Climate Change and Plant Abiotic Stress Tolerance || Climate Change: Challenges for Future Crop Adjustments

Part One

Climate Change

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Climate Change and Plant Abiotic Stress Tolerance, First Edition. Edited by Narendra Tuteja and Sarvajeet S. Gill.� 2014 Wiley-VCH Verlag GmbH & Co. KGaA. Published 2014 by Wiley-VCH Verlag GmbH & Co. KGaA.

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Climate Change: Challenges for Future Crop Adjustments

Jerry L. Hatfield

Abstract

Climate change will affect all agricultural areas over the coming years;however, this effect will not be equally distributed spatially or temporally.Increasing temperatures of 2–3 �C over the next 40 years will expose plants tohigher temperatures throughout their life cycle and also increase the atmo-spheric demand for water vapor, adding to the stress because of the increasedrate of crop water use. Coupling the effect of temperature with a more variableprecipitation pattern creates a combination of temperature and moisture stresson crop plants. This will affect our ability to increase water-use efficiency(WUE) in crops in order to produce more grain or forage with less water. Onepositive aspect of climate change is that rising carbon dioxide increases therate of photosynthesis and also decreases the rate of transpiration, leading toincreased WUE. Our challenge will be to determine how to extrapolate theseeffects to whole canopies and into management systems that take advantage ofthis effect. The changing climate will not only affect growth and developmentof plants, but also the quality of the product. In evaluating the effect of climateon plants we need to include the direct effects of perennial plants becauseadaptation strategies for these production systems will be more complex thanin annual crops. To ensure an adequate food and feed supply required to meetthe needs of 9 billion people requires a transdisciplinary approach to developinnovative strategies to manage our crop production systems to reduce oreliminate the impact of climate change.

1.1

Introduction

Climate has always impacted agriculture throughout the world. The quest for astable food supply and the ability to feed the family has prompted many inno-vations in terms of cropping systems, crop selection, and cultivation practices.Our modern world is confronted with two unique challenges during the twenty-

Climate Change and Plant Abiotic Stress Tolerance, First Edition. Edited by Narendra Tuteja and Sarvajeet S. Gill.� 2014 Wiley-VCH Verlag GmbH & Co. KGaA. Published 2014 by Wiley-VCH Verlag GmbH & Co. KGaA.

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first century: an increasing global population with an ever-increasing demand forfood and increasing climate variability. Climate change is occurring at rates neverexperienced before by modern agriculture and will place constraints on ourcapabilities to continue to observe the increasing trend in grain production aroundthe world. Hatfield [1] showed that the increasing trends in grain productionacross the United States were offset by climate stresses (variation in precipitation)during the growing season, while Lobell et al. [2] observed that grain productionlevels around the world were already being affected by warming temperatures.These variations in climate are already impacting crop production, and theimportant question is what will be the future impact of our changing climate andwhat adjustments will crops make to cope with these changes?The challenge for agriculturalists will be to adjust crop production to the chang-

ing climate to cope with increasing temperatures, more extreme events in tempera-ture, more variable precipitation, reduced solar radiation through increasedcloudiness, increased evaporative demand, and increased carbon dioxide (CO2). Inthis chapter, we will explore the potential adjustments crops will have to make tocope with climate change.

1.2

Climate Change

Climate change will occur throughout the world and affect all agricultural areas;however, the degree of impact will be different depending upon the specific region.Projected increases in CO2 to reach 550 mmolmol�1 by 2050 [3] throughout theworld certainly seem possible given the current trends. However, changes in CO2

are not the primary concern in the future climate because of the positive impact ofCO2 on plants. A future challenge for crop production caused by increasing CO2 isthe observation that fast-growing species (e.g., weeds) are responding with greatergrowth than cultivated plants [4].Climate change will occur throughout the world for the next 30–50 years [3].

These changes will entail increases in air temperature of 2–3 �C by 2050 underestimates of reduced emissions of greenhouse gases. These changes will not beuniform throughout the world, with some regions showing increased warmingmore than others. General statements on the changes in climate expected havebeen provided by Meehl et al. [5], in which they state that heat waves are projectedto become more intense, more frequent, and last longer than what is beingexperienced in the world today. These heat waves would have short-term durationsof a few days with temperature increases of over 5 �C above the normal tempera-tures. It would be the summer period in which these heat waves would have themost dramatic impact. Their projections revealed that daily minimum tempera-tures will increase more rapidly than daily maximum temperatures, leading to aincrease in the daily mean temperatures. Plants will be subjected to conditions inwhich the nighttime temperatures will be warmer and this will affect respirationrates more than photosynthetic rates.

