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Field Crops Research 134 (2012) 95104
Contents lists available at SciVerse ScienceDirect
Field Crops Research
jou rn al h om epage: www.elsev ier .com/ locate / fc r
ugarcane for water-limited environments: Theoretical assessment
of suitableraits
.G. Inman-Bambera,, P. Lakshmananb, S. Parkc
CSIRO Plant Industry, Building 145 - ATSIP, James Cook Drive,
James Cook University Douglas Campus, Townsville, QLD 4811,
AustraliaDavid North Plant Research Centre, BSES Limited, 50 Meiers
Road, Indooroopilly, QLD 4068, AustraliaCSIRO Ecosystem Sciences,
Clunies Ross Street, Black Mountain, Canberra, ACT 2601,
Australia
r t i c l e i n f o
rticle history:eceived 13 July 2011eceived in revised form 27
April 2012ccepted 13 May 2012
eywords:ooting depthranspiration efficiencyrop
simulationPSIM
E
a b s t r a c t
In Australia water stress is estimated to cost the sugar
industry an average of $260 million (AUD) perannum in lost
production. With the predicted increased frequency of drought
events the industry is nowconsidering breeding for drought
adaptation after water stress inflicted yield losses of more than
$400million in the years 20022004, in one region alone. Defining
drought adaptation broadly, including bothshort and long periods of
water stress, we took the first step in improving sugarcane for
such conditionsby assessing the potential benefits of a number of
traits in a simulation study. The APSIM-Sugarcanemodel was used to
simulate the biomass yield response to traits that may confer
adaptation to droughtin a range of climates, some extremely dry at
times, and in a shallow and a deep soil. Among the traitsstudied,
increased rooting depth resulted in 021% increase in mean dry
biomass yield depending onthe climate and soil type. This trait was
more beneficial in the shallow than the deep soil which had
asmaller fraction of additional stored water to offer the more
vigorous root system. The simulations showedthat breeding for
reduced stomatal or root conductance (conductance) would increase
biomass yield byabout 5% only in the driest climates and better
soils. Other traits which conserved water such as leaf andstalk
senescence were generally unsuccessful in conferring adaptation to
the water-limited productionenvironments considered. Simulations
indicated that increased transpiration efficiency (TE) at the
leaflevel would nearly always help to improve sugarcane biomass
yields in water-limited environments
if the increased TE arose from up-regulation of intrinsic water use
efficiency. However if increased TEwas increased through reduced
conductance, which effectively reduces VPD during transpiration,
yieldscould be reduced in high rainfall climates and shallow soils
and they could increase in moderate rainfallclimates and deeper
soils. Thus increased rooting depth, increased intrinsic water use
efficiency and toa lesser extent, reduced conductance leading to
increased TE, are suggested as the best traits to consider
clon
for selection of sugarcane
. Introduction
Water is the single most limiting factor worldwide to
theroductivity of rainfed crops and those with supplementary
irri-ation (Juenger et al., 2005). It is likely to further
constrain croproduction where seasonal rainfall is predicted to be
more vari-ble, and/or decline (Campos et al., 2004). In Australia,
the 2002rought reduced farm output by nearly 30% (Horridge et al.,
2005).
n the sugarcane industry alone, water stress is estimated toost
an average of $260 million per annum in lost
productionInman-Bamber, 2007). These losses occur despite nearly
60% of
Corresponding author. Tel.: +61 7 47538587; mobile: +61
0424759841.E-mail addresses: [email protected],
[email protected] (N.G. Inman-Bamber),
[email protected]. Lakshmanan), [email protected] (S.
Park).
378-4290/$ see front matter. Crown Copyright 2012 Published by
Elsevier B.V. All rittp://dx.doi.org/10.1016/j.fcr.2012.05.004
es in water-limited environments in the tropics and
sub-tropics.Crown Copyright 2012 Published by Elsevier B.V. All
rights reserved.
the industry receiving supplementary irrigation (Inman-Bamberand
McGlinchey, 2003). All sugarcane-growing regions in
Australiaexperience extreme seasonal and annual variability in
temper-atures and rainfall largely due to the influence of the El
NinoSouthern Oscillation (ENSO) (Nicholls and Kariko, 1993). It is
there-fore questioned whether developing drought tolerant and
wateruse-efficient sugarcane varieties may reduce these
productivitylosses.
Whilst there appears to be clear benefits from breeding
cultivarswith improved drought-tolerance and water use efficiency
(WUE)traits, the list of potential traits associated with drought
toleranceand WUE is extensive (Ludlow and Muchow, 1990). Further,
thetraits expressed are known to depend greatly on the
environment
in which the crop is grown, the specific climate experienced
dur-ing crop growth and the management strategy imposed on thecrop
(Chapman et al., 2000). The large genotype environment(G E)
interactions for crop yield mean that breeding programs
ghts reserved.
dx.doi.org/10.1016/j.fcr.2012.05.004http://www.sciencedirect.com/science/journal/03784290http://www.elsevier.com/locate/fcrmailto:[email protected]:[email protected]:[email protected]:[email protected]/10.1016/j.fcr.2012.05.004
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9 d Crop
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6 N.G. Inman-Bamber et al. / Fiel
ust select for the specific secondary physiological traits
associ-ted with particular drought tolerance and WUE strategies
deemedptimal for a defined water-stressed environment (Tardieu,
2003).
Some degree of drought adaptation has been reported for aumber
of sugarcane cultivars. These showed drought avoidanceraits such as
leaf rolling, early stomatal closure, leaf sheddingnd reduced leaf
area, rather than traits that confer drought tol-rance
(Inman-Bamber and Smith, 2005). Leaf rolling and earlytomatal
closure are highly reversible. In a cultivar with thesewo traits,
CO2 assimilation and transpiration would be reduceduring a dry
period, but would then resume quickly when watertress was relieved
either through increased soil water content oreduced atmospheric
demand for water. Cultivars vary consider-bly in regard to stomatal
response to soil water deficits (Saliendrand Meinzer, 1989;
Inman-Bamber and Smith, 2005). Two cultivarsubjected to water
stress reduced their projected leaf width sim-larly through leaf
rolling (Inman-Bamber and de Jager, 1986) bututherford (1989)
suggested that leaf rolling may vary considerablymongst sugarcane
genotypes.
