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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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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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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

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

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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

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

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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

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for the Red dermosol in the driest sites, Towers (+7.3 1.0%) and

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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

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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

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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 (%)


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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 ( ).

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Biomass yield response to reduced conductance (%)


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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


80 100 120 140 160 60 80 100 120 140 1600.0

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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 ( ).

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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

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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

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