4 1 Climate Change: Challenges for Future Crop Adjustments

Variability in precipitation is expected to increase over the next decades [5]. Usingthe current ensemble of climate models, the prediction is for precipitation togenerally increase in the areas of regional tropical precipitation maxima (e.g., themonsoon regimes) and over the tropical Pacific in particular, with generaldecreases in the subtropics and increases at high latitudes as a consequence of ageneral intensification of the global hydrological cycle. This will lead to increases inthe global average mean water vapor, evaporation, and precipitation. Coupled withthese increases in precipitation is increased variation among seasons and amongyears. The intricate feedbacks between the land surface and precipitation will createconditions in which convective storms may decrease because of lack of evaporationfrom the surface. Agriculture will have to contend with this increased variation aspart of the production system. Precipitation changes will be the most difficult topredict in long-term climate scenarios; however, the expectation of increasedvariation in precipitation among years, shifts in precipitation totals, and increasedintensity in precipitation events creates a general statement that precipitation willbecome an increasing unknown in terms of agricultural systems; since agriculturalproduction is dependent upon adequate and timely water supplies, small changescould have dramatic effects.Changes in precipitation will not directly relate to available water for plant

growth because of the role soil plays in absorbing and storing precipitation foruse by crops. Soil water-holding capacity varies among soils from sandy soilswith 1 millimeter of available water per centimeter of soil to clay soils with2 millimeters of available water per centimeter of soil. This is furthercomplicated by the seasonality of the precipitation patterns and the crop beinggrown at a given site. As an example, a Mediterranean climate with precipitationduring the winter months would not be able to supply adequate soil water for asummer crop because of the inability of the soil to supply all of the waterrequired to meet the water demands of this crop.Agricultural production is driven by solar radiation and there are expectations

that climate change will affect this fundamental resource for plant growth.With increases in water vapor and concurrent increases in cloud cover therewould a decrease in incoming solar radiation. Observations from a global studyby Stanhill and Cohen [6] showed there has been a 2.7% reduction per decadeduring the past 50 years, with the current solar radiation totals reduced by20Wm�2, which these authors refer to as “global dimming.” In a more recentstudy across the United States, Stanhill and Cohen [7] found that after 1950there has been a decrease in sunshine duration, with more sites in theNortheast, West, and Southwest showing decreases. They suggested that moredetailed solar radiation records will be required to quantify the temporalchanges in solar radiation related to cloudiness and aerosols. Reduction in solarradiation in agricultural areas in the last 60 years as revealed by models [8] isprojected to continue [9] due to increased concentrations of atmosphericgreenhouse gases and the feedbacks from atmospheric scattering. A recentstudy on solar radiation by Medvigy and Beaulieu [10] examined the variabilityin solar radiation around the world, and concluded there was an increase in

1.2 Climate Change 5

solar radiation variability that was correlated with increases in precipitation variabilityand deep convective cloud amount. There will be changes in the solar radiationresources under climate change and this will affect the agricultural system.Another change in the climate is the increase in atmospheric demand for water

caused by the increasing temperatures. Since the saturation vapor pressure is adirect function of air temperature, then as the temperature increases, atmosphericdemand will increase. One of the major components in the atmospheric demandfor water vapor is the saturation vapor pressure which can be estimated as followsas proposed by Buck [11]:

e� ¼ 0:61121 exp17:502Ta

T a þ 240:97

� �: ð1:1Þ

where e� is the saturation vapor pressure (kPa) and Ta is the air temperature (�C).There is an exponential increase the saturation vapor pressure as the temperatureincreases. The saturation vapor pressure has a major role in the crop water demandas shown in the Penman–Monteith model for actual crop evapotranspiration(Equation 1.2):

lEta ¼DðRn � GÞ þ mrCp½e�ðzÞ��

rah

Dþ cðravþrcÞ=rah; ð1:2Þ

where lEta is the latent heat flux from the canopy (W m�2), l is the latent heatof vaporization (J kg�1), D is the slope of saturation deficit curve (kPa C�1), cis the psychrometric constant (kPa C�1), Rn is the net radiation (W m�2), G isthe soil heat flux (W m�2), mrCp m is the molecular weight of air (kg mol�1),r is the density of air (kg m�3), Cp is the specific heat of air J kg�1 C e�(z) isthe saturation vapor pressure at height z, e(z) is the actual vapor pressure atheight z, rah is the aerodynamic conductance for sensible heat transfer (ms�1), rav is the aerodynamic conductance for water vapor transfer (m s�1), andrc is the canopy conductance to water vapor transfer (m s�1). This approachwas originally described by Monteith [12] and is one of the most appliedequations for crop water use today. Changing atmospheric demand as part ofthe change in climate patterns will have a direct impact on a plant’s ability towithstand temperature stresses and variable precipitation patterns.The collective changes in the climate throughout the world will have profound

impacts on crop production. These effects have already occurred in terms ofannual yields as demonstrated by a number of previous analyses. Interannualvariations in crop yields have been related to precipitation patterns [13] andtemperature [2]. The trends in crop yields demonstrate the capacity of technologyto continue to develop crop varieties; however, the primary question is howwell crops will adapt to the increasing variability in climate in the short term(10–30 years) overlain with the increasing trends in temperature and CO2. Tocontinue to increase productivity there will have to be adjustments in cropsto cope with these changes in the environment and these will be discussedthroughout the remainder of this chapter.


1.3

Crop Responses to Climate Change

There are a wide range of species and potentially wide range of responses toclimate change. However, the challenge will remain as to how we begin to under-stand the dynamics of crop response to climate parameters both in the short andlong term. In this chapter we will assess how different climate parameters willhave to be evaluated in order to enhance future crop adjustment.

