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Chapter 24

The Potential of Climate Change Adjustmentin Crops: A SynthesisRobert J. Redden, Shyam S. Yadav, Jerry L. Hatfield, Boddupalli M. Prasanna,Surinder K. Vasal, and Tanguy Lafarge

Introduction

The big challenge for agriculture in the twenty-first century onward is to feed the world popula-tion. There is unprecedented population growth,raising the question whether there is a limit towhat the world can sustain.

The world population of 6.8 billion now isexpected to exceed 9 billion over 2040–2050(http://en.wikipedia.org/wiki/World_population,2010), and to be still rising by the end of thiscentury. The paradox is that provision ofadequate and evenly distributed nutrition abovecurrent levels not only alleviates hunger but alsoreduces infant mortality and extends longevity,further adding to population growth.

Besides the need for shelter, health and hy-giene, a sustainable economy, and communityinfrastructure, the absolute basic requirement isfor adequate food to be locally available for in-dividual energy and nutrition needs. Will theagricultural systems of this world be capable ofmeeting this challenge? This is a very complexissue not only of more than caloric and proteinrequirements but also of food diversity and bal-anced nutrition for quality of life. The incomeelasticity of demand is higher for animal than

for plant foods. Generally, as peoples incomesrise, so does their consumption of meat, milk,eggs, and fish. An increased demand for animalfeed provides an indirect pathway from fodderthrough conversion of animal products to food,with lower conversion efficiency to food than forplants eaten directly. Thus, linked to the rise inpopulation, the demand for feed will also rise.In particular, this is occurring in the developingeconomies of South and East Asia with over halfof the world’s population.

A doubling of food/feed production willbe required by 2050 to provide for the pro-jected population at a satisfactory level (http://www.wfp.org/content/world-must-double-food-production-2050-fao-chief, Chapter 1, thisbook). This goal will continue to rise rapidly, aspopulation growth continues past 2100.

Climate change adds much more uncertaintyand complexity to this challenge for agricul-ture. Already extremes of high temperature anddrought are having impacts on production of an-nual crops in mainly rainfed agriculture. Themost vulnerable growth stage for grain crops is atanthesis and during grain filling. In some regionsdrought stress can be relieved by irrigation fromrivers and ground water; however, temperatures

Crop Adaptation to Climate Change, First Edition. Edited by Shyam S. Yadav, Robert J. Redden, Jerry L. Hatfield,Hermann Lotze-Campen and Anthony E. Hall.c© 2011 John Wiley & Sons, Ltd. Published 2011 by John Wiley & Sons, Ltd.

482



THE POTENTIAL OF CLIMATE CHANGE ADJUSTMENT IN CROPS 483

above 35◦C will threaten seed-set in most crops,and failure often occurs by 40◦C.

Impact of global warming on crops

The impact of global warming has already beenstated, and can be observed by the frequency ofnatural calamities in different countries acrossthe continents. These calamities are the results ofdrought, temperature fluctuations, high rains andfloods, high CO2 emission, melting of glaciers,and wild fires around the globe. Examples are nu-merous geographically; some recently occurredones are listed below.

Drought

Drought is one of the most important threatsamong the natural hazards/calamities to peo-ple’s livelihoods, socioeconomic development,food production system, and nutritional security.When it occurs, it generally affects a broad agroe-cological region for seasons or years at a time.As per the global database of disasters main-tained by the Centre for Research on Epidemi-ology of Disasters, drought accounted for only4.2% of the total natural disaster events (428 outof 10,186 disasters) during the past four decades(1970–2009). Africa has the maximum numberof droughts and also the maximum deaths due todroughts, but Asia suffered the maximum eco-nomic loss and also the maximum number ofpersons affected due to droughts (SAARC KabulWorkshop 2010). The impact of drought at globallevel is shown in Table 24.1.

The figures in Table 24.1 indicate that eco-nomic losses due to drought at a global level arevery high.

High temperature and wild fires

The global projections suggest that temperatureis rising and will continue to rise in future due tohigh CO2 emission, deforestation, industrializa-tion, and environmental pollution. Due to hightemperature, ice, snow, and glaciers are melting;sea level is increasing; changes in the rainfallpatterns are occurring; and wild fires in differ-ent countries like Australia, America, and Russiaare frequently occurring and damaging croppingsystems and ecological environments. A recentexample of these impacts was evident in Au-gust 2010 when wild fires in Russia broke allthe records of the past 130 years, and tempera-ture in Moscow city of Russia exceeded 38◦C.Wheat exports had been stopped by Russia andemergency was declared in the country. Thesefires have spread to neighboring countries likeGeorgia and Ukraine. The ban on wheat exportsby Russia has created a major social crisis inEgypt (www.bbc.com). Thus, such extreme cli-mate events are not only damaging crops andagricultural production, but also affecting mil-lions of people globally.

Rains and floodsIn the recent years, there have been drasticchanges in the rainfall and storm patterns thatare visible in the form of floods and tornados on

Table 24.1. Continental Contrasts—Impact of Drought—1970–2009.

