final grain yield. This phenomenon

is also observed under field conditions, but may be attributedto wheat and/or other cereals that were cultivated on soil withan extremely low K supply during the pre-anthesis period ofgrowth. However, the alleviating effect of added K, even athigh rates, was not significant in the Baque et al. (2006)study. In other cases, as in Fig. 6, the wheat canopy shows ahigh adaptive plasticity to water and K stress. As reported byGrzebiszet al. (2009), wheat plants that were adequatelysupplied with K during vegetative growth, but especially dur-ing early stages of grain development (milk stage), are ableto reduce the stress severity and in turn increase grain yield.At this particular stage, the most decisive nutritional factor indetermining the grain yield was the K concentration of theflag and third leaves. It is well documented that the flag leaf isimportant for covering grain-development requirements byassimilate production (Akmal et al., 2000; Grzebisz, 1988,Inoue et al., 2004; Xu and Yu, 2006). The reported impor-tance of the third leaf in yield determination emerged onlyunder stress conditions as noted by Grzebiszet al. (2009).The year-to-year variability of K concentration in both wheatleaves explained 75% and 51% variability in the grain yield:

(1) flag leaf: Ya= 2.349 K + 4.0 forR2 = 0.75,n= 9,andP< 0.1%;

(2) third leaf: Ya= 2.877 K + 4.71 forR2 = 0.51,n= 9,andP< 5%;

where:Ya, grain yield (t ha1); K, K accumulation (kg K ha1).

A K-fertilizer effect on grain-yield performance is possible,provided that the soil K pool, which is a primary source of

plant K, is sufficiently large. According to Cakmak (2005),high K accumulation in the leaves enables a plant to extendits capability to produce and transport assimilates from theleaves to the grain. This hypothesis is supported by a higherthousand-grain weight (TGW), which was achieved in trialswith artificially imposed drought when fertilized with K (Baqueet al., 2006;Grzebiszat al., 2009;YangandZhang, 2006).

3.2.2 Maize

The seasonal course of maize-canopy growth is differentfrom wheat, which results from the number of plants estab-lished after sowing. Therefore, the cob is the plant part withthe highest potential sensitivity to water-deficiency stress, ascorroborated by a high year-to-year variability in the kernelnumber per plant (Otequiand Bonhomme, 1998). The maincharacteristics of K accumulation in maize are highly similarto other cereals (Novk and Vidovic, 2003). In general, ashortage of water negatively affects the rate of K accumula-tion during early stages of maize development (Fig. 7). Thehighest disturbance in plant growth, which occurred during

the vegetative period, resulted in decreasing amounts ofaccumulated K and nitrogen. At the end of this period, thedaily rate of K accumulation was 13 lower under suboptimalgrowth conditions, as imposed by a water shortage (Heck-man and Kamprath, 1992). Plants experienced a milddrought during the vegetative period of growth, which couldcompensate for the K-uptake rate during the grain-filling per-iod. In spite of this K compensation effect, the grain yield ofmaize was 25% lower.

The first classical symptom of water deficiency in maize isleaf rolling (Kadiogluet al., 2012). During the course of theday, rolling appears in mid-day, increasing towards noon andrecovering in the evening. This plant behavior results fromdecreased leaf turgor, which is a consequence of temporarywater deficit. As reported byPremachandraet al. (1993), theK supply can significantly decrease both the degree of leafrolling and its length. Water shortage during the vegetativeperiod of the maize life cycle results in a significant reduction


Figure 7:Characteristics of K-accumulationdynamics in maize during two contrastingseasons (basic data adapted fromHeckmanandKamprath, 1992). DY, OY = drought andoptimum water conditions during vegetation.



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in plant growth, revealing a stunted canopy at the beginningof anthesis (BargandFrackowiak-Pawlak, 2008).