1.3.1

Temperature Responses

1.3.1.1 Annual Crops

Responses of annual crops to a changing climate are partially dictated by thetemperature ranges of the specific crop being studied. The temperature ranges fordifferent crops have been summarized [1]. The general consensus observed forannual crops is that rising temperatures will increase the rate of development,causing smaller plants. Since the harvest index (grain yield/total biomass) is rela-tively constant within a given species, this will lead to reduced grain yield. This is theessence of the conclusion arrived at by Lobell et al. [2]. The projected increases in airtemperature throughout the remainder of the twenty-first century shows that grainyields will continue to decrease for the major crops because of the increasedtemperature stress on all major grain crops [1]. Beyond a certain point, higher airtemperatures adversely affect plant growth, pollination, and reproductive processes[14,15]. However, as air temperatures rise beyond the optimum, instead of falling ata rate commensurate with the temperature increase, crop yield losses accelerate. Forexample, an analysis by Schlenker and Roberts [16] indicated yield growth for corn,soybean, and cotton would gradually increase with temperatures up to 29–32 �C andthen decrease sharply as temperature increases beyond this threshold.Crop simulation models show that continued increases of temperature will lead

to yield declines between 2.5% and 10% across a number of agronomic speciesthroughout the twenty-first century [1]. Other evaluations of temperature on cropyield have produced varying outcomes. Lobell et al. [2] showed estimates of yielddecline between 3.8% and 5%, and Schlenker and Roberts [16] used a statisticalapproach to produce estimates of wheat, corn, and cotton yield declines of 36–40%under a low emissions scenario, and declines between 63% and 70% for highemission scenarios. A limitation in their approach was the lack of incorporation ofthe effects of rising atmospheric CO2 on crop growth, variation among cropgenetics, effect of pests on crop yield, or the use of adaptive management strategies(e.g., fertilizers, rotations, tillage, or irrigation).Evaluations of the impact of changing temperature have focused on the effect of

average air temperature changes on crops; however, minimum air temperaturechanges may be of greater importance for their effect on growth and phenology [1].Minimum temperatures are more likely to be increased by climate change overbroad geographic scales [17], while maximum temperatures are affected by local

1.3 Crop Responses to Climate Change 7

conditions, especially soil water content and evaporative heat loss as soil waterevaporates [18]. Hence, in areas where changing climate is expected to causeincreased rainfall or where irrigation is predominant, large increases of maximumtemperatures are less likely to occur than will be the case in regions where droughtis prevalent. Minimum air temperatures affect the nighttime plant respiration rate,and can reduce biomass accumulation and crop yield [1]. Even as climate warmsand minimum average temperatures increase, years with low maximum tempera-tures may more frequently be closer to achieving the temperature optimum, whichwill result in higher yields than is the case today during years when averagetemperatures are below the optimum. Welch et al. [19] found this to be the casefor a historical analysis of rice in Asia – higher minimum temperatures reducedyields, while higher maximum temperature raised yields; notably, the maximumtemperature seldom reached the critical optimum temperature for rice. As futuretemperatures increase, they found maximum temperatures could decrease yieldsif they rise substantially above the critical zone.One of the more susceptible phenological stages to high temperatures is the

pollination stage. Maize (Zea mays L.) pollen viability decreases with exposure totemperatures above 35 �C [20–22]. There is an interaction of temperatures andvapor pressure deficit during the duration of pollen viability (prior to silk reception)because pollen viability is a function of pollen moisture content, which is stronglydependent on vapor pressure deficit [23]. Temperatures of 35 �C compared to 30 �Cduring the endosperm division phase reduced subsequent kernel growth rate(potential) and final kernel size, even after the plants were returned to 30 �C [24].Temperatures above 30 �C damaged cell division and amyloplast replication inmaize kernels, which reduced the strength of the grain sink and ultimately yield[25]. Maize is not the only plant to exhibit sensitivity to high temperatures; forexample, in rice (Oryza sativa L.) pollen viability and production declines as thedaytime maximum temperature (Tmax) exceeds 33 �C and becomes zero whenexposed to Tmax above 40 �C [26]. Current cultivars of rice flower near mid-day,which makes Tmax a good indicator of heat stress on spikelet sterility. Theseexposure times occur very quickly after anthesis and exposure to temperaturesabove 33 �C within 1–3 h after anthesis (dehiscence of the anther, shedding ofpollen, germination of pollen grains on stigma, and elongation of pollen tubes)causes negative impacts on reproduction [27]. Current observations in rice revealthat anthesis occurs between about 9 to 11 a.m. in rice [28], and exposure of rice tohigh temperatures may already be occurring and increase in the future. There isemerging evidence that differences exist among rice cultivars for flowering timeduring the day [29]. Given the negative impacts of high temperatures on pollenviability, there are recent observations from Shah et al. [30] suggesting flowering atcooler times of the day would be beneficial to rice grown in warm environments.They proposed that variation in flowering times during the days would be avaluable phenotypic marker for high-temperature tolerance. A recent study onsoybean (Glycine max L. Merr.) revealed that the selfed seed set on male-sterile,female-fertile plants decreased as daytime temperatures increased from 30 to 35 �C[31]. This confirms earlier observations on partially male-sterile soybean in which