No. of events Total killedTotal affected

(millions)Damage

(millions US$)

Africa 184 553,000 266.8 4,817America 97 <100 47.2 15,433Asia 100 >5,000 1,293.0 27,620Europe 34 <10 10.4 18,561Oceania 13 <100 8.0 10,103Total 428 558,000 1,625.4 76,533

Source: EM-DAT, Centre for Research on Epidemiology of Disasters, Leuven.



484 CROP ADAPTATION TO CLIMATE CHANGE

a global level. For example, devastating floods inAugust 2010 in Pakistan, China, and India killedmore than 3000 people, and millions of hectaresof cultivated land and crops have been lost. Thesefloods have broken the records of more than 80years in Pakistan (www.bbc.com). Interestingly,the rains and floods in northern India in the monthof August and September 2010 also broke therecords of 100 years (www.Indiatv.com). Recordflooding in the central part of Iowa during August2010 (with a combination of high temperaturesand excessive rainfall) are outside of the normalpatterns for both temperature and precipitationat this time of year and impacted maize and soy-bean yields.

It is possible that the ongoing research mayshow a link between climate change and the in-cidence of volcanic eruptions and earthquakes.

Crops that have been bred for prevailing trop-ical and mid-latitude environments will be ex-posed by 2050 to average temperature increasesof 1◦C on the optimistic scenario and 3◦C onthe worst case scenario which now appears morelikely (IPCC 2001). These will be accompaniedby an even greater frequency of extreme devi-ations, including extreme temperature stressesand variations in water availability through morevariable precipitation. Crops need to be bred foranticipated rises in temperatures and to be moretolerant of extreme stresses. Since these are out-side of current typical crop environments, man-aged stress environments are now important op-tions for international breeding programs. Inter-national Maize and Wheat Improvement Cen-ter (CIMMYT) Mexico scientists screen elitebreeding lines for heat tolerance by delayedsowing into summer with irrigation. Droughtstress screening is also managed by withhold-ing irrigation at critical growth stages (see Chap-ter 3.8, this book). A similar approach is usedby the CIMMYT maize breeding program andby private maize breeders such as Monsantoin southern Africa (Australasian Plant Breed-ing 14th Conference 2009, Cairns, Queensland).This genetic buffering to future temperature anddrought stresses can be built into crop breeding

programs; however, relatively little research hasbeen done on establishing managed screeningenvironments for increases in greenhouse pollu-tants, particularly CO2, while regionally ozonelevels near industrial areas in the northern hemi-sphere and SO2 in China will also be important.

CO2 levels, currently 380 ppm, are ex-pected to double by 2100 (http://www.climatechangeinaustralia.gov.au/documents/resources/TR_Web_Ch4.pdf). A CO2 fer-tilization effect of increased response tophotosynthesis is expected in all crops, perhapseven with a bonus of increased nitrogen fixationin legume crops. However, Free Air Carbondioxide Elevation (FACE) studies indicate thatthe production penalty of increased temperatureabove moderate increases will exceed the CO2

fertilization effect (IPCC Synthesis report,Climate Change 2007). Yet, there may be aphysiological ceiling to grain production intoday’s cereal varieties that may be adaptedto current CO2 levels. Genetic variability forimproved response to the projected high CO2

has not been assessed comprehensively inany crop. A capacity for improved harvestindex under high CO2, which could raise yieldpotential, will require a managed environment.As yet no breeding program has enhanced CO2

facilities to conduct the required screening forCO2 stimulated productivity.

Just as the breeding of the green revolutionwheats was done for high input irrigated targetenvironments to identify the best yield poten-tial, the screening environment now needs up-dating for the new agricultural environment ofincreased temperature and CO2 levels. This callsfor a shuttle breeding program encompassing thescreening of CO2 responsiveness and identifica-tion of molecular markers for the relevant quan-titative trait loci (QTLs), combined with selec-tion for temperature/drought stresses in managedfield environments (see Chapter 3.8, , this book).After CO2 responsive parents are identified, andsuitable marker-assisted selection (MAS) proce-dures available, the crossing and early generationselection could be carried out in the field under




current CO2 concentration. Only the advancedelite selection would need to be confirmed forCO2 response in enhanced CO2 facilities, withcomplementary heat and drought stress toler-ances already selected.

The goal of increased food production in 2050will utilize a combination of three strategies:

� a switch to crop species with regional adapta-tion to important stresses, especially toleranceto heat and drought stresses,

� intraspecific genetic improvement of existingcrops, and

� agronomic improvements in production forrainfed and for irrigated farming systems, in-cluding strategies for; water efficiency, re-duced tillage, targeted fertilizer applicationetc., which are not addressed in this book,but which are complementary to geneticimprovement.