The key period of yield formation in maize, which is called thecritical window, extends from tasselling (BBCH 51) to thewatery stage of kernel development (BBCH 71; DAndreaet al., 2008). During this period, maize is sensitive to water,K, and N supply. All these factors affect the kernel number

per plant (KNP), (DAndrea et al., 2008; Otequi and Bon-homme, 1998;Subediand Ma, 2005). It is therefore neces-sary to assume that lower K uptake negatively affects thedegree of yield-component development. This hypothesishas been corroborated by Barg and Frackowiak-Pawlak(2008), who showed that the kernel number per plant signifi-cantly responded to K application. However, the effect ofapplied K fertilizer was year-specific, revealing a positivetrend during dry year 2006 (+ 0.5 t ha1) and semidry year2007 (+ 0.5 t ha1), but not during 2005, an optimum formaize growth. The same conclusion can be drawn from thework of Damm et al. (2013), who found a 1.0 t ha1 yieldincrease of an already high-yielding crop in response to Kapplication during the semidroughty 2009 season. A studyconducted bySubediand Ma (2009) in Canada showed thatthe K shortage reduced maize grain yields by 13%.

3.2.3 Sugar beet

Sugar beet is an example of a crop with a high dependenceon K supply during the growth season. As reported by Barget al. (2002), the variability in beet yield can be 96%, whichmay be explained by the variability in K accumulation at har-vest. In this study, the yield response to K fertilization was ob-served in years with drought and semidrought during thesummer months. Beet yields were high even in years withdrought. The highest yield, which was approximately 90 t

ha1, was achieved in 1998; however, the yields did not showany response to increasing K rates. Analogous conclusionshave been drawn byMilfordet al. (2000) under British condi-tions. The same trend of seasonal beet response to applied Kwas reported byDammet al. (2013).

The study on dry-matter accumulation by sugar beets in re-sponse to K management showed a high dependence on the

weather pattern. As presented in Fig. 8, the absolute rate ofbeet growth was affected both by the weather within a givenyear and K fertilization. In 2001, the water supply during thesummer months amounted to 235 mm, while it was only105mm in 2003. The absolute rate of dry matter for beetgrowth was higher in the NPK treatment, regardless of theweather. The degree of difference between non-K and K-ferti-lized treatments was much higher in 2001, when the watersupply was ample. However, during the dry conditions of2003, the maximum rate of beet and absolute growth was sig-nificantly delayed. This finding is most likely the key reasonfor yield depression in relation to the summer water deficit.

A study on the response of sugar beets to induced water andK stress during the summer months showed a significant de-pendence on the K supply. As reported by Musolf et al.(2004b), plants grown under conditions without K applicationhad much lower concentration of K and nitrogen, both in theleaves and young roots, at the 6th-leaf stage. It is worth men-tioning that an insufficient supply of nitrogen in beets resultsin a slower rate of canopy growth. As a consequence, plantsare not capable of efficiently absorbing solar energy, which iscritical to their final yield (Malnouet al., 2006;Milfordet al.,1985). Early K stress impacts the performance of the sugarbeet canopy, which is the primary reason for its high sensitiv-ity to water stress. As presented in Tab. 2, the artificiallyimposed water stress affected K accumulation in plants in


0

5

10

15

20

25

30

35

40

45

50

40 57 77 92 113 134 155 175

days from sowing / d

absoluterateofs

toragerootsgrowth/kgha

-1d-1

NPK2001 NP2001 NPK2003 NP2003year and treatment

Figure 8:Effect of natural drought and limiting K supply on absolute rate of storage-root growth in sugar beet (adapted from Grzebiszet al.,2012).