complete sterility was observed when the daytime temperatures exceeded 35 �Cregardless of the night temperatures and concluded that daytime temperatureswere the primary influence on pod set [32]. These studies have implications for thedevelopment of hybrid soybean, but provide even more understanding of the rolethat warm temperatures have on the pollination phase of plant development.Similar responses have been found in annual specialty crops in which

temperature is the major environmental factor affecting production with specificstresses, such as periods of hot days, overall growing season climate, minimumand maximum daily temperatures, and timing of stress in relationship to deve-lopmental stages, having the greatest effect [33–37]. When plants are subjected tomild heat stress (1–4 �C above optimal growth temperature), there was moderatelyreduced yield [38–41]. In these plants, there was an increased sensitivity to heatstress 7–15 days before anthesis, coincident with pollen development. Subjectingplants to a more intense heat stress (generally greater than 4 �C increase overoptimum) often resulted in severe yield loss up to and including complete failureof marketable produce [33,41–44]. There is evidence that temperature effects onyield loss vary among crops and among cultivars within crops. Tomatoes underheat stress fail to produce viable pollen while their leaves remain active. The non-viable pollen does not pollinate flowers, causing fruit set to fail [42]. If the samestressed plants are cooled to normal temperatures for 10 days before pollinationand then returned to high heat, they are able to develop fruit. There are some heat-tolerant tomatoes that perform better than others related to their ability tosuccessfully pollinate even under adverse conditions [38,45].One of the major concerns is that air temperature does not equate to leaf or canopy

temperatures, so the actual temperature the plant experiences may be different thanthe air temperature. This was first observed by Tanner [46] who noticed that the leaftemperature differed from the air temperature, and further expanded by Wiegandand Namken [47] to relate leaf temperature to plant water status. Throughout the last50 years there have been continuing refinements in the use of leaf or canopytemperatures to quantify plant water stress. Leaf or canopy temperatures of well-watered canopies are often 3–5 �C less than the air temperature because of theevaporative cooling induced by transpiration from the leaf surface. However, asavailability of soil water decreases the leaf temperature more closely tracks the airtemperature and under extreme water stress can exceed the air temperature. Thedynamics of this process were reviewed in Hatfield et al. [48] and approaches toquantifying crop water stress using canopy temperature were compared.One of the interesting observations from research on canopy temperatures has

been the discovery of the thermal kinetic window for plants based on the thermalstability of metabolic enzymes [49]. It was proposed that plant leaves have anoptimum temperature and shown that in cotton (Gossypium hirsutum L.) carbonassimilation as measured with leaf photosynthetic chambers peaked at a canopytemperature, Tc, of 29 �C while the fluorescence assay was optimal between 28 �Cand 30 �C, and yield was maximum at 26 �C [50]. When the Tc values exceeded28 �C there was a decrease in yield. These findings have implications under climatechange due to the increasing air temperatures and more variability in soil water

1.3 Crop Responses to Climate Change 9

availability. These linkages were more fully developed in [51] where the authorsdescribed both the energy balance responses at the leaf level and canopy level tochanging air temperatures. Under well-watered conditions plants would be able tomaintain leaf temperatures near their optimal range; however, as soil waterbecomes limiting then leaf temperatures would no longer be maintained within aplant’s optimal range. The concept of the thermal kinetic window would alsoprovide an explanation for why changing maximum air temperatures have less ofan impact on plant growth and development compared to minimum airtemperatures. During the day, increasing maximum temperatures would bemoderated by increasing crop water use, which in turn would maintain leaftemperatures within a given range; however, at night with no transpirationoccurring leaf temperatures would be in equilibrium with the air temperaturebecause the only energy exchange will be long-wave radiation. If we develop thisfurther in mathematical form as shown in the following equation then theserelationships demonstrate the linkages between canopy and air temperature:

Stð1� alÞ þ Ld � esT4l ¼

rCpðT l � TaÞra

þ rCpðe� � eaÞcðrs þ raÞ ; ð1:3Þ

where St is the incoming solar radiation (W m�2), al is the albedo of the leaf orcanopy, Ld is the incoming long-wave radiation (W m�2), e is the emissivity of theleaf or canopy, s is the Stefan–Boltzmann constant, Tl is the leaf or canopytemperature, ra is the aerodynamic conductance (m s�1), and rs is the canopyconductance resistance (m s�1). During the night and with stomata closed, theright-hand term becomes zero, and the leaf and air temperature become equal.This effect has been observed by Bernacchi et al. [52] for soybean in which theyobserved differences in canopy versus air temperatures during the day induced bystomatal closure, but no difference at night. During the night, leaf respiration willcontinue and will be a direct function of the air temperature; as temperatureincreases there will be an increase in the respiration rate. This is an aspect of plantresponse to climate change that has not been extensively evaluated.Perennial crops have a more complex relationship with temperature than annual

crops. Many perennial crops have a chilling requirement in which plants must beexposed to a number of hours below some threshold before flowering can occur.For example, chilling hours for apple (Malus domestica Borkh.) ranges from 400 to2900 h (5–7 �C base) [53], while cherry trees (Prunus avium) require 900–1500 hwith the same base temperature [54]. Grapes (Vitis vinifera L.) have a lower chillingthreshold that other perennial plants with some varieties being as low at 90 h [55].Increasing winter temperatures may prevent chilling hours from being obtainedand projections of warmer winters in California revealed that by mid-twenty-firstcentury, plants requiring more than 800 h may not be exposed to sufficient coolingexcept in very small areas of the Central Valley [56]. Climate change will impact thechilling requirements for fruits and nut trees.Perennial plants are also susceptible to exposure to warm or hot temperatures