Crop options with climate change

The change of crop profiles will vary by re-gion and preferred crops, but will be affected bygenerally shorter growing seasons with globalwarming, and wider variations in temperature.Thus, both earliness and greater crop reliabilitywill be sought in tropical and temperate latitudesof low to medium elevation. Yet for some re-gions with high altitudes or high latitudes, thelength of growing season may be increased withmore frost-free days, and a moderate increasein temperature may favour crop production (seeChapter 4, this book). In other regions, a mod-erate temperature increase may initially favourproduction (Chapter 1.2, this book) but then fur-ther temperature increase and increased vari-ability would result in a greater frequency ofheat stress events. Adaptation needs by cropsfor future food security have been reviewed byLobell et al. (2008), with modeling indicat-ing high vulnerability for southern Africa, WestAfrica, and South Asia. Crop improvement pri-orities include wheat, rice, millet, rapeseed, andSoybean in South Asia, maize in South Africa,

sorghum, groundnut, and yams in West Africa.A strategy of switching to less impacted crops isalso suggested as a viable adaptation option.

The choice of cereal crops for different agroe-cological conditions is displayed in West Africa.Among tropical cereals, the shorter season pearlmillet is better adapted to the heat and droughtstress of the sub-Saharan Sahel region thansorghum, which is more widely cultivated inthe wetter Savannah zone, while maize is bet-ter suited to the higher rainfall Savannah rain-forest zone as well as being more responsive tofertilizer inputs. Where fertilizer distribution hasbeen established in association with local creditschemes, maize has been more profitable thansorghum and has become a major crop in WestAfrica. Rice growing has expanded mainly inthe rainforest zone, irrigated and semiflooded, inNigeria, Guinea, Mali, and Ivory Coast. Wheatproduction is limited to the cool dry season inthe northern Savannah.

Common beans (Phaseolus vulgaris) are amid-elevation tropical legume, and less tolerantof heat stress than lima beans (Phaseolus luna-tus) and cowpea (Vigna unguiculatus) cultivatedin low-elevation tropics. The latter are grownin West Africa from the coastal rainforests tothe Sahel, and landraces with short maturity andheat tolerance have been identified (Chapter 16,this book). Soybeans have been recently intro-duced to Nigeria, with production exceeding0.5 million ton, after pest disease and nitrogenfixation problems were solved with research.A form of tofu is used in local recipes (http://www.scienceinafrica.co.za/2001/may/tofu.htm).

Very recently, hybrid early maturing pi-geon pea has been introduced to East Africa(http://www.icrisat.org/newsroom/latest-news/happenings/happenings1416.htm#1r). This isan attractive and more reliable food optionthan the traditional local maize. There are othersuch examples where targeted adaptive researchhas accelerated the introduction of a new cropat a faster rate than previously, when it tookmany decades for the reciprocal interchange ofcrops between the “old” and “new” world in the




sixteenth century. Climate change could becomethe trigger for a new wave of crop substitutionsaround the world.

Among the cereals in the temperate zone,barley is regarded as faster maturing and moreheat and drought tolerant than bread wheat(Hadjichristodoulou 1984; Rees et al. 1991) thatin turn has wider adaptation than durum wheat.Oats are comparatively adapted to the high al-titude and cool temperate zones, as too is ce-real rye. Lane and Jarvis (2007) examined rela-tive changes in area of different crops with the“Ecocrop” model and indicate that with globalwarming cold weather crops such as wheat(18%) and rye (16%) suffer significantly de-creases in suitable areas compared with bar-ley (2% gain), while large gains are predictedfor pearl millet (31%), chickpea (15%), andsoybean (14%).

Among the temperate legumes, in the winterrainfall zone of southern Australia, peas are bestadapted to semiarid zones and have versatility inend use. The presence of widespread saline sub-soils may favour peas over chickpeas, whereaschickpeas are widely grown in India on reservesoil moisture after the monsoon main crop sea-son where deeper rooting plus better adaptationto warm temperature are advantages that makeit more attractive with predicted climate changes(Lane and Jarvis 2007; Chapter 9, this book).

Changes in the relative areas of crops withclimate change have been examined for temper-ature and moisture effects, but not for CO2 re-sponsiveness. The scale of research and breedingfor heat and drought stresses in wheat and maizeat CIMMYT and in rice at IRRI reflects theirimportance as major food staples. Even thoughcowpea is a minor crop, there has been researchinto genetic variation for tolerance of high nighttemperature and production of pollen, followedby the breeding and release of a heat stress tol-erant variety (Chapter 16, this book).

Genetic variation for heat stress tolerance, un-der major gene control in some cases, has beenfound in wild tomato, pepper, cabbage, commonbean, and mung bean (Chapters 15 (beans) and

18 (vegetables), this book). It is likely that moregenetic variation for tolerance of heat stress willbe found in other crops, especially in land racesfrom stress-prone regions, as for cowpeas andfor rice (Chapter 13, this book), and in the wildrelatives that often have wider variation than inthe domestic gene pool. Abiotic stress tolerances(especially for heat and drought) are found inwild relatives of rice, potato, and in both domes-tic and wild gene pools of wheat and commonbean as cited in this book. But the reality is thatfor many crops these abiotic stress toleranceshave yet to be screened in the wild relatives,where the limited research has been concentratedon screening for major gene control of pest anddisease resistances, which are easier to introgressinto the domestic gene pool than quantitativelyinherited traits. Research in the related wild genepools of wheat show these to be an importantsource of variation for abiotic stress tolerances,and also for improved yield potential (Chapter 6,this book).