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July. However, a significant response was noted only in thetreatment that was fertilized with K. The same conclusionwas found for nitrogen. The harvested yield of sugar beetshad a significant response to both experimental factors, butin this particular case, K was the dominant factor and waterwas minor. The regression analysis, which was based on Kaccumulation at harvest and was averaged over years, hadmuch higher yield predictability (YSR) when using the amount

of accumulated K (AK) in the beets in comparison to leaves,as in the independent value:

(1) leaves: YSR= 0.078AK+ 31.88 forR2 = 0. 44,n= 8,andP 5%;

(2) roots: YSR= 0.282AK+ 32.21 forR2 = 0.63,n= 8,andP 1%.

The above equations corroborate a hypothesis of higher beetsensitivity relative to the leaves with a K shortage during thecritical yield-performance stage. These results are in agree-ment with the opinion of Kenteret al. (2006), who stressedthe important but not dominant effect of water shortage dur-ing July and August on the performance and yield of thesugar beet canopy.

4 Potassium as a root-growth-inductionfactor

In general, the pattern of root-system development during thegrowth season for particular crop species is consistent andunder genetic control (Taylor and Klepper, 1978). However,this pattern undergoes changes throughout consecutivevegetative stages that depend on the soil physical properties,water and mineral-nutrient content throughout the soil profile

(SmuckerandAiken, 1992). Soil-profile moisture plays a dualrole and is considered to be a plant-growth resource and thesource of life for growing roots. The rate of nutrient uptake bycrop plants is governed by the rate of growth, so the demandis stage-dependent. In general, it is assumed that roots growintensively in the top soil at the beginning of the growth per-iod. As a rule, this soil layer contains sufficient amounts ofnutrients to cover the requirements of the developing seed-lings (Kinget al., 2003). In contrast, the amounts of waterand available nutrients in the upper soil layer continuouslydecreases during crop development, which forces plants toextend their roots towards moist and nutrient-rich zones ofthe soil profile (Forde and Lorenzo, 2001). Therefore, the

plant-oriented pattern of root-system growth undergoes achange to become more soil-oriented throughout the lifecycle, i.e., it becomes more and more dependent on thewater and particular nutrient distribution throughout therooted soil profile (Kinget al., 2003; Van Noordwijkand vande Geijn, 1996). Under the limited supply of resourcesrequired by the growing plant, roots tend to grow faster in soillayers, which creates more favorable conditions with an

ample water and nutrient supply (Taylorand Klepper, 1978).As a consequence of the lack of soil property uniformitythroughout the growing process and over the course of theseason, roots are not homogenously distributed in the soilprofile (SmuckerandAiken, 1992). However, the general pat-tern of wheat-root distribution along the soil profile was highlyconserved, in spite of the different varieties that were culti-vated over an 18-year period, as presented recently byZhang et al. (2009). This study corroborates an opinion onthe domination of plant-oriented patterns in root-systemgrowth, which were only slightly modified by the impact ofexternal growth factors.

The chemical characteristics of soil K in response to its exter-

nal supply are well described in many papers, but they arenot the subject of this conceptual review. However, the effectof K application on physical soil properties is little recognizedin the scientific literature. The main reason for this data scar-city is the low number of experimental fields with differen-tiated levels of K and related studies on this subject. It is nec-essary to note that a reliable estimation of an impact of a par-ticular nutrient on soil properties is possible, provided that itoccurs for a long time period. As reported by Dammet al.(2013), a 14-year period with constant rates of K applicationsignificantly improved the structure of a top soil. The mostimportant remark from this work refers to the content of soilwater, which increased across the whole range of the water-

retention functions up until the permanent-wilting point. Thelong-term application of K, which is dependent on the fertilizerrate, significantly changed its available content in the soil. Asshown in Fig. 9, the content of water that is available to barleyplants showed a significant, linear response to increasing Kcontents in the topsoil. In the case of winter wheat, theamount of available water in this layer increased the most, upto 6 mm, when compared to the K-non-fertilized treatment.The positive effect of K application on soil structure as a soil-aggregating agent has been reported byHamzaand Ander-son (2003) and Holthusenet al. (2010). Souliet al. (2007)explained the stabilizing effect of K on soil structure, pointingout its glue-like effect on soil elementary particles.


