50Brassica Crop Species: Improving Water Use Efficiency:Challenges and OpportunitiesConstantine Don Palmer, Wilf Keller, Jas Singh, and Raju Datla

The Brassica species occupies a large portion of the world�s economically importantcultivated crops. These include vegetables, oilseeds, condiments, and forages. Thesecrops are grown globally under a wide range of climatic conditions. With currentconcerns for food and energy security, expanded use of crop products, and environ-mental stewardship, there is a pressing need to improve yield through greaterefficiency of resource utilization. Water availability is the most limiting factor tocrop productivity andwith the predicted scarcity, due to climate change and increasednonagricultural demand, improving water use efficiency (WUE) in crop productionis an imperative. Consequently, increased carbon assimilation per unit of water usedby Brassica crops must not only be realized but this carbon must also be efficientlypartitioned into the harvested product. Thus, these plants need to be equipped withthe genetic capacity to extract more water from the soil under water-limited condi-tions,fixmore carbon, and transpire less water. There is natural genetic variability forWUE and this can be used for screening germplasm to identify better genotypes.Evaporative demand is the driving force for water loss andWUE can be improved byincreasing transpiration efficiency (TE), alteration in crop phenology, increasedcarbon fixation, and increased harvest index (HI) by greater partitioning of assim-ilates into harvestable product. Modification of root architecture, leaf morphology,and stomata conductance are important targets for developing cultivars withimproved WUE. Drought tolerance is closely associated with WUE and factorscontributing to maintenance of metabolic function under water-limited conditionscontribute to improved WUE. Studies on Arabidopsis have contributed to significantadvances in our understanding of WUE and drought tolerance. The use of geneticengineering and genomic tools has allowed for the incorporation of identified geneticfactors for improving WUE and drought tolerance traits and will be vital to thedevelopment of new Brassica cultivars. The carbon fixation machinery, a vitalcomponent in yield, will require adjustments to deal with anticipated water deficitsin order to take advantage of increases in atmospheric carbon dioxide as a result ofclimate change. Manipulation of assimilate partitioning and selection of genotypes

Improving Crop Resistance to Abiotic Stress, First Edition.Edited by Narendra Tuteja, Sarvajeet Singh Gill, Antonio F. Tiburcio, and Renu Tuteja� 2012 Wiley-VCH Verlag GmbH & Co. KGaA. Published 2012 by Wiley-VCH Verlag GmbH & Co. KGaA.

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with capacity to store water-soluble carbohydrates in stems that can be remobilized toharvestable structures are targets for improving WUE. To maintain high yields ofBrassica crops with economic use of water will require substantial increases in ourunderstanding of the biological processes associated with growth under water-limited conditions. The availability of potential gene targets from the present andfuture discoveries including rapid advances coming from application of genomictechnologiesmay provide a valuable resource base for development of superiorWUEBrassica crops in the coming years.

50.1Introduction

The genus Brassica contains a number of species of agricultural importance that arewidely grown as oilseeds, vegetables, and forages and that are well adapted to a rangeof climatic conditions. Brassica oilseed crops are an important component of theglobal vegetable oil market and are valuable sources of condiments and vegetableprotein. At present, canola qualityBrassica oilseed crops are grown on over 25millionha worldwide [1] with Brassica napus having the largest area. B. oleracea L. contains anumber of varieties that are grown for consumption as vegetables [2]. In addition touse as a food, there are a number of secondary metabolites that are of medicinal andnutraceutical importance [3]. A number of Brassica species, including B. napus spp.biennis L. (forage rape) and B. rapa L. (turnip) are of importance as forages [4].Increased yield is generally themain focus of improvement of agricultural crops andwith the projected increase in demand for food and the use of food crops for energyand industrial feedstock, increased yield is now an imperative.Brassica crops can be avital component of any strategy aimed at ensuring food and energy security.Consequently, there must be efficient use of input resources and maximization ofyield. In other words, over all plant performance must be improved. While yield is afunction of the genetic component of the plant, there are a number of environmentalconstraints including biotic and abiotic stress, which affect thefinal outcome. For anybiological system,water is a vital component and in the case of agriculture, about 70%of the available freshwater is used in crop production [5–7]. It is now recognized thatincreased urbanization and the impact of climate change on water use and precip-itation, will likely reduce the amount of water available for agriculture [8, 9]. Asignificant portion of agricultural water use is in crop production [10], and to increaseyield, plants must be more efficient in water use. In this chapter, the focus will be onthe status of water use efficiency in Brassica crop species and ways to enhance thisefficiency without compromising yield. We will draw liberally from published workon water use efficiency (WUE) in other crop species such as cereals. Water useefficiency is interrelated to plant performance under water-limited conditions andthis necessitates discussion of this topic in the context of drought tolerance and plantgrowth under water-limited conditions.