In this book, we have attempted to examinethe capacity for adaptation to climate changein a wide range of crops, and to assess theavailability of genetic resources to meet thischallenge. The aggregate research on this topicvaries greatly between crops, precluding a well-informed comparison among crops. It is to beexpected that genetic potentials for tolerances toabiotic stresses will differ among domestic andwild gene pools. Chickpeas, for instance, haveundergone repeated postdomestication geneticbottlenecks to severely reduce genetic diversityin this crop (Abbo et al. 2003). Wild relatives assources of diversity for abiotic stress toleranceswill be a more important research priority in thefuture. However, for faba bean with no knownwild relatives, the options may be more limited.

A wider search for sources of stress tolerancein distantly related species will be more diffi-cult but perhaps necessary, to buffer food pro-duction against climate change. In Chapter 27(this book), Xue and McIntyre review the po-tential to tap into the component physiologicalmechanisms and gene regulatory systems with




the use of biotechnology tools. Such transgenicapproaches will open up new opportunities andperspectives for improved productivity toward adoubling of food production by 2050.

Genetic diversity within crops foradaptation to climate change

Will genetic improvement of crops be needed foradaptation to climate change? There is a widerange of conclusions among the crop experts inthis book from very little adjustment for cassava;except in local phenology, storage roots tolerantof rainfall, and herbicide tolerance; to major ad-justment for rice, cowpeas, maize, and sorghumfor the warmer and drought prone environmentsin semiarid subtropical regions.

Legume crops

The legume crops are well placed to doubly ben-efit from elevated CO2, with increased photo-synthetic carbon fertilization directly enhancinggrowth, and indirectly enhancing nitrogen fix-ation. However, there are also likely to be dif-ferences among species (West et al. 2005). Theincreased availability of nitrogen would supportboth the expansion of the plant sink for vegeta-tive growth and for grain yield, and more efficientrubisco-mediated carbon fixation. The legumeswill likely have a more significant role in farm ro-tations as an alternate source of nitrogen for othercrops. However, there is wide genetic variationamong the legumes in environmental adaptation,end use, and diversity for tolerance of abioticstresses.

In cowpea, the vegetative stage is more tol-erant to heat stress at 41◦C/24◦C under 14–15 hour days than the reproductive stage (Chap-ter 15, this book). High night temperature re-duced pod-set with 4.4% yield decline per degreeCelsius above 15◦C night temperature, but notthe maximum day temperature. Pollen viabilityand pod-set are most sensitive to night heat withdaylength interaction. The domestic gene poolhas major gene variation for flower and for pod

production, and some accessions have both setsof genes. Extreme stress selection environmentscan be used to identify parents and breed for heatstress tolerance. A heat stress tolerant cultivarbred in California also had grain yields equal tothe best check under moderate conditions. Heattolerant selections were semi-dwarfed and hadgreater harvest index. Delayed leaf senescencehelps survival under drought, and can be com-bined with heat tolerance for yield benefits.

Heat tolerant edible pod cowpeas bred in In-dia had both floral bud and pod-set tolerances toheat stress and were also semi-dwarf. Progenyof parents selected in extreme environments inCalifornia, then screened for agronomic traitsin Ghana, led to two cultivars. Heat stress af-fects reproductive development in heat sensi-tive lines making them less responsive to ele-vated CO2, whereas heat tolerant lines have largeincreases in pod production. At elevated CO2

(700 ppm), pod yield increased by 45% in onestudy. In the tropics, useful heat-tolerance traitsare high pod-set and maintenance of many seedper pod. Cowpea has genetic variation for heattolerances at different growth stages, some beingsimply inherited, and there is potential for indi-rect selection. Currently, cowpea adaptation ex-tends into the low rainfall zones of West Africa,with adaptation for short-day responsiveness un-der sorghum intercropping with flowering andseed-set after the end of the rainy season in sa-vannah Nigeria (Steele and Mehra 1980).

Soybeans have genetic variance for water-useefficiency (WUE), rooting depth and mass, pho-toperiod control of phenology, and adaptive par-titioning to roots under water deficit (Chapter17, this book). It has a high tolerance of heatwith seed size maximized at 23◦C, compensa-tion of smaller seed at 28◦C with an increase inseed number, and yield decline above 32◦C. Thegrain-filling period is reduced at high tempera-tures. Trait combinations for “stay-green” longergrain filling, rate and depth of root growth, andWUE have cumulative effects on yield. Responseto CO2 enrichment is greater under droughtstress. Stomatal conductance is reduced 40%




with doubling of CO2; however, transpiration re-duces by 10% and the canopy is 2◦C warmer. Thelatter may result in heat stress in tropical regions.Besides improved WUE, elevated CO2 increasesphotosynthesis leading to increased partitioningto roots for water uptake and nitrogen fixation.Soybeans are adapted to the current range of hightemperatures in the tropics and subtropics. Butwith climate change bringing even higher tem-perature extremes, novel sources of genetic vari-ation may become important from the primaryand secondary gene pools, and even the tertiaryGlycine gene pool that is also indigenous to theharsh environments of Australia.