similar to annual plants. These responses and the magnitude of the effects aredependent upon the species grown. Exposure to high temperatures, above 22 �C,


for apples during reproduction increases the fruit size and soluble solids, butdecreases firmness as a quality parameter [57]. In cherries, increasing thetemperature 3 �C above the mean decreased fruit set, when the optimumtemperature is 15 �C for fruit set [58]. Exposure of citrus (Citrus sinensis L. Osbeck)to temperatures greater than 30 �C increased fruit drop when the optimumtemperature range is 22–27 �C [59]. During fruit development when the tempera-tures exceed the optimum range of 13–27 �C with temperatures over 33 �C there isa reduction in Brix, acid content, and reduced fruit size in citrus [60]. Temperaturestresses on annual and perennial crops have an impact on all phases of plantgrowth and development.

1.3.1.2 Major Challenges

Temperature is a fundamental parameter affecting plant growth and development.There are a number of major challenges for crops to be able to withstand increasingtemperatures around the globe. As an example of the potential impacts of changingtemperatures, the recent evaluation of increasing temperatures in India on wheat(Triticum aestivum L.) by Ortiz et al. [61] illustrates the effects on future production.One of the major challenges will be to evaluate on a global scale the potential impactsof increasing temperatures on crop production and potentially viable adaptationstrategies that could be implemented to avoid crop production declines. Projectingthe impacts of increasing temperatures will identify areas and cropping systems withpotential vulnerabilities to changing climate. This is an exercise that would providean overview of anticipated problems; however, it does not derive a very detailed viewof the potential physiological and genetic adjustments required to develop morerobust germplasm with tolerance to temperature stresses.A survey of the literature as shown in the previous sections reveals that there are

many potential aspects of temperature response that need to be addressed. One ofthe first is the differences among germplasm in their temperature responsesduring the vegetative and reproductive phases of development. One aspect that hasnot been examined very clearly is the recovery of plant physiological responses totemperature stresses, and the relationship between air temperature and canopytemperature relative to the physiological responses. Understanding these basicprocesses will advance our understanding of how plants react to temperaturechanges. Development of efforts to link physiologists with molecular biologists andagricultural meteorologists would be valuable to help quantify temperature effectson plant growth and development.

1.4

Water Responses

Water availability to plants is critical to physiological functions within plants, andthe linkage between precipitation patterns and soil water-holding capacity governsthe potential response to climate change. One of the biggest challenges facing cropproduction emerges when more variable precipitation throughout the growing

1.4 Water Responses 11

season is coupled with a degraded soil with limited water-holding capacity. Whileprecipitation changes and the uncertainty in precipitation amounts are a majorfocus in climate change, the critical role soil plays in the infiltration, storage, andrelease of water to growing plants is generally overlooked. The overall challengeconfronting agriculture is how to increase water-use efficiency (WUE) in produc-tion systems in order to produce more per unit of water transpired.WUE provides us a framework for evaluating how climate change will impact

agricultural production. There are two scales from which we can examine WUE,the canopy scale or the leaf scale, and each of these provides us different insightsinto the linkage between the plant response and the environment. Hatfield et al.[62] reviewed the current state of knowledge about the role of soil management onWUE and an earlier review by Tanner and Sinclair [63] examined the principlesunderlying WUE. The basic equation for WUE (kg ha�1 mm�1) is:

WUE ¼ YET

ð1:4Þ

where Y is the crop yield (kg ha�1), and ET is the crop water use as a combinationof transpiration from the leaves and evaporation from the soil surface (mm); therecan be a change in WUE either through changes in Y or ET. This approachdescribes the canopy level process in which there are techniques available tomeasure ET. At the leaf level, the dynamics of the system become more insightfulin terms of explaining the linkages between the physiological reactions and thephysical environment because we can relate physiological parameters to CO2

uptake and leaf transpiration as a simple expression of leaf CO2 exchange relativeto transpiration. Other methods include measurement of differences in RuBisCOactivity or rate of electron transport [64]. These physiological studies provide someinsights into plant responses to environmental changes. If we examine WUE at theleaf level then we can express these relationships as:

WUEl ¼ Pl

LEl¼

f ½CO2��raþrsþrm

raLePa

el � eara þ rs

� � ; ð1:5Þ

whereWUEl is the WUE at the leaf level, Pl is the photosynthetic rate (mg CO2 m�2

s�1), LEl is the evaporation rate from a leaf (mg H2O m�2 s�1), f is the conversionof CO2 from ppm to g cm�3 (1.67� 10�9), [CO2] is the ambient CO2 concentration,ra is the aerodynamic conductance for an individual leaf (m s�1), rs is the stomatalconductance (m s�1), rm is the mesophyll conductance (m s�1), ra is the density ofair (g m�3), L is the latent heat of vaporization, e is the ratio of molecular weightsof water vapor and air, Pa is the atmospheric pressure (kPa), el is the vapor pressureof the leaf at leaf temperature, and ea is the actual vapor pressure of the airsurrounding the leaf. The gradient of water vapor between a leaf and the atmosphereis affected by the internal leaf water vapor pressure (e; kPa) which is tightly coupledto leaf temperature (T; �C) and can be calculated from Teten’s equation, e¼ 0.61078� exp(17.269�T/(Tþ 237.3)). Factors affecting energy balance and leaf or canopytemperature will directly affect water vapor pressure inside the leaves and its water