Cultivar differences in leaf shedding in response to water
stressave been observed (Inman-Bamber and de Jager, 1986). This is
aore drastic response to water stress than leaf rolling or
stomatal
losure and recovery would be relatively slow. Stalk senescence
isnother option for the crop to reduce water-use during extendedry
periods. There are few studies on the nature of water stressnd
stalk senescence but experience shows that crops lose stalksnd
stools during severe drought, eventually requiring a replant-ng
program. Stalk population in two cultivars remained unaffecteduring
a period without irrigation long enough to reduce
biomassccumulation by 70% (Inman-Bamber, 2004), suggesting the
needor severe stress for stalk loss to occur.
Clearly a mechanistic approach to conventional crop improve-ent
programs requires knowledge of how plants manage
ompeting requirements to assimilate carbon and conserve
waterhroughout their growth and development. By capturing cur-ent
scientific understanding of the physiological determinants ofrop
growth and development, mechanistic and eco-physiologicalrop models
provide a useful tool for integrating physiologicalnderstanding
into empirical plant breeding procedures. Such anpproach can enable
a quantitative assessment of the impact of spe-ific traits on crop
yield (Tardieu, 2005), aid the design of optimaldeotypes for target
environments (Shorter et al., 1991; Chapmant al., 2002), facilitate
an assessment of the variation in the G Enteraction for
quantitative traits such as yield, and determine
theepresentativeness of the short-term climate and associated
yieldsbtained in plant breeding trials, to more long-term historic
climatend crop performance (Chapman et al., 2002).
In this paper we conduct a quantitative assessment of thempact
of specific traits such as conductance, rooting depth,
leafenescence and transpiration efficiency (TE) on biomass yield
ofugarcane to support a largely field-based breeding program
inearch of drought tolerance mechanisms and related traits in
dif-erent production conditions. To the best of our knowledge this
ishe first study of its kind for sugarcane.
. Materials and methods
In this simulation study, the APSIM-Sugarcane module (v 7.2)as
used in conjunction with the Agricultural Production Systems
IMulator (APSIM) crop modelling system (McCown et al.,
1996;eating et al., 2003). The model is designed to simulate cane
yield,
ucrose yield, crop biomass, water use, crop nitrogen uptake
andartitioning of carbon and nitrogen for a uniform field of
sugarcaneLisson et al., 2005). The APSIM-Sugarcane module was
devel-ped from detailed growth analysis experiments, 14 of which
were
s Research 134 (2012) 95104
conducted along the east coast of Australia (18.429.5S), four
onthe east coast of South Africa (26.229.5S) and one in
Hawaii(21.5N). The experiments included five varieties and various
levelsof N and water regimes (Keating et al., 1999). The Australian
exper-iments were done largely with the commercial variety Q117
andthe South African experiments with NCo376 and N12.
Coefficientsof determination for model predictions compared to
observeddata (n > 150) were 0.79 for leaf area index (LAI), 0.93
for drybiomass yield and 0.83 for sucrose yield (Keating et al.,
1999).Q117 is therefore one of the varieties that has been
characterisedcomprehensively in terms of its physiology and crop
growth anddevelopment and was used at the default variety (clone)
for thelong-term simulations in this study.
APSIM-Sugarcane was used to simulate reduced water use aswould
occur with early stomatal closure, rapid leaf shedding andearly
stalk senescence in response to increased water stress.
Twoadditional traits for which there is some evidence of variation
insugarcane genotypes are transpiration efficiency (TE)
(Saliendraet al., 1996) and rooting depth distribution (Smith et
al., 2005).These were also varied within realistic limits in the
APSIM sim-ulations.
Two published field experiments (Inman-Bamber, 2004) wereused to
verify simulations of rapid leaf shedding responses towater stress.
We reported only biomass yields in this study becausein some
situations, water stress was so severe that simulatedcrops never
advanced enough to accumulate significant amountsof sucrose. In
addition, biomass is increasingly becoming thestarting point for
integrated sugarcane-based industries that maydeliver sugar,
ethanol, electricity and high-value products in future(Waclawovsky
et al., 2010).
2.1. Field experiments
Full details of the two experiments used for verification of
theleaf senescence trait were provided by Inman-Bamber (2004)
andonly essential details are presented here.
The two experiments were planted simultaneously on 15 June1998
at Kalamia Estate, Ayr, Queensland (14725E, 1932S). Bothexperiments
were of a randomized split plot design with cultivarsQ96 and Q124
as the sub-plot treatment and irrigation (wet) orsuspended
irrigation (dry) as the whole plot treatment. Sub-plotsize was 45 m
10 rows, 1.5 m apart. There were five replicationsand hence 20
sub-plots in each experiment.
Experiment 1 was ratooned on 28 April 1999 in preparation
forimposition of a stress treatment in September to December
andexperiment 2 was ratooned on 15 November 1999 in preparationfor
imposition of a stress treatment after the annual wet
season(usually December to April). Both crops were maintained
follow-ing standard crop management practices, other than
irrigation.Lodging was minimal in these trials. Standard industry
irrigationmanagement was applied to all plots until it was time to
imposewater stress to half of the plots in each experiment. For the
firstexperiment the dry plots were irrigated on 21 September 1999
forthe last time and for the second experiment, dry plots were
notirrigated at all after the wet season. Biomass, LAI and stalk
popu-lation were monitored at 23-week intervals through
destructivesampling as described by Inman-Bamber (2004).
The experiments were simulated using standard (default)
set-tings in APSIM and canopy characteristics for one of the
varieties(Q96) used in the experiment. The other variety was Q124
but itsuccumbed to orange rust in the second experiment and was
there-fore excluded (Inman-Bamber, 2004). Settings for the
maximum
area of successive leaves of Q96 recognised the reduction in
leafarea in older leaves even when plants were well irrigated
(Inman-Bamber, 2004). Leaf numbers 1, 14, 30 and 40 were assigned
areasof 50, 550, 550 and 350 cm2 and served as inflection points
for the
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2
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N.G. Inman-Bamber et al. / Fiel
xtrapolation of the area of other leaves. Climate data was
obtainedrom an automatic weather station installed within 2 km from
theite.