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50.2Yield

There is now the realization that yield of all crops must be increased to meetgrowing demand and this must be achieved in an environmentally sustainablemanner. In the case of Brassica crops, like other crop plants, yield is the portion ofbiomass that is partitioned into seed, leaf, stem, root, or floral buds. To realize thegenetic potential of a crop, as reflected in yield, environmental factors thatnegatively affect yield must be controlled. The major limitation to crop yield iswater availability or drought [11]. To improve yield, with anticipated decline inwateravailability for agriculture, plantsmust be equippedwith built-in genetic capacity touse water more efficiently. In cases where water is the limiting factor to plantgrowth, crop yield (CY) is a function of WU�WUE�HI (WU¼ total water use,WUE¼water use efficiency, and HI¼ harvest index (harvestable biomass/totalbiomass)) [12]. Assuming these three terms are independent, an increase in any oneis reflected in CY increase [13, 14].

50.3Water Use Efficiency

The termwater use efficiency is used to detail the amount ofwater per unit of biomassproduced. This ratio of biomass to evapotranspiration is generally expressed asWUE(biomass)¼ (TE/1) þ (Es/T). TE is transpiration efficiency, which is the dry matter/transpiration,Es is thewater lost to evaporation from the soil, andT is transpiration bythe crop [15]. This means that to increaseWUE, TEmust be increased or Es reduced.In general, there are several management strategies to reduce soil water evaporationsuch as increasing ground cover, improving plant vigor, and optimizing nutrientstatus of the plant [15]. Improving transpiration efficiency, TE, can also be achievedthrough management, as less water will be transpired by crops that accumulatemaximum biomass under cooler conditions. This is the case as the main driverfor transpirational water loss is the saturated water vapor deficit. Breeding for plantswith increased transpiration efficiency is desirable as this trait is under geneticcontrol [16–18]. Plants with pubescent leaves are likely to transpire less water. Alsoplants with thick cuticles generally have reduced leaf water loss. Genetic variation incuticular water loss has been reported [18, 19], and selection for genotypes withreduced water loss by this route may improve overall TE. Selection for improved TEmay be hampered by the accuracy with which this can be measured efficientlyand conveniently. The close relationship between C13O2 and C12O2 discriminationduring photosynthetic carbon assimilation and TE was established and isconsidered a reliable breeding tool for selecting genotypes with high TE, at leastin C3 plants [20, 21]. This technique has been used effectively in selecting wheatgenotypes with high TE [22]. The advantages and limitations of this technique havebeen discussed [22, 23].