use. There are several methods that can be used to evaluate WUE; the importance ofthese approaches is to be able to more completely understand the relationshipsbetween productivity and water use, and how these may be affected by climatechange.CO2 concentrations continue to increase with general agreement that CO2 levels

will increase to near 450mmolmol�1 (ppm) over the next 50 years [65]. Since cropwater use (leaf transpiration, LEl) is determined by crop physiological andmorphological characteristics [66], and is described by Equation (1.2), from whichan assessment of the role changes in leaf stomatal aperture and conductance forwater vapor loss, vapor pressure gradient between the ambient air and substomatalcavity, and canopy morphology and plant size can be quantified.The coupling between canopy growth and water use throughout the season is

dependent upon the rate of growth, and the atmospheric and soil conditions.Changes in the canopy size and increases in leaf area are proportional to growthrate and transpiration [67]. When plants begin to develop sufficient canopy sizewith an increase in mutual shading within a plant canopy, transpiration begins toincrease at a diminishing rate with increasing leaf area index (LAI) and approachesan asymptotic plateau with LAI > 4m2m�2, causing a decoupling of transpirationfrom changes in LAI [67–69]. One of the effects of a projected doubling ofatmospheric CO2 from present-day levels will increase average C3 species growthon the order of 30% under optimum conditions [70–73] with the expectation thatconcentrations near 450 mmolmol�1 would increase C3 plant growth on the orderof 10%. Increases in growth can lead to an increase in the duration of leaf area,which will directly affect total seasonal crop water requirements. Crops or varietiesadapted to the higher temperatures and plants with an extended growing seasonwill increase the overall crop water use with no change in any of the physiologicalparameters. However, a direct effect of increasing atmospheric CO2 is to inducestomatal closure, causing a decrease in the rate of water vapor transfer from thecanopy. Reduced stomatal conductance affects water vapor transfer more thanphotosynthesis because changes in stomatal conductance are the major factorcontrolling transpiration. Observations from chamber-based studies evaluating theeffects of elevated CO2 on stomatal conductance have shown that doubling CO2

reduces stomatal conductance by nearly 34% [74]. There have been somedifferences observed between C3 and C4 species. Morison [75] found an averagereduction of nearly 40%, while Wand et al. [76] observed across multiple studies onwild C3 and C4 grass species, grown with no stresses, elevated CO2 reducedstomatal conductance by 39% in C3 and 29% in C4 species. Significant differencesin stomatal conductance of two C3 and C4 species were found from free-air CO2-enrichment experiments where daytime CO2 concentrations were increased frompresent concentrations to 550–600 mmolmol�1. Ainsworth and Long [77] did notobserve significant differences in stomatal conductance of two C3 and C4 specieswhen they summarized results from free-air CO2-enrichment experiments wheredaytime CO2 concentrations were increased from present concentrations to 550–600 mmolmol�1 with an average reduction in stomatal conductance of 20%. Insoybean, a doubling of CO2 created a reduction in conductance of 40% [78,79].


Increases in atmospheric CO2 concentration to nearly 450 mmolmol�1 as estimated[65] by 2040–2050 will likely cause reductions of approximately 10% in stomatalconductance. The magnitude of these CO2 increases with their resultant effect onstomatal conductance, when considered in terms of the energy balance in thewhole canopy, should lead to decreases in transpiration and potentially positiveimpacts on WUE.Increasing CO2 effects on stomatal conductance will increase water conservation

at the leaf level; however, these effects may not be as evident at the canopy orecosystem scale [80]. Elevated CO2 has been observed to increase ET from canopies[81]. There are compensatory effects that occur as a result of increased foliagetemperature caused by the reduction in conductance and the increased leaf areadue to CO2 enrichment, leading to negligible to small changes in ET [82].Observations from soybean grown in controlled environment chambers underambient and doubled CO2 exhibited a 12% reduction in seasonal transpiration and51% increase in WUE [83]. In controlled environment chambers there has been anincrease in canopy temperatures (e.g., 1–2 �C (soybean), 1.5 �C (dry bean), and 2 �C(sorghum)) to doubled CO2 [82,84–86]. For different crops grown under increasedCO2 there has been a decrease in transpiration (e.g., wheat (8% [87]; 4% [88,89]),cotton (8% [90]; 0% [91]), soybean (12% [92]), and rice (15% [93])). Increases in airtemperature will further offset the positive impacts of increased CO2 withobservations at 24–26 �C showing an increase in rice WUE of 50%, declining as airtemperature increased [93]. These observations illustrate that changes in WUE arepossible under a changing climate; however, what is not understood is the linkageamong physical variables (e.g., air temperature, wind speed, vapor pressure deficit)and physiological variables (e.g., stomatal conductance, photosynthetic rates, respira-tion) to be able to understand the interactions among these variables to determine themost viable approach to enhance WUE from a physiological and genetic basis.There are a few studies beginning to emerge that have compared different