.2. Reduced conductance
APSIM-Sugarcane does not simulate stomatal conductance asuch so
the impact of reduced stomatal conductance was simulatedy modifying
experimentally determined root water extractionoefficients (KL) for
sugarcane (Inman-Bamber et al., 2001). Theres some evidence that
root resistance and/or root signals, regulateesistance to water
flow though the soilplantatmosphere con-inuum (Saliendra and
Meinzer, 1989) so the variation of KL toimulate additional
reduction in transpiration and CO2 assimilationuring water stress,
has some foundation. Bulk canopy resistancearied from 34 to 57 s m1
in studies on sugarcane by Grantz andeinzer (1991) and McGlinchey
and Inman-Bamber (1996) thus
50% reduction in conductance (1/resistance) seemed plausiblen
our simulation study. Standard and reduced values for KL weresed to
compare standard and high resistance to water vapour andO2 flux. A
50% reduction in KL (Table 1) reduced transpiration bybout 20%.
Photosynthesis was reduced in proportion to transpi-ation when
atmospheric demand for water exceeded root waterupply (Keating et
al., 1999).
.3. Simulation of deep root distribution
The observed differences in rooting depth between modern
cul-ivars are not large and some at least can extract water to 3 m
andossibly more (Smith et al., 2005). The water balance module
(Soil-at) in APSIM divides the soil into layers (Table 1) to
simulate waterux in one dimension only but in two directions, with
gravity andapillarity involved (Keating et al., 1999). Roots were
allowed toxplore one additional layer in each of the two soils
(Yellow chro-osol and Red dermosol) simulated in this study. Deeper
rootsere also made more efficient by increasing the lower limit
(LL)
o which water could be extracted from each soil layer beneath
theurface layer (Table 1). The LL for the surface layer was not
alteredo avoid confounding the root depth and vigour traits with
emer-ence rate which in the model depends on the amount of
water
vailable in the top layer. Hypothetical clones with shallow andess
vigorous roots had access to 10% less water stored in the soilayers
where roots were present and 22% less water over all layersor the
shallow soil (60 mm and 76 mm for shallow and deep roots,
able 1olume fraction for lower limit of water availability,
drained upper limit, saturation andater extraction coefficient (KL)
in simulated layers for two soils (see Inman-Bamber et a
Yellow chromosol (poor soil)Layer 1 2 3
Soil depth from surface (mm) 200 400 500 Lower limit shallow
roots 0.082 0.083 0.099 Lower limit deep roots* 0.082 0.077 0.094
Drained upper limita 0.162 0.136 0.141 Saturationa 0.289 0.247 0.25
Saturated conductivity coefficient 0.8 0.8 0.8 KL standarda 0.10
0.10 0.08 KL reduced 0.05 0.05 0.04
Red dermosol (excellent soil)Soil depth from surface (mm) 200
400 500 Lower limit shallow roots 0.171 0.235 0.280 Lower limit
deep rootsa 0.171 0.225 0.27 Drained upper limita 0.282 0.325 0.371
Saturationa 0.358 0.379 0.402 Saturated conductivity coefficient
0.5 0.5 0.5 KL standard 0.10 0.10 0.08 KL reduceda 0.05 0.05
0.04
a Based on measurements by Inman-Bamber et al. (2000, 2001).
s Research 134 (2012) 95104 97
respectively) and 10% less water for the deep soil (258 mm
and287 mm for shallow and deep roots, respectively). The
defaultsettings in APSIM-Sugarcane allow the rooting front to
advance10 mm d1 until the stalk elongation phase starts and then
at15 mm d1 thereafter (Keating et al., 1999). The model assumes
thatrooting depth penetration is reduced when soil water content in
aparticular layer falls below 25% but we removed this limitation
inour study.
2.4. Simulation of rapid leaf senescence
In APSIM sugarcane, water stress reduces photosynthesis in
pro-portion to the extent to which root water supply fails to
meetatmospheric demand for water. The ratio of root water supply
towater demand is called the stress factor for photosynthesis (SP
orswdef photo in APSIM code). When SP is less than 1.5, green
leafarea is reduced each day by the fraction 0.01(1.0 SP) to
simulatestandard leaf senescence. To simulate rapid leaf senescence
the dieback fraction was increased to 0.03(1 SP) per day based on
the leafarea loss observed under water stress in the second
experimentof Inman-Bamber (2004). Initial rate of leaf senescence
during theonset of water stress was more than three times greater
in one cul-tivar of soybean than another (Lawn and Likoswe, 2008)
and it isseems from the field experiment (Inman-Bamber, 2004) that
leafsenescence could vary this much in sugarcane. Inman-Bamber
andde Jager (1986) concluded that cultivar N11 was better adaptedto
water stress than another cultivar, NCo376, because leaf
areadeclined more rapidly during stress and increased more
rapidlyafter stress was relieved than was the case for NCo376.
HoweverSmit and Singels (2006) found no difference in the rate of
decline inleaf area between N22 and NCo376 during a period of water
stress.
2.5. Early stalk senescence
Stalk senescence is known to occur in sugarcane but this
processwas omitted in the sugarcane module of APSIM because the
exactcauses were not known at the time (Keating et al., 1999).
Lodg-ing was later found to be one of the causes of stalk
senescence(Singh et al., 2002) and a procedure for reducing stalk
popula-tion was introduced to the sugarcane module. A senescence
rate
up to 0.1 stalks m2 d1 was reported by Park and Attard (2005)and
Park et al. (2005) when attempting to explain reduced growthin
ageing sugarcane and a mid-range value of 0.05 stalks m2 d1
due to water stress is conceivable. Assuming a typical final
saturated conductivity coefficient and standard, reduced and
deep values for rootl., 2000). All units are fractions except for
soil depth.