Table 2: Accumulation of K by sugar beet around the transition point (TP) of canopy growth as imposed by different water and potassiumregimes in kg ha1.

Water treatments Leaves Storage roots Total

K+ K K+ K K+ K

Rain-fed 216.9ab 88.9c 61.7a 20.1d 278.7ab 109.1c

Irrigation 237.0a 78.4c 55.0b 16.5d 292.1a 94.9c

Drought 172.5b 88.0c 43.5c 18.9d 219.1b 106.9c

1 based onMusolf(2003); numbers marked with the same letters are not significantly different. Irrigation and drought were imposed in July.



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The second aspect of K management refers to its availabilityto crop plants under various soil water regimes. The uptakeof nutrients from the soil solution is governed by two mainprocesses: (1) the transport of ions from the soil solution tothe root surface, and (2) root growth into soil patches that arerich in nutrients. The rate of K-ion diffusion towards the rootdepends on the K-concentration gradient between the rootsurface and the soil solution. As a consequence of the lowmobility of K ions in soil solution, K concentration near theroots decreases very quickly. However, the degree of the

decrease depends on its content in the bulk soil and soilmoisture. Therefore, any process that decreases the K con-centration in the soil solution lowers the supply of K to theaboveground parts of the plant (Jungk and Classen, 1997;Gthet al., 1989). On the other hand, a decrease in soil watercontent may result in increasing K concentrations in the soilsolution. At the same time, a decreased K-ion-transport rateto the root has been observed when the soil moisture drops(Clarkson, 1981;Barber, 1985;Gthet al., 1989).

Plants have evolved numerous regulatory mechanisms towater-deficiency stress to control water loss from the leaves.The primary one is abscisic acid (ABA), the key signal mole-

cule that is produced in the dehydrating root (Shao et al.,2009). It is transported in the xylem water stream to theleaves, and in turn induces stomatal closure (Luan, 2002;Shaoet al., 2009). In light of recent scientific advances, theincreasing concentration of endogenous ABA is now knownto differentially affect the shoots and roots (Taizand Zeiger,2006). The rate of shoot growth is significantly limited, butroots show an opposite response, which increases the rate ofextension. In addition, potassium deficiency increases ABAsynthesis in root cells as documented by Schraut et al.(2005) for maize and Vysotskaya et al. (2008) for durumwheat. On the other hand, root cells that are exposed totemporary shortage of K at the root surface induce high-affi-

nity K transporters (Ashleyet al., 2006;Shaoet al., 2007). Itcan therefore be concluded that the negative impact of endo-genous ABA production in dehydrating roots on shoot growthis partly controlled by soil K supply. Therefore, the uptake ofthe required amount of K by a water-deficient plant dependson both the rate of root-system growth and an adequate Ksupply (Kuchenbuch et al., 1986; Shao et al., 2009). Thishypothesis is in agreement with the Cushman mechanisticmodel of K uptake by plants. According to this model, theextension of root length, followed by the initial K concentra-

tion in the soil solution, is the dominant factor of K uptake bythe growing crop (Barber, 1985).

Crops that are cultivated under rain-fed conditions are sub-ject to frequent wettingdrying cycles during the vegetativeperiod. Under these conditions, nutrient and water supply toroots on one side and their growth rate on the other side aresignificantly disturbed. The main issue of interest is theimpact of a particular nutrient on root extension. Most pub-lished scientific papers on this subject focus on nitrogen andphosphorus (King et al., 2003; Van Noordwijk and Geijn,1996). There are limited data on K, especially regarding its

impact on the root system when the plant is subjected to awater deficit. In addition, reports dealing with the K supply toa crop plant and the resulting root-system responses are con-tradictory. The most frequently cited work byTennant(1976)implicitly indicated that a lack of K in the growth medium ofwheat significantly decreased the number of seminal rootsand their length, and at the same time entirely halted the de-velopment of nodal roots. In addition, the root growth, espe-cially in nodal roots, were considerably reduced in responseto a half rate or a doubled rate of the required amount of K.These conclusions are contradictory to results presented byDrew(1975), who showed prolific growth of barley roots in re-sponse to increasing rates of applied K.