genetic material for their WUE. Van den Boogaard [64] observed significantdifferences in WUE between two wheat cultivars and also observed that WUEincreased with decreased water supply under a high nitrogen treatment in their potstudy. Baodi et al. [94] utilized a combination of statistical methods and pathanalysis to evaluate the relationship between leaf WUE and physiobiochemicaltraits for 19 wheat genotypes. Their measurements included photosynthesis rate,stomatal conductance, transpiration rate, intercellular concentration of CO2, leafwater potential, leaf temperature, wax content, leaf relative water content, rate ofwater loss from excised leaf, peroxidase, and superoxide dismutase activities.Photosynthesis rate, stomatal conductance, and transpiration rate were the mostimportant leaf WUE variables under natural rainfall conditions. They concludedselections for high leaf WUE wheat under natural rainfall could be obtained byselecting breeding lines with a combination of high photosynthesis rate, lowtranspiration rate, and low stomatal conductance. One of the major challenges willbe develop effective methods to compare genetic material for WUE and thephysiological and genetic basis for differences in order to develop improved plantresources capable of responding to climate change.


Modifying plant resources is only a part of the way toward increasing WUE.There is the potential for increased plant productivity through enhanced CO2

concentrations; however, there have not been detailed studies that have demon-strated if increased growth translates into increased grain or fruit yield. The effortshave concentrated on annual crops and there is little information on how perennialor vegetable crops respond in terms of yield under increased CO2. This leaves someuncertainty as to the amount of change in the Y term in Equation (1.4) to assesswhether WUE could be enhanced through genetic selection or crop management.This also leaves the ET term in Equation (1.4) under some uncertainty due toclimate change because ET is dependent upon a combination of factors: soil wateravailability, atmospheric water vapor demand, and the plant species. Hatfield et al.[95] found both spatial and temporal variation in ET and CO2 exchange for corn andsoybean across central Iowa with variations due to differences in atmosphericconditions, rainfall distribution from convective storms, and soil water-holdingcapacity. One of the major determinants to soil water-holding capacity is the soilorganic matter (SOM) content and Hudson [96] found a linear relationship betweenthese variables.A critical parameter in WUE will be the availability of soil water to the plant; the

uncertainty in the precipitation amounts under climate change means there is theprojection for greater extremes in precipitation events [65]. Increased uncertainty inprecipitation amounts throughout the growing season will create scenarios inwhich the available soil water may not be adequate for optimum plant growth.Coupling the variation in precipitation with the increased atmospheric demand forwater from the plant will cause the likelihood of water stress in the plant toincrease. One avenue for increasing WUE would be to increase the available waterfor transpiration (T ) and decouple the evaporation (E ) component out of ET. Thiscan be achieved through the use of mulches to create a surface layer on the soil toreduce the energy impinging onto the soil. This effort was reviewed in Hatfieldet al. [21] and this could have an impact early in the growing season when there isless than complete ground cover and the soil surface is exposed to direct sunlight.Later in the growing season under complete canopy there is less energy at the soilsurface to drive the evaporation process at the surface; however, mulches stillremain effective in reducing soil water evaporation.The critical factor in increasing or maintaining WUE under climate change will be

to increase the amount of water available within the soil profile. Variation in cropproduction across fields has been related to soil water availability during the grain-filling period [97]. Variations of yields across fields have been associated withenhanced soil erosion and soil water availability, and high-yielding managementzones in a corn/soybean rotation were associated with poorly drained level soil types,whereas low-yielding zones were associated with eroded soil or soil on more slopingareas [98]. The difference in soil water-holding capacity among soils was the primaryfactor affecting the total seasonal water patterns in maize and if we extrapolate theresults from [96], then removal of organic matter from soil will reduce soil water-holding capacity. This reduction in organic matter content is a result of soildegradation and throughout the world this problem has not been associated with the


potential consequences of climate change. Lal [99] observed throughout the world,and especially in the tropics and subtropics, soil degradation is a major threat toagricultural sustainability and environmental quality. After a recent survey, Nyssenet al. [100] reported that nearly all of the tropical highlands (areas above 1000mabove sea level covering 4.5 million km2) are degraded due to medium to severewater erosion. Conversion of farmland from the original pasture in the Horquinsands caused significant decreases in crop yield and poorer soil properties afterconversion to cropland [101]. A consequence of population increase is the demandfor food and Kidron et al. [102] suggested that abandoning the traditional practice of10–15 years of cultivation followed by 10–15 years of fallow with a continuouscropping practice increased the rate of soil degradation in Mali, West Africa. SOMcontent displayed the strongest relationship to soil degradation and soil managementpractices that accelerated the removal of SOM increased the rate of soil degradation.Observations throughout the world would suggest that soil degradation is occurring;however, there has been little attention given to the linkage between soil degradationand susceptibility to climate change. Wang et al. [103] observed that differences insoil structure and saturated hydraulic conductivity were related to cropping systemsand degradation of soil structure throughout the soil profile caused maize yieldreductions as large as 50%. This decrease in yields was attributed to the shallow rootgrowth and limitations in water availability to the growing plant during the growingseason. Impacts of poor soil structure on plant growth and yield can be quite large,and continued degradation of the soil resource will have a major impact on theability of the plant to produce grain, fiber, or forage.Intensive cultivation for over 50 years in the subhumid and semi-arid Argentinean