4 5 6 7 8600 800 1000 1300 1600
0.115 0.115 0.115 0.115 0.1510.111 0.111 0.111 0.111 0.1110.151
0.151 0.151 0.151 0.1510.236 0.236 0.236 0.236 0.2360.8 0.8 0.8 0.8
0.80.07 0.06 0.05 0.05 0.050.035 0.03 0.025 0.025 0.025
600 800 1500 2300 31000.327 0.329 0.333 0.333 0.4120.317 0.319
0.323 0.323 0.3230.42 0.416 0.412 0.412 0.4120.427 0.425 0.423
0.423 0.4230.5 0.5 0.5 0.5 0.50.07 0.06 0.05 0.05 0.050.035 0.03
0.025 0.025 0.025
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98 N.G. Inman-Bamber et al. / Field Crops Research 134 (2012)
95104
Table 2Traits and treatments with up to five variations or
levels used in 640 factorial combinations in the simulation.
Trait or treatment Level 1 Level 2 Level 3 Level 4 Level 5
Rooting depth and vigour Standard ReducedRoot water extraction
coefficient (KL) Standard Reduced DeepLeaf senescence rate (m2 d1)
0.01(1-Sp) 0.03 (1-Sp)Stalk senescence (% stalks m2 d1 when
leaf area index falls below 0.5)0.0 0.5
Intrinsic water use efficiency (g kPa kg1) 8.7 9.6Soil Yellow
chromosol
(shallow)Red dermosol(deep)G0
pGslrae
2
dp
T
w(SfiayIko1iI(ew
ac(bmTua1sd(ae
aoutl
Climate BundabergFraction of maximumminimum
saturation vapour for VPD estimation0.75
opulation density of 10 stalks m2 without water stress (Bell
andarside, 2005), early stalk senescence was invoked in half of
theimulations by reducing the stalk population by 0.5% per day
wheneaf area index (LAI) declined to a low value of 0.5 after
havingeached a value of 2.0. The loss of stalks then further
reduced leafrea and the accumulated amount of biomass but not
radiation usefficiency.
.6. Transpiration efficiency (TE)
In the APSIM model, daily transpiration (T) is proportional
toaily biomass accumulation (W) multiplied by mean daily
vapourressure deficit (VPD).
= W VPDk
(1)
here k is an empirical constant equal to transpiration
efficiencyTE) at a VPD of 1 kPa (Keating et al., 1999; Sinclair et
al., 2005).inclair et al. (2005) called k the intrinsic water use
efficiency coef-cient, a phrase we adopted here. Keating et al.
(1999) reported
k of 8.0 g kPa kg1 when simulating a number of growth anal-sis
experiments mentioned above, mainly with Q117. Howevernman-Bamber
and McGlinchey (2003) obtained 8.7 g kPa kg1 for
from Bowen ratio energy balance and biomass accumulation workn
one sugarcane cultivar (Q127). It is reasonable to assume that a0%
variation in k could be found in the sugarcane germplasm sim-
lar to the variation implied for sorghum by Hammer et al.
(1997).n our study TE values of 8.7 and 9.6 g kPa kg1 were
comparedTable 2). This is a smaller range than the one used by
Hammert al. (2005) (8 to 10 g kPa kg1) for a similar simulation
exerciseith sorghum.
TE defined as W/T depends on the diurnal variation in VPDnd T
particularly in drying soil conditions when partial stomatallosure
limits T during periods of high VPD. Meinzer and Grantz1990)
presented evidence indicating that T in sugarcane is limitedy
coordinated root and stomatal conductance which reached aaximum of
about 1.4 mmol s1 per plant with 0.8 m2 leaf area.
hus maximum transpiration rates may be limited to 0.6 mm h1sing
the maximum conductance of Meinzer and Grantz (1990) and
leaf area index of 5 m2 m2 (Muchow et al., 1994;
Inman-Bamber,994). Under water limited conditions or under high VPD
(or both)tomata start to close further limiting T and W (CO2
assimilation)uring the time of day when evaporative demand and VPD
are highInman-Bamber et al., 1986). Mean daily TE as defined will
increases relatively more transpiration occurs towards the morning
andvening when VPD is low (Sinclair et al., 2005).
The effect on effective VPD by limiting T during the day
wasnalysed using hourly records of relative humidity and
radiation
btained from an automatic weather station at BSES. Eq. (1) wassed
to derive T on an hourly basis assuming 1.8 g MJ1 for radia-ion use
efficiency (Keating et al., 1999) and 8.7 g kPa kg1 for k.
Theong-term mean VPD was obtained as the sum of hourly T VPD
argett Farleigh Charters Towers Pongola.67
divided by total daily T with T either not limited at all or
limitedto 0.5 mm h1 (Sinclair et al., 2005). In APSIM-Sugarcane,
VPD isderived as a fraction of the difference in saturation vapour
pres-sure at minimum and maximum daily temperatures. The
defaultfraction is 0.75 and this was reduced by 11% (to 0.67),
which is the% reduction in long term VPD when T was limited to 0.5
mm h1.It was assumed that reduced conductance could lead to
reducedeffective VPD which would increase TE.
Thus TE was varied in two ways, firstly by a 10% increase in
thedefault intrinsic water use efficiency (k) and secondly by
combiningreduced conductance and reduced VPD in the APSIM
model.
2.7. Climate
Three distinct climatic regions of the sugarcane industry
inAustralia (southern, central and dry tropics) were selected
fromthe range of conditions where drought resistance traits could
be ofbenefit. The SILO database operated by Queensland Climate
ChangeCentre of Excellence
(http://www.longpaddock.qld.gov.au/silo/)provided daily radiation,
rainfall, class A-pan evaporation, andmaximum and minimum
temperatures from 1960 to 2009 for theBSES Ltd experimental farms
near Bundaberg (southern region),Farleigh Cooperative Sugar Mill
and the Gargett Post Office in theMackay region (central region),
and for Charters Towers (Towers)in the dry tropics. The dry region
of Pongola in South Africa wasalso considered in order to broaden
the study. Climate data for thePongola region was obtained from the
South African SugarcaneResearch Institute through their web-enabled
weather network(http://portal.sasa.org.za/weatherweb).