Beans (P. vulgaris) have an optimum tem-perature for growth of 17.5–23◦C, with re-duced pollen viability, flower and pod-set above30◦C/20◦C day per night temperatures (Chapter16, this book). Heat tolerance will be more im-portant than drought tolerance for adaptation toclimate change. Although there is genetic vari-ation for heat and for drought stress, includ-ing a deep rooting trait, wider genetic variationfor these occurs in the secondary gene poolsfor tepary (Phaseolus acutifolius) and runner(Phaseolus coccineus) beans. Such interspecificcrossing may be the required strategy for abioticstress tolerances and also for the new insect-pestand disease resistances with climate change.

Pea is the world’s third most important grainlegume, with multiple end uses as vegetable andgrain foods, and as fodder, silage, and grainfeed (Chapter 7, this book). However, researchon responses of crop plants to combination ofboth heat and drought stresses and to elevatedCO2 associated with climate change has beenquite limited. Heat stress reduces photosynthesisand seed-set, flower abortion may occur above27◦C. High CO2 mitigated the reduction of leafwater potential and of height in response todrought, but there is little knowledge of inter-action of heat stress with elevated CO2. Semi-leafless types have greater WUE and more tol-erance of drought than normal leaf types. Thereis wide genetic diversity for abiotic stresses indomestic and in wild primary gene pools. Pea

has advanced molecular technologies for link-age maps and gene identification. The potentialexists to identify alleles for tolerance to bothheat and of drought stresses, and to breed forresponsiveness of biomass/grain yield and nitro-gen fixation to elevated CO2. However, there isno active breeding as yet.

More studies are needed of genetic diver-sity for heat and for drought tolerance in fababean, for which no wild relatives are known,thereby limiting the scope for genetic improve-ment (Chapter 10, this book). There is geneticvariation for dehydration through stomatal con-trol and for depth of rooting. Subtropical regionssuch as Bangladesh have heat tolerant cultivars,provided that nights are cool and soil moistureis nonlimiting (30◦C/15◦C), and Mediterraneancultivars may tolerate up to 32◦C. In the absenceof drought, large yield increases are obtained atelevated CO2, though whether this is associatedwith increased nitrogen fixation is unclear. Po-tential interactions between elevated CO2 andheat and drought stresses have not been investi-gated. The scope for adaptation to climate changeis largely unexplored.

Drought may be mitigated in chickpea withgenetic variation for deeper roots (Chapter 9, thisbook). Chickpea also has superior drought toler-ance in the tertiary gene pool and flowering/pod-set heat (>35◦C) tolerance in the domestic genepool. Elevated CO2 increases pod-set and yieldin chickpea and is likely to increase nitrogen fixa-tion. Markers for root and drought traits are beingidentified, but the application of MAS for stresstraits has yet to be realized. Climate change mayhave greater indirect effects on chickpea produc-tion through effects upon pests and diseases.

Due to the allocation of more photosynthateto production of protein in legumes, the produc-tivity levels in legumes are poor in comparisonto cereals. Mostly, the legumes are rainfed crops,and good agronomic management under rainfedenvironment play an important role in enhancingcrop yields. Good tillage under rainfed environ-ments can improve WUE and nitrogen fixation inlegume crops (Al-Tawaha et al. 2010). In legume




crops, limited success has been achieved in cropimprovement program to develop multiple re-sistant, widely adapted, high-yielding cultivarsfor rainfed environments. There is a need to de-velop and create new gene pools in these crops,possessing multiple resistances and wide adap-tation for stress environments to meet the chal-lenges of global warming. Climate change mayhave greater direct and indirect effects on legumeproduction through changes in the pests anddiseases.

Thus, for legume crops, genetic variation forheat tolerance varies widely, from large oppor-tunities in the domestic gene pool for cowpeato limited in chickpea, to considerable opportu-nity in the secondary gene pool as with commonbeans or no opportunity for faba bean. Geneticdiversity for drought tolerance is often availablethrough such traits as deep rooting. However,very little is known on genetic diversity for re-sponsiveness to CO2, both directly for photosyn-thesis and indirectly for stimulation of nitrogenfixation. This may have significant implicationsfor future farming systems.

Nonlegume crops

In other crops, nitrogen fixation is mostly ab-sent, except via endophytes in sugarcane but ap-parently confined to Brazil. With increased rateof photosynthesis with climate change, mineralnitrogen levels may be relatively diluted in non-legumes to result in reduced levels of protein.Climate change effects are more direct, with CO2

enhanced photosynthesis, a rise in mean and inextreme temperatures for crops, and greater un-certainty of rainfall. For many crops, there islittle knowledge of response to CO2, nor of in-teractions with temperature and drought stresses.