50.3 Water Use Efficiency j1303

50.4Drought Tolerance and WUE

Water availability is perhaps the most important factor limiting plant growth andcrop yield. Given the prospect of future drought and water scarcity for cropproduction, as a consequence of climate change, the crop plant growth will likelyoccur under suboptimal water condition. Compared to other abiotic stressesaffecting plant growth, drought is the one with the most far-reaching negativeconsequences [23]. Thus, a fundamental knowledge of how plants sense anddevelop mechanisms to adapt to water-limiting conditions is an imperative.Although WUE in crop production has received considerable attention, there isa widely held view that it is effective use of water, EUW, and not WUE, which isimportant in over all water conservation [24–26]. This concept is regarded as theimportant aspect of achieving drought tolerance. This involves assessment ofcritical periods of crop growth that are sensitive to water stress. Althoughmodification of agronomic practices such as a modification of crop phenologyand deficit irrigation through partial root zone drying, PRD, among others, geneticmanipulation using transgenics may be required to address the maintenance ofcrop yield under water-limited conditions. Partial root zone drying may be usefulin improving WUE as it probably maintains plant water status through hormonalsignals that regulate stomatal function [27]. Two recent publications havehighlighted the importance of farming system management in association withplant genotype to improved productivity in water-limited environments [26, 28].Under conditions of water limitation, the main concern for improved crop yieldwill be maintenance of growth and biomass accumulation and effective strategiesmust be directed to this end. Modification in root and shoot traits may be essentialfor maintaining plant water status under drought conditions. For example, deeproot system with increased lateral branching will be able to tap soil water at greaterdepths. This deep root system may also contribute to increased nutrient uptakeresulting in more growth and biomass accumulation and consequently increasedWUE. Breeding for drought tolerance through modification of root traits isdesirable, but there are technical difficulties in accurately defining the phenotypes,as roots of crop plants are largely inaccessible for convenient observations. The useof closely or tightly linked molecular markers may assist in introgression ofdesirable WUE traits from wide germplasm [29]. Stomatal and epidermal con-ductances are important shoot traits relevant to drought tolerance and WUE asabout 90% of water uptake is lost by transpiration [30]. Therefore, minimizing thisloss is vital. However, gas exchange must be optimized to allow carbon assimi-lation without excessive water loss. Breeding for leaf pubescence density will likelyincrease leaf reflectance causing lower leaf temperature and lower water loss athigh irradiance [30]. This may also increase leaf boundary layer resistance andenhance photosynthesis [31] contributing to increased WUE. Under conditions ofdrought, plant cells may maintain turgor and growth by active accumulation ofsolutes, a process called osmotic adjustment (OA). These solutes can be carbohy-drates, amino acids, or sugar acids [32]. In soybean, a positive correlation was

1304j 50 Brassica Crop Species: Improving Water Use Efficiency: Challenges and Opportunities

found between the rate of decline in relative water content and relativeOA [33]. Theeffect of OA on yield is debatable [34–36], but it may allow maintenance ofmetabolic machinery for growth resumption under favorable water conditions.However, a positive correlation between seed yield and OA was observed inBrassica species [37, 38]. There was also a close association between stomatalconductance, canopy temperature, and OA in the same species [39, 40]. Thesefindings suggest that species differences or the type of osmolyte may affectphysiological and biochemical responses to OA. As a strategy for maintainingturgor under conditions of water deficit by using OA, there may be a reduction inbiomass as significant amount of energy can be expended in the production ofosmolytes.

50.5Stomatal Water Loss

Stomata are vital to carbon assimilation, and because CO2 uptake is tied to watervapor loss an important aspect of WUE is to strike a balance between minimizingwater loss while maximizing CO2 assimilation. Therefore, there must a deeperunderstanding of the regulatory factors that control transpiration efficiency (theratio of CO2 assimilated to water transpired). ABA is well known to function in theregulation of stomatal aperture [41], and while this hormone is a factor in transpi-ration efficiency, there are several others such as cell wall composition, G protein,GPA1, RD20, a stress-inducible Caleosin, and the ERECTA gene product that arereported to be involved in the regulation of transpiration efficiency [15, 42–45]. Thediscovery and characterization of the ERECTA gene has potential benefits fortranspiration efficiency improvement in crop plants as it affects both photosynthesisand transpiration [15]. Similarly, expression of the geneHARDY in rice significantlyimproved WUE through root and shoot modification [46]. There is natural geneticvariation in transpiration efficiency in several plant species including Brassicas andthis could be exploited to improve WUE in Brassica crops. There are other factorssuch as hydrogen sulfide and the enzymemyrosinase, which appear to be involved assignaling components in hormone-induced stomatal closure and may be importantto TE [47, 48]. Stomatal density regulation is also a factor in TE as there may be anoptimum density for improvedWUE [49]. There is also evidence indicating an ABA-independent stomatal aperture regulatory pathway [50], which underscores thecomplexity of the stomatal regulatory process.Understanding the factors controllingstomatal density will also be vital to identifying genotypes with improved transpi-ration efficiency [51, 52].