Although mean annual rainfall (MAR) for Mackay is reasonablyhigh
(1647 mm), drought has inflicted severe yield losses of morethan
$400 million AUD in the years 20022004 (Inman-Bamber,2007). The
Bundaberg/Maryborough region has one of the lowestannual rainfalls
(1022 mm for BSES Bundaberg) in the Australiansugarcane belt.
Despite the availability of modest amounts of irri-gation water,
the region suffers more from drought than any otherproduction area
in Australia (Inman-Bamber, 2007). Sugarcane isnot grown
commercially at Charters Towers with only 655 mmMAR and no
irrigation, but this region may represent the type ofclimate that
might be experienced with climate change with a pos-sible 40%
reduction in rainfall by 2070 (Park and Attard, 2005) orthe type of
climate where sugarcane may be grown for biofuel tohelp reduce CO2
emissions in future (Waclawovsky et al., 2010).Pongola in South
Africa has a similar MAR (648 mm) but irrigationis available to
support commercial sugarcane cultivation.
2.8. Other model settings
A typical crop cycle was simulated using the APSIM
sugarcanemodule. This consisted of a crop planted in April and
harvestedin July followed by four 13-month-old ratoon crops. The
final stalkpopulation was 10, 10, 9, 8 and 7 stalks m2 for each
successive crop
http://www.longpaddock.qld.gov.au/silo/http://portal.sasa.org.za/weatherweb
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N.G. Inman-Bamber et al. / Field Crops Research 134 (2012) 95104
99
Fig. 1. Measured leaf area index for wet ( ) and dry ( )
treatments (bars denote2sa
iefafAaMnlgfrwcopbetw
3
3
ettffew
FSc(
Fig. 3. Average total monthly rainfall (bars), mean monthly
temperature (lines) and
SEM). Simulated leaf area index for wet () and dry treatments (- -
- -) usingtandard senescence settings and using a high leaf
senescence rate for wet ( ),nd dry treatments ( ).
n the planting and ratooning cycle to recognise, at least to
somextent, the loss of vigour in later ratoons. The soil was
assumed to beallowed between harvesting the fourth ratoon crop in
Novembernd replanting in April. This cycle was repeated from 1960
to 2009or the Australian sites and between 1967 and 2009 for the
Southfrican site. Crop residue was left on the soil surface to
simulate
green cane trash blanket which is a common practice in theackay
and Bundaberg regions. The residue, water balance and
itrogen balances were continued (not reset) for the entire
simu-ation period. An irrigation of 40 mm was applied to help with
cropermination and to acknowledge the likely and logical choice
ofarmers to plant into moist soil. The time limit for germination
wasemoved but not the thermal time or the cumulative stress
limitshich can cause the crop to fail. If these limits were
exceeded, the
rop was harvested and the ground then lay fallow till the
nextpportunity for planting in April. APSIM crop modules restrict
rootenetration when the soil is dry but we removed this
restrictionased on our unpublished observations of root penetration
intoxtremely dry soils and saprolite. A total of 640 combinations
ofhe various traits and treatments (Table 2) and a total of 9382
cropsere simulated.
. Results
.1. Simulation of field experiments
The simulation of LAI (Fig. 1) and biomass (Fig. 2) illustrate
theffect of varying levels of sensitivity to water stress in
relationo leaf senescence. Using standard APSIM settings, simulated
LAIowards the end of the crop was low for experiment 1 and high
or experiment 2 compared to observed LAI but the simulated
dif-erence between the wet and dry treatments was realistic for
bothxperiments. Measured LAI of the dry treatment in experiment 2as
bounded by simulations with standard settings and with the
ig. 2. Measured biomass for wet () and dry () treatments (bars
denote 2 SEM).imulated biomass for wet () and dry treatments (- - -
-) using a standard leaf senes-ence rate and using a high leaf
senescence for wet ( ), and dry treatments
).
radiation (symbols) for Bundaberg (solid bars, line, symbol),
Farleigh(diagonal bars, , ), Gargett (checked bars, , ), Pongola
(stripedbars, , ) and Charters Towers (open bars, , ) from 1960 to
2009.
higher level of leaf senescence, indicating that the simulated
vari-ation in leaf senescence was realistic. In the simulation,
conditionsfor stalk senescence causing loss of leaf area did not
occur evenwith higher leaf senescence trait. In the field
experiments the waterstress treatments imposed were not
sufficiently severe to result instalk death (Inman-Bamber,
2004).
Simulated biomass was generally too high for experiment 1 andtoo
low for experiment 2, although final estimates of biomass wereclose
to measured biomass in the case of the wet treatment in
bothexperiments (Fig. 2). High sensitivity of leaf senescence to
waterstress had only a small negative effect on biomass
accumulationin both experiments because variations in LAI above a
value of 2have a diminishing effect on radiation interception which
is 53, 68and 78% with LAI values of 2, 3 and 4, respectively (based
on anextinction coefficient of 0.38; Keating et al., 1999).
Comparing measured and simulated responses to water stressfor
the two experiments, indicated that the drought avoidancemechanism
of rapid leaf senescence is potentially available whilethe trait
for early loss of stalks may not be immediately available inthe
local germplasm if it turns out to be a useful mechanism. How-ever,
Smit and Singels (2006) found that one South African variety(N22)
was considerably more susceptible to water stress in regardto stalk
senescence than another (NCo376).
3.2. Climate of target sites
Mean monthly rainfall totals for JanuaryApril were 1.52.5times
higher at Farleigh and Gargett in the Mackay region thanat the
other sites (Fig. 3) and exceeded monthly evaporation totals(data
not shown). In all other cases, rainfall totals were lower
thanevaporation totals for all months and sites. Water stress is
likelyto be severe in spring particularly in Charters Towers where
radi-ation, temperature and evaporation are both high and rainfall
islow. Spring temperatures were considerably lower at Pongola
thanat the other sites and rainfall was relatively high so water
stress atthis time of year could be less severe at Pongola than at
the othersites.