Bread wheat and rice are major world staples,which have received the greatest attention forboth sources of abiotic tolerances and breedingfor climate change.

For wheat, physiological and morphologicaltraits are addressed for drought tolerance (Chap-ter 6, this book). Synthetics or hexaploid wheat

reconstituted from combining durum wheat (AB genomes) with the Triticum tauschii source ofthe D genome have provided genetic variation fordeeper roots and maintenance of seed weight dur-ing drought. The domestic wheat gene pool hasmajor gene control for osmotic adjustment, phe-nology adjustment with photoperiod and flow-ering genes, stem thickness and internodes, andleaf morphology. The “stay-green” avoidance ofleaf senescence derives from synthetics, confer-ring heat tolerance. However, QTLs for stress tol-erance tend to be influenced strongly by the ge-netic background as well as environments. Sus-tainable conservation agriculture needs changesin breeding trial environments to respond togenotype × farming system interactions. Thereis good opportunity to breed for genetic adjust-ment to climate change in wheat, for more un-predictable rainfall and increased temperatures,combined with a fertilizing effect of elevatedCO2.

Responses of barley to climate change willbe similar to wheat with both crops showing apositive response to increasing CO2. Exposureto extremes in temperature and lack of adequatesoil water will cause these two crops to show re-duced yield through reduced tillering, head size,kernels per head, and weight per kernel. Bothcrops show an interaction to climatic parame-ters with available nitrogen and development ofmanagement systems for climate stress will needto account for nitrogen availability. Since thesecrops are grown in areas with potential exposureto more extreme conditions, quantifying the ef-fects of drought and options to overcome droughtthrough genetics and management will increasethe stability of wheat and barley production.

Rice responds to elevated CO2 levels; how-ever, increased photosynthesis and decreasedstomatal conductance is down-regulated at matu-rity to a 10% photosynthetic gain compared with30% at mid-tillering, with 15–30% biomass/yield gain (Chapter 12, this book). Down-regulation of photosynthesis in later growthstages is greater in grasses than in legumes es-pecially if N supply is limiting. Yield response




to CO2 fertilization is greater in hybrid than inconventional rice, possibly reflecting a greatersink strength. This is also greater in the indicathan the japonica types. Stomatal conductance isreduced with elevated CO2 and WUE increased,leading to an increase in canopy temperature. Atemperature increase of 2◦C will increase yield incool areas, but reduce yield in dry season tropicsand other warm environments, aggravated by el-evated CO2 reducing transpiration. Above 32◦C,the temperature reduction of photosynthesis andof yield is greater than the predicted CO2 en-hancement. Genetic variation is found in Iranianlandraces and some cultivars for high temper-ature tolerance at anthesis, and in the primarygene pool for root traits conferring drought tol-erance. Abiotic stress tolerant genes are presentin wild rice and in Oryza glaberrima. Unknownissues with climate change include overall sinkregulation and partitioning of photosynthate andnitrogen uptake. There are good prospects to se-lect for adaptation to climate change, with widerexploitation of land races from extreme environ-ments and of wild relatives.

Maize is not as responsive to increasing CO2

because of the C4 metabolic pathway (Chapter14, this book). Temperature increases, as pro-jected under climate change, will decrease thelife cycle and duration of the reproductive stage.The projected reduction in grain yield will bepartially offset by the increases in CO2 but maybe affected more by the extremes in temperatureand precipitation. Temperature effects on polli-nation and kernel set may be one of the criticalresponses related to climate change (Banzigeret al. 2000; Lobell et al. 2011). The greatestloss in productivity of maize is predicted forsouthern Africa (Lobell et al. 2008). CIMMYT(International Maize and Wheat ImprovementCenter) and IITA (International Institute onTropical Agriculture) breeding programs havedelineated six to eight mega-environments insouthern Africa (Banziger et al. 2004; Setimelaet al. 2005). However, end-use characteristics,colour preferences, and other factors may oftenprevent the direct use of lowland-adapted vari-

eties to replace the varieties adapted to the mid-elevation or upland mega-environments withhigher temperatures as a result of climate change.Breeders will need the capacity to rapidly movestress tolerance traits into germplasm preferredby people in the target environment they serve.

Genetic variation is present in maize fordrought and other stress traits and can be ex-ploited in superior performing genetic back-grounds (Vasal et al. 1999; Banziger et al. 2000).Most abiotic stresses including drought are com-plex, and rapid progress is not easily attainable.Lines that are tolerant to drought alone mayor may not perform well when screened undercombination of drought and high temperature(BM Prasanna, personal communication). Jointscreening for both drought and heat stresses isrequired, especially for enhancing adaptation ofcrops to environments which are prone to boththe stresses.

For drought, it is desirable to identify a fewkey traits or secondary traits that can facilitate se-lection. In maize, anthesis-silking interval (ASI)is considered a good secondary trait to facili-tate selection for drought tolerance (Borras et al.2007; Edmeades et al. 2000). It is also an im-portant trait for other stresses. Inbreeding, high-density, and pre-flowering biotic stresses canadversely affect ASI. Ear number and seed-setare additional traits that could facilitate droughtwork.