In addition to stomatal water loss, epidermal water loss through the cuticle can besignificant to overall TE, and in cotton WUE was negatively correlated with leafepidermal conductance [18]. Also, cuticular wax composition may influence thetranspiration barrier properties of the cuticle [53]. Consequently, screening Brassicagenotypes for variation in cuticular wax composition and epidermalwater loss shouldbe considered while breeding for improved WUE.

50.5 Stomatal Water Loss j1305

50.6Water Acquisition

Root characteristics will largely determine the efficiency of water uptake under water-limited conditions and root size influences both yield andWUE [54]. Plantswith deeproot system will be able to tap water from deeper soil layers, compared to those withshallow root system. Root length and degree of branching are characteristics thatshould be considered and examined in breeding for water acquisition traits. Asoutlined in Section 50.4, it is technically difficult to phenotype roots for selectionpurposes, and the use ofmolecular tagging andmeasurement of canopy temperatureare suggested as alternatives for selecting root-related traits [55, 56]. The developmentof a phenotyping platform for root systems should advance selection for roottraits [57]. Water uptake can also be enhanced by breeding for preferential root OAto sustain growth and water uptake under deficit conditions. An important aspect ofroot function in WUE is root–shoot signaling under water deficit conditions. ABAappears to play a role in this process andWUE can be improved by such techniques asroot-deficit irrigation (RDI) or partial root drying (PRD) [58]. However, other factorssuch as hydraulic and pH signaling may be involved [59]. These methods aredesigned to increase stomatal sensitivity to ABA to effect partial closure and reducedwater loss while CO2 assimilation is maintained. The process is complicated by theinfluence of other hormones such as ethylene that counteracts theABA response [58].

50.7Carbon Assimilation

Improvement in WUE will ultimately depend on net carbon gain through photo-synthetic carbonfixation. Thismeans that factors such as internal CO2 concentration,activity of the main CO2 fixation enzyme Rubisco, and photorespiratory carbon lossmust be optimized [60]. It is generally accepted that C4 plantsmaintain higherWUE,compared to C3 plants, such as Brassica species, as a consequence of the ability toconcentrate CO2 at the site of fixation. As a result, there is interest in expressing C4carbon fixation pathways in C3 crop plants [61]. Modification of the catalytic activityand specificity of Rubisco have also been suggested [61–64]. While this is a potentialtarget for improving net carbon gain in Brassica crop species, due considerationshould be given to carbon partitioning into harvestable products as this has asignificant impact on photosynthetic efficiency. There is evidence indicating theimportance of sucrose transporters, SUTs, in carbon partitioning [65]. These trans-porters could be manipulated in Brassica crop species for enhanced carbon gain.Stem storage and remobilization of water-soluble carbohydrates (WSCs) to repro-ductive structures during grain filling increased harvest index [66]. Brassica geno-types with this characteristic could make a significant contribution to WUE andincreased yield. Improving carbon fixation efficiency in Brassica oilseed species isvital as these species generally require greater biomass to produce the same amountof storage products as do carbohydrate storage crops. For increased biomass,

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consideration must be given to factors contributing to high-yield potential underwater-limited conditions. Increased understanding of how plants reorder metabo-lism tomaintain growth under water deficit conditions will be vital to development ofdrought-tolerant and high-yield cultivars capable of improved efficiency in the use ofwater. For example, invertases have been shown to play a vital role in carbohydratemetabolism and yield under conditions of stress [42, 67]. This suggests thatknowledge of how plants adapt various metabolic processes to maintain growthunder unfavorable conditions will be vital to the development of genotypes forefficient use of input resources.

50.8Importance of other Growth-Limiting Factors to WUE

There are many factors such as adequate supply of nutrients, pests and diseases, soilconditions, temperature, and abiotic stresses that may influence crop performanceand WUE. Consequently, the impact of these conditions on the performance ofBrassica crops must be taken into account in the development of cultivars withimproved WUE. Improvement in nitrogen nutrition increased WUE throughincreased photosynthesis [68, 69]. Improved nitrogen nutrition may also be involvedin ABA-induced root–shoot signaling and stomatal regulation [70]. Soil microfloramay have a positive effect on WUE by both improving nutrient acquisition andproviding signal molecules for stomatal regulation [58]. It is likely that soil mycor-rhizae may contribute to WUE and this is an area that should be explored as it mayoffer complimentary and synergistic improvements.