3.3. Biomass yields
The range in biomass yields was large for all sites (Fig. 4).
Simu-lated dry biomass yields could be less than 10 t ha1 in the
mostfavourable site, Farleigh, and greater than 30 t ha1 in the
leastfavourable site, Towers. Although MAR was similar for Towers
andPongola, yields were greater at Pongola because of the better
dis-
tribution of rain through the year and the lower radiation
andtemperature resulting in less water stress. Mean biomass yield
forTowers, Pongola, Bundaberg, Gargett and Farleigh was 11, 13,
30,34 and 44 t ha1 respectively.
-
100 N.G. Inman-Bamber et al. / Field Crop
Biomass yield (t ha-1)
0 3 0 6 0 9 0 0 . 0
0 . 1
0 . 2
0 . 3
0 . 4
0 . 5
0 . 6
0 . 7
0 . 8
0 . 9
1.0 F
r a c t i o n o
f D
a t a
Fig. 4. Cumulative frequency distribution of biomass yield for
all treatments forfiG
3
asmaeMsicttrlbaT
for the Red dermosol in the driest sites, Towers (+7.3 1.0%)
and
F(
ve climates Charters Towers ( ), Pongola ( ), Bundaberg (
),argett ( ) and Farleigh ( ).
.4. Increased rooting depth
Increased rooting depth was beneficial to biomass yield in
nearlyll situations (all years and all treatments other than
climates andoils) simulated with properties of a poor soil type
(Yellow chro-osol) (Fig. 5a). A deep and more vigorous rooting
trait was not
s beneficial in the good soil because the increase in soil
waterxploited was not as great (10% compared to 22% for the poor
soil).ore vigorous roots could also be a disadvantage in some
circum-
tances where they encourage greater exploitation of water
storedn the soil leaving little or none available for the
subsequent ratoonrop. Deeper roots in the Red dermosol were more
beneficial forhe wetter sites Gargett and Farleigh because there
was more addi-ional water to exploit. For this soil at Gargett and
Farleigh, deeperoots increased biomass yield in 80% and 90% of all
crops simu-ated respectively while in the other sites only 50% of
the crops
enefitted from this trait (Fig. 5b). The average yield increase
from
deeper and more vigorous root system in the shallow soil
forowers, Pongola, Bundaberg, Gargett and Farleigh was 18.0
0.5,
Yellow chromosol
Biomass yield response
a
0 50 100 150 2000.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
Fra
ction o
f D
ata
ig. 5. Cumulative frequency distribution of biomass yield
response to increased root dept), Gargett ( ) and Farleigh ( ).
s Research 134 (2012) 95104
15.2 0.3, 19.2 0.2, 19.1 0.2 and 18.9 0.2%, respectively andfor
the deep soil the mean benefit was 5.4 0.6, 4.2 0.3, 9.2 0.4,13.7
0.4 and 13.7 0.4% for these sites, respectively.
3.5. Increased leaf senescence rate
The biomass yield response to increased leaf senescence ratewas
mostly negative with yield losses of up to 40% for both the poorand
good quality soil (Fig. 6). Rapid leaf senescence was of benefitin
less than 10% of situations. The negative affects were greater
inthe shallow than in the deeper soil because of the limited
storagecapacity of the shallow soil and the more frequent loss of
leaf area.In the deeper soil the negative effects were less serious
in the wetterclimates than in the drier climates because leaf
senescence was lessprevalent there. Generally this strategy failed
to save enough waterto offset yield loss through reduced radiation
capture except in avery limited number of situations.
3.6. Early stalk senescence
Stalk senescence initiated in about one third of all
simulationsfor both soils but in many cases this had little effect
on biomassyield because stalk senescence occurred shortly before
harvesting,or it occurred in situations where there was little
recovery in LAIregardless whether stalks senesced or not. The loss
of stalks whenLAI declined to 0.5 through water stress was
generally unsuccessfulin helping crops get through dry conditions
(Fig. 7) despite the stalkpopulation being fully renewed at the
start of each crop. A smallproportion (
-
N.G. Inman-Bamber et al. / Field Crops Research 134 (2012) 95104
101
Yellow chromosol Red dermosol
Biomass yield response to rapid leaf senescence (%)
60 70 80 90 100 110 120 0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
Fra
ctio
n o
f D
ata
60 70 80 90 100 110 120
Fig. 6. Cumulative frequency distribution of biomass yield
response to rapid leaf senescence for two soils and five climates,
Towers ( ), Pongola ( ), Bundaberg( ), Gargett ( ) and Farleigh (
).
Biomass yield response to early stalk senescence (%)
Yellow chromosol Red dermosol
50 70 90 110 130 150 50 70 90 110 130 1500.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
Fra
ction o
f D
ata
Fig. 7. Cumulative frequency distribution of biomass yield
response to early stalk senescence for two soils and five climates,
Towers ( ), Pongola ( ),Bundaberg ( ), Gargett ( ) and Farleigh (
).
50 100 150 200 2500.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
Fra
ction o
f D
ata
50 100 150 200 250
Biomass yield response to reduced conductance (%)
Yellow chromosol Red dermosol
FB
ig. 8. Cumulative frequency distribution of biomass yield
response to reduced conduundaberg ( ), Gargett ( ) and Farleigh (
).
ctance for two soils and five climates, Towers ( ), Pongola (
),
-
102 N.G. Inman-Bamber et al. / Field Crops Research 134 (2012)
95104
Yellow chromosol Red dermosol
80 100 120 140 160 60 80 100 120 140 1600.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
Fra
ction o
f D
ata
60
Biomass yield response to increased intrinsic water use
efficiency ( k) (%)
F nspiraT Farle
ccrG(wif
3(
atgodcdtrmf
Fs
ig. 9. Cumulative frequency distribution of biomass yield
response to increased traowers ( ), Pongola ( ), Bundaberg ( ),
Gargett ( ) and
onservation of water within the life of the crop and for the
nextrop was mostly insufficient to offset the loss of yield
througheduced CO2 assimilation. For the wetter climates of
Bundaberg,argett and Farleigh reduced conductance was less
detrimental
1.9 0.2%) in the deep than the shallow soil (14.7 0.2%). Thisas
because of the greater amount of water that could be conserved
n the deeper soil for later use, either within the crops
duration oror the next ratoon crop.