Both direct and indirect breeding options canbe deployed considering the robustness of popu-lation and hybrid research activities (Edmeadeset al. 1998; Vasal et al. 1997). For manageddrought stress screening, a rain-free season isa must, with evaluation of genotypes/breedingmaterials under no stress (control), intermedi-ate stress, and severe stress conditions. Use ofselfed progenies and inbred testers in popula-tion improvement efforts is preferred to non-inbred progenies to help improve heritabilityof the trait. Choice of stratified mass selectionor full-sib family selection or modified half-sibreciprocal recurrent selection depends on man-power, physical facilities, available testing sites,




and resources allocated to this activity (Vasalet al. 1997).

The drought tolerant source Michoacan 21(“Latente”) has been known for several decades.Because of poor agronomic performance andother undesirable attributes, it has been difficultto exploit and transfer this trait to other geneticbackgrounds. CIMMYT researchers selectedthis trait to the widely grown Tuxpeno popu-lation adapted to lowland conditions. Progressfrom eight cycles of recurrent full-sib familyselection was encouraging and demonstratedthat tolerance of water deficits at floweringand during grain filling can be improved at nocost to performance in well-watered conditions(Bolanos and Edmeades 1993; Edmeades et al.1993). Gains averaged 90 kilograms per hectareper cycle across all sites at a mean yield levelof 5.6 tons per hectare with yield potential in-creased in both well-watered and drought stressenvironments (Byrne et al. 1995).

An important recent finding is that droughtselections also perform well under low-N condi-tions (Banziger et al. 2002). Also, a number ofgood drought-tolerant lines have been selectedfrom inbred line evaluation trials with no previ-ous history of selection for drought (Vasal et al.1997, 1999, 2000, 2001). Hybrids tolerant todrought were also identified, though such hy-brids were never previously selected for drought.Thus, there are good opportunities in maize tobreed for abiotic stress tolerances.

For tropical vegetables, heat tolerance willbe the most significant challenge from climatechange (Chapter 18, this book). Heat tolerantmechanisms include leaf hairiness and wax,heat shock proteins, specific isozymes of cata-lase/peroxidase, with the focus on dissection oftolerance traits to components highly correlatedwith heat tolerance.

In tomato, there is genetic variation forheat stress tolerance for pollen productionand viability, possibly under major gene con-trol, and genetic variation for fruit set underhigh temperature. Novel sources of heat tol-erance have been introgressed from the wild

tomato species Solanum chilense and Solanumpennellii.

In pepper, high temperatures reduce fruit size,pollen fertility, and growth. High temperatureincreases floral abscission, but there is geneticvariation for fruit set. Pepper breeding lines withheat tolerance have been developed with selec-tion both in optimal and in warm conditions.

There is major gene inheritance in cabbage forheat tolerance. This is expressed by anthers ac-cumulating proline under increased temperature,more and longer/thicker leaves, greater water up-take, and increased superoxide dismutase, cata-lase and ascorbic acid peroxidise. A heat-shocktolerance gene has been isolated.

Mung beans are adapted to high temperaturegrowing conditions, and have good genetic vari-ation for abiotic stress. Presence of early genepool in cultivated varieties, resistance againsthigh temperature, and effective WUE has madethis crop suitable for wider cultivation.

There is good capacity to deploy genetic vari-ation for heat stress in a range of vegetables.

Climate change models suggest a 32% dropin potato production globally (Chapter 11, thisbook). Temperature changes (rises) are expectedto be more critical for potato production thanprecipitation changes. Minimum temperaturesare the most limiting at high latitudes, andmaximum temperatures in the subtropics. Each10◦C increase in temperature doubles respira-tion rate and accelerates leaf senescence. Nighttemperature >17◦C restricts tuber initiation,and the optimal temperature for tuber bulkingranges 14–22◦C. Heat tolerance genes mayregulate phloem unloading, sucrose and starchmetabolism in tubers, and possibly detoxifi-cation of reactive oxygen species involved instarch synthesis and sink strength regulation.Tuber initiation is more susceptible to heat stressthan carbon partition, but some wild speciestuberise at 30–40◦C.

Potato has genetic variation for droughttolerance and for WUE, which may reflectstomatal behavior, improved water uptake,desiccation tolerance, or reduced transpiration.




Genetic variation in root length possibly is aidedby osmotic adjustment, with correlation to tuberyield. Commercial potato has a narrow geneticbase. Three to seven backcrosses are neededto transfer a major dominant gene from wildtuber-bearing relatives to domestic cultivars,due to problems with different ploidy levels.Systematic screening of wide germplasm isneeded with priority to ecologic sources forspecific adaptations (GIS climatology) forfocused trait screening of target germplasm.Whole genome sequences are available for anal-yses of linkages and of epistatic interactions,in polyploids. There is wide scope to utilizegenetic diversity for heat and drought stress,and response to CO2 is largely unresearched.A very large expansion of breeding effort isneeded to adjust to climate change.