Y0-6 = 0.01K + 20.01

R2= 0.65, P < 0.05

Y18-24 = 0.013K + 19.67

R2= 0.83, P < 0.01

21,2

21,4

21,6

21,8

22

22,2

22,4

22,6

22,8

23

23,2

23,4

100 120 140 160 180 200 220 240 260

content of available potassium, K/ mg (kg soil)-1

availablewatercapacity,

Y/%v.

0-6 18-24soil layers, cm

Figure 9:Available water capacity as a function of soil available K content (basic data adapted from Dammet al., 2013).



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These earlier findings by Drew (1975) have been recentlycorroborated by Perna and Menzies (2010). Their workshowed that the morphology of the maize root system under-went proliferation in the presence of banded or fractional Kfertilization (when a small part of soil is mixed with K fertili-zer). A study conducted by Valadabadiand Farahani(2010)on the effect of water-deficit impact on C4-crop root-systemlengths showed a crop-specific response to applied K. Maizeextended its root system in the presence of K when givenoptimum water conditions. The growth of a millet root systemwas mainly controlled by water deficits, and only sorghumshowed a high response to increasing rates of K. Andersenet al. (1991) found higher root densities in the subsoil in re-

sponse to increasing K rates (1991) when working with barley

grown in the field, but under controlled water conditions.However, this extra root growth appeared only during oneyear of the study and did not show any significant impact onthe grain yield.

A long-term field study byDammet al. (2013) with winter bar-ley, spring barley, maize, and sugar beet showed a significantbut crop-specific response of the root system to K manage-ment. The most important conclusion in this paper refers tothe increasing rate of root extension in early stages of eachcrops growth in response to high amounts of available soil K.It is necessary to stress that higher rates of root-system


Y/K- = 42.489e

R2= 0.94

Y/K+ = 45.084e

R2= 0.96

0

10

20

30

40

50

60

0 0,1 0,2 0,3 0,4 0,5 0,6 0,7

depthofr

ooting/cm

K-

K+

Figure 10: Effect of long-term Kfertilization on asymptotic root dis-tribution in sugar beet with depth (bvalues calculated by means of Galeand Grical function (Liet al., 2006;basic data adapted from Dammet al., 2013).

0

2

4

6

8

10

12

14

16

18

K- K+

potassium treatments

accumul

ationofnutrient/kgha-1

N K

nutrientsK/N = 1.5

K/N = 1.8

a

b

a

b

Figure 11:Effect of long-term K fertilization on nitrogen and K accumulation in sugar beet at BBCH 16 (basic data adapted fromMusolf, 2004b).Analysis of variance conducted for each nutrient separately; respective columns marked with distinct letters are significantly different.



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growth were revealed during the year with prevailing semi-drought conditions. As shown in this paper, all crops culti-vated in plots with high content of both available and recentlyapplied K grew faster than the control K treatments, i.e.,withmedium attainable K content and without freshly applied K.As shown in Fig. 10, the fractional distribution of sugar beettrue roots at the full-rosette stage showed a significant re-sponse to differences in soil K status. Plants grown on soil

that was rich in available K could increase the fraction ofactual roots in the deeper soil layers as a consequence of thefaster growth rate. This phenomenon is a classic example ofa drought-avoidance strategy that was used by sugar beetplants in response to a mild water-stress deficiency (Farooqet al., 2009;Monneveuxand Belhassen, 1996; Shaoet al.,2009).