When the potential consequences of climate change to crop productivity andagriculture in general are taken into account, it is clear that the sustainability willdepend on the use of advance technologies to increase productivity. Breeding forincreased productivity must include consideration of efficient use of all inputresources [71]. There is little debate that the significant advances in our knowledgeof biology in general, but specifically microbial and plant biology, should be theplatform for addressing sustainability of agricultural systems. To improve WUE inBrassica crop species will require sustained advances in our knowledge of how theseplants function under water-limited conditions and their interaction with otherenvironmental stresses. The status of our knowledge of plant biology has improvedour understanding of gene expression and genetic networks responsive to droughttolerance and water deficit [72–75]. There is a significant body of knowledge availableon the physiological and biochemical aspects of plant response to drought, and therole of ABA in these processes has been advanced [76–78].However, themechanismsof action of these gene products remain to be established [79]. The application ofgenetic engineering tools has resulted in the development of transgenic crop plantsexpressing genes conferring tolerance to drought and increased WUE [76]. Some ofthese genes exhibit undesirable side effects, which may limit broader usage.However, regulated expression may circumvent these negative outcomes. WildBrassica species may contain genes conferring improved WUE and drought toler-

50.8 Importance of other Growth-Limiting Factors to WUE j1307

ance. These can be introgressed into crop species by conventional breeding.However, genetic engineering offers a better option for the development of cropswith such traits as access for desirable genes is potentially unlimited. This also offersan excellent platform for increasing our knowledge of gene function in a variety ofgenetic backgrounds. The overall aimof drought tolerance and efficiency ofwater useshould be to maximize yield under water limited conditions. To this end, it isimportant to know how plants maintain homeostasis and metabolic activity undersuch conditions. To gain this knowledge, gene function cannot be viewed in isolationbut must be viewed in the context of interacting genetic and metabolic networks in awell-integrated comprehensive systems approach [80, 81].

50.9Water Use Efficiency in Brassica Species

Brassica crop species are adapted to a range of environments and like in other cropspecies yield is heavily influenced by water availability. Many areas of Brassica cropcultivation are drought prone and tomeet increasing demand forBrassica products, itwill be necessary to expand cultivation into less favorable areas. Therefore, effectivewater use for yield maintenance is vitally important. In spite of the importance ofthese species, there are not many reported studies ofWUE. In a comparison ofWUEand stomatal conductance in Moricandia and Brassica species, McVetty et al. [82]attributed lower WUE in the latter species to higher stomatal conductance. Studieswith B. oleracea revealed a number of quantitative trait loci (QTL) for variation in leafconductance and photosynthetic assimilation rate [83]. These findingsmay be usefulin breeding for improvedWUE [82]. Variation in tolerance to drought, attributable todifferential osmotic adjustment, among Brassica species has been reported [84].There are genotypic differences in response to drought stress, which may be relatedto OA [84–86]. These studies indicate availability of a rich source of genetic variationfor drought tolerance and WUE in Brassica species that can be exploited in breedingfor both drought tolerance and WUE.

50.10Conventional Breeding for WUE in Brassica Crop Species

This invariably involves breeding for drought tolerance and growth under water-limiting conditions. With the availability of germplasm, conventional breeding canmake a significant contribution to improvement in WUE and drought tolerance.Physiological and morphological traits related to WUE can be identified and theirinheritance determined. Carbon isotope discrimination (CID), which measures theratio of 13C/12C in the plant tissues compared to the air, is an indirect measure ofWUE and can be used to detect genetic variation for TE in plants. This could providean effective tool for screening the germplasm. Screening can also be done bymeasuring leaf ash content, LASH, and K content [87, 88]. Ash content is generally