Sugarcane has high levels of ploidy, n =10 for Solanum officarum, n = 8 for Solanumspontaneum, and modern cultivars have 100–130chromosomes with 80% ex officinarum, 10%spontaneum, 10% recombined between species(Chapter 21, this book). S. spontaneum con-tributes not only tolerance to abiotic stresses,rationing ability, and pest/disease resistances butalso low sucrose and high fiber. It is found inwidely diverse habitats, jungles/deserts/swampsand freezing high altitudes, and is a potentialsource of genetic variation for abiotic stresses ofclimate change. Other wild relatives of the Sac-charum complex may be sources of tolerancesto abiotic stresses, plus vigour and disease resis-tances. As a C4 crop, response to increased CO2

is modest, with less stomatal conductance and re-duced water consumption but increased leaf tem-perature. A range of WUE and yield responseshave been found. Drought tolerance mechanismsinclude more vigorous roots, early stomatal clo-sure, leaf rolling and leaf senescence; however,some drought avoidance traits negatively affectbiomass yields. Transpiration efficiency varieswidely among interspecific hybrids. A study ofunselected clones in Australia gave similar re-sults across many regions but lower yields forthe Ord River region that is 3◦C warmer. Sugar

content is lower at high temperatures. More studyis needed of physiological responses, and of ge-netic diversity, for heat stress. There has beenvery little study of CO2 response. The Ord Rivermay be an analogue environment for climatechange. The topic has been little explored.

Delayed senescence “stay-green” has majorgene inheritance in sorghum (Chapter 14,this book). Sorghum, a C4 crop, has muchgreater WUE than wheat, and is droughttolerant with good recovery in subsequentrains. “stay-green,” maturity variation, andosmotic drought tolerance can be selected byscreening alternate generations in drought and inhigh-yield environments. Regulatory genes fordrought tolerance have been mapped and bothgenotypic and cytoplasmic variation for heattolerance also provides the possibility for MAS.There appears to be good scope for adaptationto climate change in sorghum.

Suitable growing regions for cassava may beincreased with warmer temperatures, whereashigher CO2 may reduce production increasecyanogenic glucosides in roots (Chapter 19, thisbook). More erratic rainfall could affect foodquality. Climate change may have greater indi-rect effects on cassava production through effectsupon pests and diseases. Reduced tillage has po-tential to improve WUE; however, this dependson future success in identifying genes for her-bicide tolerance. Thus, cassava requirements foradjustment to climate change are different fromgrain crops.

Brassica species include important oilseedcrops as well as vegetables (Chapter 22, thisbook). The focus of this book is on field cropsand hence oilseeds. Climate change may affectoil content, oil quality, and seed yield. Increasesin temperature and moisture stresses will havenegative effects and little is known of responsesto CO2. The main oilseed species are polyploid,and extensive variation for abiotic stresses isavailable from the domestic diploid parents andtheir wild relatives. Although certain interspe-cific crosses present difficulties, there is a largescope for selecting adaptation to climate change




with resynthesis of the polyploid crops. Bras-sica juncea is better adapted to heat and moisturestress than Brassica napus, the current principaloilseed crop. Possibly, it may be replaced by B.juncea as a consequence of climate change. Ma-jor diseases such as blackleg in Australia mayincrease in severity. Thus, breeders have a widerange of options for genetic mitigation of climatechange.

In many crops, the primary and secondarygene pools have yet to be explored for toler-ance of heat and of drought stresses, and of-ten little is known of heritable responsiveness toCO2 fertilization. There may also be complexenvironmental interactions of photoperiod withheat tolerance, and of CO2 response with bothdrought and temperature tolerances. Clearly, cli-mate change raises new challenges in crop breed-ing, and prompts rethinking of diversity betweencrops for regional adaptation, such as geneticvariation for earliness, and especially toleranceto heat stress. Predicted high temperature ex-tremes in the tropics and temperate zones areexpected to exceed those in current analogueenvironments in the worst case climate changescenarios.

Conclusions

Climate change will have large impacts on globalproduction and productivity of crops, unless ur-gent and concerted measures are taken for en-hancing the adaptation of the crops and the crop-ping systems. Associated with global warming,natural calamities are increasing and having anegative impact on overall agriculture produc-tion globally. The intensive screening of avail-able genetic resources including wild speciesand landraces, against drought, temperatures ex-tremes, flooding/waterlogging and elevated CO2

levels is needed for the identification of suit-able donors and varieties across crops. New plantbreeding approaches and biotechnological toolsand techniques must be identified and utilized inthe development of new material with adaptationto climate changes, and for commercial cultiva-

tion globally. To mitigate the adverse impacts ofclimate change, crop adaptation must go hand inhand with mitigation measures. Relevent tech-nologies highlighted in this book, may be po-tentially adopted by the scientific and farmingcommunities, national and international institu-tions and governments, and private organizationsworldwide.

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