The acclimation mechanism of crop plants to a decreasingsupply of water and nutrients showed a significant depend-ence on the K supply. Sugar beets that were grown on soilrich in K developed a much deeper root system at the full-rosette stage (BBCH 41). As a consequence, plants growingwith high available soil K had yields that were 12% higherwhen compared to the K-nonamended crop. These findingsare in agreement with the study on sugar beet responses toinduced potassium and water stresses (Musolf,2003;Musolfet al., 2004b). Young beet plants were grown on soil with amoderate K content in comparison to those with an amplesupply of K and were not able to take up a sufficient amountof both K and nitrogen (Fig. 11). As a result, the plant biomassof both the leaves and storage roots was much lower, whichnegatively affected the beet yield.

5 Concluding remarks

Water-deficiency stress is irregular in temperate regions,

which has unforeseeable results throughout crop-plant devel-opment of varying duration and severity (Ludlow andMuchow, 1990; Monneveux and Belhassen, 1996). There-fore, the efficient water management of any farming systemshould be oriented towards increasing the resistance of cropsto potential water stress. However, the degree of a cropplants response to water deficit is stage-dependent (Marty-niuk, 2008). Yield losses occur when water stress negativelyimpacts the dry-matter-accumulation rate, in turn inducing achange in its partitioning among the plant organs during de-velopment progress (Bingham, 2001;Farooqet al., 2009). Asa consequence of water deficits, the development of key yieldcomponents is disturbed, so it has been believed that any

growth factor that increases the uptake of water and/or itsinternal efficiency use can decrease the extent of water-stress effects on the plant-growth rate (Debaekeand Abou-drare, 2004;Ritcheand Basso, 2008). Potassium is a plant-growth factor that should be carefully considered on the basisof its physiological function in the plant, and as one of themost important amendments that can be applied by farmersto cope with water-stress deficiency (Cakmak, 2005).

This review has evaluated the alleviating effects of K applica-tion on the basis of yield increases. The positive response ofselected crops to K fertilizer is mostly revealed under condi-tions of mild water deficit. The drought resistance increase is

a result of adequate K supply to a crop during its life cycle.The water-stress-alleviating action of K is summarized as fol-lows:

(1) The higher the rate of root-system growth at early stagesof crop performance, the higher the growth rate of above-ground plant parts.

(2) The greater the canopy biomass of a crop at its earlystages of growth, the lower the losses of unproductive evapo-rated water.

(3) The denser the root system in a larger volume of occupiedsoil, the higher the amount of resources available for the plantcrop.

(4) The better the K supply to a crop at the critical stages ofyield formation, the higher the probability of yield potentialrealization.

The acclimation mechanisms of a particular crop plant to

temporary water shortage can be strengthened by K applica-tion. Its antistress action may be manifested through theincreasing rate of root-system growth into humid parts of thesoil profile in response to a decreasing water supply (Dammet al., 2013;PernaandMenzies, 2010).

The yield gain that results from K application, which is consid-ered to be the fractional part of the harvested yield, is mostlikely soil-specific, as documented in this paper with exam-ples from cereals, maize, and sugar beet. The alleviatingeffects of an adequate external K supply under mild waterdeficiency has also been documented for sorghum (Asghari-pourandHeidari, 2011), and sorghum and millet (ValadabadiandFarahani, 2010).

It could be hypothesized that the K-induced growth of rootstowards humid zones of the soil profile in response to adecreasing soil water supply primarily depends on the nitro-gen status of the plant. As documented in many papers,nitrate is transported to the root surfaceviamass flow (Clark-son, 1981). However, under conditions of decreasing soilwater content, diffusion becomes a more and more importantroute for nitrate to flow to the root surface (Gonzalez-Dugoet al., 2010). It is possible to state, when taking into accountthe values of diffusion coefficients for NO3 and K ions, (2.7 106, 2.8 108 cm2 s1, respectively), that nitrate develops adepletion zone around the root several times faster than K