1308j 50 Brassica Crop Species: Improving Water Use Efficiency: Challenges and Opportunities

negatively associated with WUE if a constant concentration of minerals in thetranspiration stream is maintained. Since this is likely to be affected by environ-mental conditions, the accuracy of genotypic comparison, they should be grown inthe same environment. This method has been used to identify QTL for WUE insoybean [87]. A potentially promising area of study is to screen the germplasm fordifferences in root architecture in relation to WUE. This is an often underexploredarea of research that could improve both yield and WUE [88–90]. Improving rootarchitecture and size will be essential to ensuring adequate transpiration duringwater deficit, which will have a positive influence on productivity [91–93]. QTL forroot architecture in rice have been identified and their effect on yield determined [88].Application of similar approaches in Brassicas may contribute to new potential genetargets for WUE traits in Brassica species.

50.11Unique Challenges to Breeding for WUE in Brassica Crop Species

As a consequence of the diversity of Brassica crop species, where the harvestableproducts include oilseeds, leaves, stems, florets, flower buds, axillary buds, andtubers, breeding for WUE is naturally challenging. This is in contrast to major graincrops such as rice, wheat, and maize where the grain is generally the product atharvest. Therefore,WUE strategies such as increasing sensitivity of stomates to ABAmay be unsuitable for leafy Brassica crops as leaf growth may be compromised [58].Larger leaves will usually transpire more water, which means that for leafy Brassicacrops increased WUE should be by means that do not compromise leaf growth.Increasing photosynthetic efficiency and enhancing crop development under con-ditions where evaporative demands are lower should improve WUE in these crops.

50.12Engineering Drought Tolerance and WUE in Brassica Crops

Although conventional breeding will undoubtedly contribute to improvement inWUE, there are substantial benefits to be derived from the application of a biotech-nological approach. By this approach, there is no limitation to the source of genes thatcan be evaluated. In contrast, conventional breeding is limited by the compatibility ofgermplasm with the genotype under improvement. This, in effect, widens the genepool for improvement in WUE and drought tolerance. Another benefit of thisapproach is the potential to gain valuable information on gene function afterexpression in the desired background. Given the importance of abiotic stress toplant growth, the literature on attempts to engineer stress tolerance in plants isvoluminous and there are a number of approaches for the identification of genesinvolved in abiotic stress [94, 95]. Themodel plantArabidopsis has been an invaluabletool in advancing our knowledge of plant biology in general and specifically somefundamental aspects of abiotic stress responses. This topic has been reviewed

50.12 Engineering Drought Tolerance and WUE in Brassica Crops j1309

recently [94] and will not be further discussed here. The ability to identify genes forspecific functions and to express them in heterologous systems through geneticengineering technology has revolutionized plant biology. Genes conferring toleranceto abiotic stresses have been evaluated in the model species Arabidopsis and in otherplants [96–100]. The attractiveness of this technology is that we can introduce only thegenes for specific traits into an elite variety without carrying along other potentialundesirable genes, as is the case with conventional breeding. In that case, severalrounds of backcrossing are normally required to remove the unwanted genes. Anexcellent example is the identification and introduction of the transcription factorNFYB2 intomaize [101]. This conferred drought tolerance,WUE, and increased yieldwhen tested under water-limited field conditions. There are a number of cases wheredrought tolerance, WUE, have been achieved by genetic approaches includingengineering of functional and regulatory proteins, and enzymes for production ofosmolytes and osmoprotectants [76, 79]. Many of these, though at the proof-of-concept stage, still show promise for incorporation into crop species to developsuperior WUE traits.

In many environments where Brassica crop species are grown yield will be heavilyinfluenced bywater availability. Therefore, effectivewater use to sustain yield is vitallyimportant. In spite of the importance of these species, there are not many studies onWUE.