.8. Increased TE through increased intrinsic water use
efficiencyk)
Increased k was beneficial in more than 95% of all cases inll
climates and both soils (Fig. 9). The yield increase was morehan
17% for 1 out of 10 simulated crops at Bundaberg and Pon-ola (Fig.
9). This is greater than the 10% increase in k becausef flow on
effects of reduced water stress and more rapid canopyevelopment.
Mean biomass yield increased most in the Pongolalimate (11.1 0.15%
for the shallow soil and 13.7 0.16% for theeep soil) and least in
the wetter Farleigh climate (9.2 0.15% for
he shallow soil and 8.4 0.15% for the deep soil) (Fig. 9).
Theesponse to increased k was intermediate in the dry Towers
cli-ate. Increased k is of no benefit when water is either
non-limiting
or photosynthesis and transpiration or it is totally limiting.
These
ig. 10. Cumulative frequency distribution of biomass yield
response to reduced VPD (henoils and five climates, Towers ( ),
Pongola ( ), Bundaberg ( ), Garg
tion efficiency combined with reduced conductance for two soils
and five climates,igh ( ).
extremes were more prevalent at Towers than at Pongola. In a
verylimited number of cases increased k was detrimental for
biomassyield because preceding crops used more water with
up-regulatedk, leaving subsequent crops with less water to start
with. Meanwhole crop TE (biomass yield/total transpiration) was
6.18 g kg1
for k = 8.7 g kPa kg1 and 6.82 g kg1 (10% greater) for
up-regulatedk = 9.6 g kPa kg1. Variation around a 10% mean response
in biomassyield to a 10% up-regulation in TE was due to periods
when waterwas not limiting or when leaf area development was
enhanced orsenescence was reduced by more favourable water
relations withup-regulated k.
3.9. Increased TE through decreased VPD combined with
reducedconductance
Mean whole crop TE was 6.13 g kg1 with the default esti-mate of
VPD and 6.87 g kg1 (12% greater) when VPD was reducedeffectively by
reduced conductance. Although whole crop TE wasincreased more by
reduced effective VPD than by up-regulated k,the yield benefits
were equivocal (Fig. 10). The benefit was greater
for the deep than for the shallow soil, due to the greater
storagecapacity of the deeper soil where reduced conductance could
be ofmore benefit than in the shallow soil in conserving water for
use insubsequent dry periods as explained above.
ce increased transpiration efficiency) combined with reduced
conductance for twoett ( ) and Farleigh ( ).
-
d Crop
(btamc1t
cg9t
4
tataaapais
iyyotrvwyGtr
a3bsbwiirraasfifgbpwtwl
N.G. Inman-Bamber et al. / Fiel
Yield benefits were substantially greater in the drier
climatesPongola, Towers) than the wetter climates (Farleigh and
Gargett)ecause of the combined effects of benefits from reduced
conduc-ance at Pongola and Towers (Fig. 8) and increased TE at
Pongoland to a lesser extent at Towers (Fig. 9). Crops in the
Pongola cli-ate and on deep soils would nearly always benefit from
reduced
onductance associated with reduced effective VPD while 9 out of0
crops at Farleigh in shallow soils would be worse off with
thisrait.
The mean response in biomass yield to increased TE and
reducedonductance in the deep soil for Towers, Pongola, Bundaberg,
Gar-ett and Farleigh climates was 13.6 0.7, 16.7 0.4, 13.7 0.4,.3
0.4 and 4.8 0.3%, respectively and the mean response forhe shallow
soil in these climates was 8.0 0.6, 3.8 0.5, 0.8 0.5,5.2 0.4 and
8.9 0.4%, respectively.
. Discussion
In a review of a range of plant traits associated with
drought-olerance, Ludlow and Muchow (1990) recommend eight traits
thatre desirable for intermittent stress conditions in modern
agricul-ure. The top three of these traits were plant phenology,
osmoticdjustment, and rooting depth. The reproductive phase is
gener-lly avoided in sugarcane production through crop managementnd
altering plant phenology to achieve better conditions for thishase
was not of interest in this study. Instead the traits identifieds
potentially useful for improving drought tolerance in
sugarcanencluded rooting depth, rate of leaf senescence, trigger
for stalkenescence, hydraulic conductance and transpiration
efficiency.
At two of the climatic regions of this study (Towers and
Pongola)rrigation is mandatory for a viable sugarcane industry.
Biomassields simulated for these regions illustrate this point with
meansields of less than 15 t ha1 at Towers and Pongola and 30 t
ha1
r more at the other sites (Fig. 4). Irrigation was introduced
rela-ively recently (in the past 20 years) in the Bundaberg and
Mackayegions but now growers believe their enterprises would not
beiable without irrigation. In this study the only irrigation
allowedas 40 mm at the time of planting the crop at the start of a
seven
ear cropping cycle. Simulated biomass yields at Bundaberg
andargett were less than 20 t ha1 (60 t cane ha1) for one out
of
hree crops (Fig. 4) thus supporting the need for irrigation in
theseegions.
A root system capable of exploring an additional 10% of
watervailable in the root zone as well as extending the rootzone
by00800 mm (one additional soil layer) would be of
considerableenefit (021% mean and 200% maximum yield increase in
ourtudy) particularly for poor soils and dry climates. Of course
thisenefit can only be realised if there is sufficient rain
periodically toet the soil to the additional depth. Rainfall during
summer (Fig. 3)
s normally adequate for this purpose and the APSIM model usedn
our study accounted for seasons when soil at depth was not
fullyeplenished with water. Smith et al. (2005) provided evidence
foroot water extraction at a depth of 2.8 m in Australia. Battie
Laclaund Laclau (2009) found surprisingly little difference in the
rate ofdvance and the final rooting depth between irrigated and
rainfedugarcane in Brazil. The root front advanced about 5 mm d1
for therst four months and then about 18 mm d1 thereafter. Roots
were
ound at a depth of 4.7 m for the rainfed crop and at 4.2 m for
the irri-ated crop suggesting that rooting depth is not greatly
influencedy water regime. However the rainfall during the 10 month
studyeriod was not particularly low (940 mm) and only 1140 mm
water
as required for the irrigated crop. Baran et al. (1974) also found
hat roots were more deeply distributed when irrigation
frequencyas reduced in sugarcane even though the rooting depth was
simi-
ar between treatments. APSIM-Sugarcane makes no allowance
for
s Research 134 (2012) 95104 103
an increase in root proliferation under moderately dry
conditionssuch as in the Brazilian experiment where withholding
irrigationresulted in 49% more intersects between roots in the
vertical gridused in the study of Battie Laclau and Laclau (2009).