(Clarkson, 1981; RaynaudandLeadley, 2004). Recent pro-gress in the study of plant-response mechanisms to N defi-ciency explains its complexity. The uptake of nitrate is con-trolled by two signaling pathways. The first local uptake isinduced by decreasing the concentration of nitrate in the soilsolution. It has been recently shown that decreasing the sup-ply of nitrate to the root encodes the activity of high-affinitytransporters. The initial sites of early nitrate shortage sensinghave been found in the lateral root cap, the root stele, and thepericycle. The second main route of N deficiency signaling islocated in the shoot and is driven by the internal status ofnitrogen or nitrate (Alvarezet al., 2012;Hoand Tsay, 2010).Therefore, a hypothesis can be formulated to explain the




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crawling-like pattern of nitrate mining that is followed by aplant root in drying soil, which most likely acts as the primarysignal of its deficiency. Next, the signal is directly transferredto the aerial parts of a plant, resulting in a change of N meta-bolism (Bingham, 2001;FordeandLorenzo, 2001).

A decreased soil water content interferes with relationshipsbetween cations in the soil solution, which in turn modifies

the intracellular concentration of Ca2+ ions in the root apo-plast (Shaoet al., 2007). Calcium acts as a sensor of environ-mental signals, including the soil-water gradient, whichaffects the processes that are responsible for plant watermanagement (Bray, 1997;Luan, 2002). Potassium deficiencyin the growth medium also affects the Ca2+ sensors, which inturn activate the high-affinity K+ transporters in cells withinthe root cortex (Maathuis, 2009; Szczerba et al., 2009).Recent investigations have also revealed that nitrate and Kdeficiency can be recognized in the growth medium by thesame molecule, protein kinase (CIPK23). Its activation by cal-cium-binding proteins is a required step to induce the high-affinity K transporters (HoandTsay, 2010).

The nitrogen and especially the nitrate status of a plant is thekey factor in regulating a series of processes that are respon-sible for the synthesis, signaling, and control of hormoneflows in the shootroot continuum. The response of youngparts of the plant to an insufficient supply of nitrogen resultsin a decreased downward transportation of auxin (Krouket al., 2011). The movement of this signal molecule within theroots has been addressed by K carrier TRH1 (Vicente-Agulloet al., 2004). As a consequence, this molecule affects therate of growth and the shape of the root system in the soilprofile (Ashleyet al., 2006;Cassonet al., 2003;Eapenet al.,2005).

The modified root system of a crop plant is a consequence ofa consecutive series of metabolic and physiological pro-cesses, which result from the action of hormones in the rootshoot continuum. Therefore, these secondary processes,which are primarily induced by nutrient deficiency in the plantmedium, have recently been termed as trophomorphogenicplant response (Forde and Lorenzo, 2001). There is mostlikely some cross-talk between water- and nutritional-defi-ciency signaling. The water supply to a plant is considered tobe the main factor in nutrient mass flow, and can induce plantnutrient-deficiency signaling, which in turn is a driving forcefor root growth into the soil layers to reach both water andnutrients.

The proposed hypothesis of water-stress deficiency controlrelies on a simple agronomic trait, i.e., the improvement ofsoil-K management. It is well recognized that a decrease insoil water content is accompanied by a subsequent decreasein transpiration rate, resulting in less mass flow of K to theroot surface, which in turn reduces K uptake in the K-nona-mended soil. Increasing K in the soil solution through fertilizerapplication compensates for this effect by providing an ade-quate K supply and maintaining root growth. Maintaining rootgrowth allows access to other mineral elements (includingnitrogen) and water. Thus, yields are greater under milddrought when K fertilizer is added. High available K content in

the soil is therefore the primary reason for the K-induced pro-lific growth of the root system in response to an increasingwater deficit in upper layers of the soil profile.

Acknowledgments

The Authors express their gratitude to the anonymousreviewer, for comments and suggestions, which improved thescientific value and readability of the paper.

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