Success in improving WUE and growth under water-limited conditions dependson an understanding of plant physiology and metabolism under conditions of waterlimitation [102]. Since the economics ofwater use is a function of carbon assimilationper unit of water used, efficiency can be obtained by addressing factors limitingcarbon assimilation along with those controlling water losses. It is well establishedthat a major consequence of reduced plant water status is impairment in thephotosynthetic machinery [79, 102]. This is the result of reduction in internal CO2

levels as stomates close in response not only to water deficit but also to disruption inoxidative homeostasis alterations in cellularmetabolism. Tomaintain photosyntheticcarbon assimilation, plants must be equipped with enzymes to alleviate oxidativestress. About 90% of the water absorbed by plants is lost in transpiration. Thismeansthat WUE can be improved by regulating stomatal water loss. The plant hormoneABA is a major factor in the regulation of stomatal aperture and reduction intranspirational water loss [41]. By uncovering the molecular components regulatingthis ABA response, it was possible to engineer drought tolerance in canola (B. napusL.) [103–105]. Negative regulation of ABA via downregulation of ERA1 has beenreported to both enhance drought tolerance and maintain productivity in Brassi-ca [104, 105]. Drought tolerance and improved WUE was also reported for canolaplants expressing a gene for lipid metabolism, PtdIns-PLC2, [106]. In another study,transgenic canola plants expressing poly (ADP-ribose) polymerase exhibited lowerreactive oxygen species (ROS) and displayed drought tolerance [107]. Transgeniccanola plants with wheat mitochondrial Mn superoxide dismutase (Mn SOD3.1)exhibited increased vigor and tolerance to abiotic stress, including drought [108].Similarly, the overexpression of LEA proteins has been shown to impart droughttolerance to Brassica seedlings [109]. However, the caveat is that it has yet to be

1310j 50 Brassica Crop Species: Improving Water Use Efficiency: Challenges and Opportunities

determined that the overexpression of abiotic stress-tolerant genes or transcriptionfactors regulating these genes does not have a negative overall yield crop productivityunder field conditions.

50.13Prospects of Improving WUE in Brassica Crops

There is general agreement that crop productivity must be increased to meetincreasing global demand [110]. It is clear that this must be achieved by a moreefficient use of input resources. This requires development of crop cultivars withenough genetic capacity for high yields with minimum input. Access to desirablegenes from natural variation in wild relatives of Brassica species will enhancedevelopment of cultivars with improved WUE. However, the application of geno-mics and tools of biotechnology will be vital to this activity [111]. Considerationmust also be given to crop interaction with biotic and abiotic stresses that impactyield in the sense of acquiring fundamental knowledge of physiological andbiochemical mechanisms underlying adaptation to stress [112, 113]. Climatechange is predicted to drastically alter crop growth environment in a largelyunpredictable manner, which is a major challenge to future crop yields [62, 63].Water availability is perhaps the most limiting factor to crop growth and yield andwith the prospect of further decline in this resource, as a consequence of climatechange, its efficient use is paramount. For Brassica crops to continue to play asignificant role in future food, feed, and energy security, there is need to improveWUE and drought tolerance in these species and to extract higher yields per unit ofwater used. While there are a number of measures proposed to conserve soilwater [24, 58, 66, 113], significant research emphasis should be on the efficiencywith which water is used for biomass generation in Brassica crops. The identifi-cation and characterization of two genes, ERECTA and HARDY [15, 46] thatenhanced WUE through multiple effects on plant morphology, should be usefulin the development ofWUE Brassica cultivars. Root–shoot signaling is emerging asan important aspect of stomatal aperture regulation [114, 115], and furtheradvances in this area should benefit plant breeding efforts aimed at the develop-ment of WUE genotypes. Regulation of stomatal water loss, improved root wateracquisition, and finding ways to effect greater efficiency of carbon assimilation andpartitioning into harvestable products under water-limited conditions, are likely tobe keys to WUE in Brassica species.

Though significant progress has beenmade in the identification of several geneticfactors implicated inWUE inplants, it is likely thatmany others involved remain to beidentified. Rapid advances in genomic technologies offer complementaryapproaches for performing global gene expression analyses to identify new factorsassociated withWUE inmodel plants such asArabidopsis and closely related Brassicacrop species. New gene discoveries coupled with insights into their function andregulation will expand potential new targets for improvement of WUE in Brassicaspecies.

50.13 Prospects of Improving WUE in Brassica Crops j1311

Acknowledgments

The Brassica research activities in genomics and abiotic stress are supported by NRCGHI program funding to RD and AAFC/CCGI program funding to JS. This ispublication # 50183 from the National Research Council of Canada.

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