If root depthpenetration is constrained in dry soil then the
benefits of a morevigorous root system as well as the benefits of a
deep soil would besubstantially diminished. However sugarcane is a
perennial cropand only part of the root system dies back after
harvesting anddeep roots from the harvested crop can be critical
for the survivalof subsequent ratoon crops (Smith et al.,
2005).
The strategy underlying an increase in leaf senescence rate
inour study was to reduce water use early in response to drought
inorder to conserve water for periods of water stress later on. The
sim-ulations suggest that regardless of soil type, climate or
ratoon datethere were very few years when this strategy benefitted
biomassaccumulation and it is questionable whether reduced leaf
area isof any benefit to sugarcane production at all, given the
extremelydry climates used in this study. Rather, scarce resources
should betargeted at strategies to build and maintain leaf
area.
In the majority of years biomass yield did not respond to a
traitfor stalk senescence triggered when LAI declined below 0.5.
Wehave little evidence as to what kind of water stress triggers
stalkdeath. Stress imposed at different stages of crop growth
reducedbiomass accumulation by as much as 70% and this was not
suffi-cient to reduce the stalk population in the cultivars Q96 and
Q117(Inman-Bamber, 2004).
Breeding for reduced stomatal or root conductance is unlikelyto
provide benefits in terms of increased crop yield in shallow
soils.However, in deep soils, where there is a greater potential
for storingwater through reduced conductance this trait will be
beneficial butcould be applied only to a limited number of climates
and soils inthe sugarcane industry.
The observations and interpretations on root depth, leaf
andstalk senescence and reduced conductance (without increased
TE)align with the analysis by Blum (2009) which suggests that
traitswhich conserve water and increase water use efficiency are
gen-erally unsuccessful for improving economic yield in
productionenvironments which may be water-limited but are generally
morefavourable for plant survival compared to environments were
sur-vival rather than production is the key issue.
Our simulations indicated that increased TE at the leaf
levelwould nearly always help to improve sugarcane biomass yieldsin
water-limited environments if the increased TE arose from
up-regulation of intrinsic water use efficiency (k). However if
increasedTE was increased through reduced conductance, which
effectivelyreduces VPD during transpiration, the yield benefits are
not at allcertain. A large water storage (deep soil) would be
required to takeadvantage of water savings from reduced
conductance. A moderateclimate such as the Pongola climate would
also be required to makemost of the increased TE which is most
beneficial when root watersupply is limiting but not close to zero
and not excessive.
TE or rather its surrogate, delta, which is derived from the
ratioof 13C to 12C captured during assimilation of CO2 by C3
plants,is not always associated with yield (Blum, 2009). 13C
discrimina-tion for measuring TE may not be suitable for sugarcane
with C4photosynthesis because of the small contribution of rubsico
to theassimilation process (Ranjith et al., 1995). However the
potentialbenefits of improved TE needs to be explored more fully
usinga model capable of integrating diurnal variation in VPD,
stoma-tal conductance, transpiration and photosynthesis as in the
workof Sinclair et al. (2005). The APSIM model chosen for our
studywas useful for considering a number of traits that may be
use-
ful in water limited environments however the daily time step
ofAPSIM allowed only an indirect link between reduced
conductanceand increased TE. TE may also be increased through an
increasein intrinsic water efficiency (k) and the APSIM modelling
was
-
1 d Crop
attawoebVbistp
A
aCRI
R
B
B
B
B
C
C
C
G
H
H
H
I
I
I
I
I
I
04 N.G. Inman-Bamber et al. / Fiel
dequate for this possibility. The 12% increase in whole crop TE
dueo reduced conductance was less than the increase in TE
derivedheoretically by Sinclair et al. (2005) who limited
transpirationt midday assuming partial stomatal closure when demand
forater was high. Transpiration and photosynthesis measurements
n soybean by Rawson et al. (1978) indicated that TE was high-st
in the morning and evening and lowest between 12 and 14 hecause of
partial stomatal closure, increased leaf temperature andPD. Water
stress further reduced TE during the middle of the dayecause of
further increases in leaf temperature and VPD. Diurnal
nteractions between transpiration, photosynthesis, VPD and
watertress have not been studied in sugarcane and our study
highlightshe need to do this in order to better understand and
model theroposed benefits of increased TE.
cknowledgements
This research was funded by the Australian Federal Governmentnd
Sugar Industry through the Sugar Research and
Developmentooperation. We thank the Queensland Department of
Naturalesources and Water and the South African Sugarcane
Research
nstitute for providing the climate data used in this study.
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Sugarcane for water-limited environments: Theoretical assessment of
suitable traits1 Introduction2 Materials and methods2.1 Field
experiments2.2 Reduced conductance2.3 Simulation of deep root
distribution2.4 Simulation of rapid leaf senescence2.5 Early stalk
senescence2.6 Transpiration efficiency (TE)2.7 Climate2.8 Other
model settings3 Results3.1 Simulation of field experiments3.2
Climate of target sites3.3 Biomass yields3.4 Increased rooting
depth3.5 Increased leaf senescence rate3.6 Early stalk
senescence3.7 Reduced conductance3.8 Increased TE through increased
intrinsic water use efficiency (k)3.9 Increased TE through
decreased VPD combined with reduced conductance4
DiscussionAcknowledgementsReferences