Pampas resulted in soil degradation leading to moderate to severe erosion [104]. Insouthern Brazil, severe soil degradation was attributed to the widespread use ofwheat/soybean or barley (Hordeum vulgare L.)/soybean double-cropping systemscoupled with intensive tillage [105]. Soil degradation is not isolated to the subtropics;in the maritime climate of the Fraser Valley in British Columbia with over 1200mmof annual rainfall, conventional tillage over a number of years has contributed topoor infiltration, low organic matter content, and poor soil structure [106].Mechanical tillage resulted in a loss of SOM leading to soil degradation across

southern Brazil and eastern Paraguay [107]. The conversion of semideciduous foreststo cultivated lands creates the potential for soil degradation and proper managementwill be required to avoid further degradation. Degradation of the soil resource occursin many different forms. In Nepal, Thapu and Paudel [108] observed watershedsseverely degraded from erosion on nearly half of the land area in the upland cropterraces. This degradation was coupled with depletion of soil nutrients, which in turnaffects productivity. In Ethiopia, Taddese [109] observed that severe land degradationcaused by the rapid population increase, severe soil erosion, low amounts ofvegetative cover, deforestation, and a lack of balance between crop and livestockproduction threatens the ability to produce an adequate food supply for the country.Soil management and climate change have not been closely linked, in their

analysis for India, Ortiz et al. [61] proposed that no-till systems that would reducesoil evaporation and prevent soil erosion would have the potential to maintain


production for wheat systems under climate change. Improving SOM andmanaging soil fertility as methods to increase the capacity of the soil to store andretain water for crop use would provide an advantage to increase the efficiency ofprecipitation for crop production. Linking crop production, soil management, andclimate change as a system to evaluate where cropping systems could become moreresilient to climate change would stabilize production and also provide for futureincreases in production because of the enhanced WUE.

1.5

Major Challenges

There are no simple solutions to determining the optimum pathway for geneticmanipulation or agronomic management of plants to adjust to climate change. Therewill be continued exposure of plants to conditions beyond their range of optimumtemperatures and the simple solution will be to change the geographical distributionof plants to accommodate these changing temperature regimes; however, exposureof plants to the extreme events that are more likely to occur presents a differentchallenge to management. The major challenges to be addressed can be divided intotwo categories: growth and development processes linked to WUE, and growth anddevelopment processes linked to quality of the forage or grain.

1.5.1

Growth and Development Processes and WUE

Growth and development of crops is driven by climate change, and one critical aspectis the interaction of increasing CO2, increasing temperatures, and soil wateravailability. Current research has often focused on the primary effects of each ofthese variables on different species; however, there are only a few studies that haveaddressed the interactions among these variables [72]. One of the critical componentslacking in the current studies is the comparison of genetic material exposed todifferent climate change parameters, and the linkage with geneticists and molecularbiologists to quantify where genetic improvements to temperature and water stresscan be made. There are emerging observations that reveal the complexity of theresponse of plants to a changing climate. The observations of Castro et al. [110]revealed that in soybean exposed to increased CO2 in a free-air carbon-enrichment(FACE) study there was a delay in the onset of the reproductive development by 3days in spite of having warmer canopy temperatures. The assumption has been thatthe warmer canopy temperatures were linked to the hastening of the phenologicalstages; however, these relationships may not be consistent across all observations.The linkage among available soil water, temperature, and CO2 across genetic

material presents a challenge to both experimentalists and crop modelers. Develop-ment of plants that are resilient to stresses will require a multidisciplinary approachin order to quantify these interactions and interpret the meaning of these responses.Understanding how to enhance WUE across different species and climate stresses

1.5 Major Challenges 17

will provide a benefit for all of humankind in terms of being able to developgermplasm and production systems capable of withstanding the climate stresses.

1.5.2

Growth and Development Processes Linked to Quality

One of the emerging challenges will be to understand and quantify the impacts ofchanging climate on forage and grain quality. There have been observations on theinteracting effects among increasing CO2, nitrogen, and water. Luo et al. [111]developed their “progressive nitrogen limitation” hypothesis to demonstrate thelinkage between CO2 enrichment and reduced plant-available nitrogen through theincreasing plant demand for nitrogen. Morgan [112] had previously shown that therewas a relationship cycling of organic matter in the soil, CO2 uptake by the plant, andthe stimulation of plant growth by increased CO2 leading to a decrease in thenitrogen uptake and a potential decrease in nitrogen content in plants. Observationsof cattle fecal chemistry confirm the proposals by Morgan [112] and Luo et al. [111]that the effects of increased CO2, increased temperature, and decreased rainfall haveresulted in a general decline in forage quality [113]. The effects of climate changeand forage quality and plant composition in different rangelands was reviewed byIzaurralde [114]. In addition to production quantity, the quality of agriculturalproducts may be altered by elevated CO2. Some non-nitrogen-fixing plants grown atelevated CO2 have shown reduced nitrogen content [77], and since nitrogen is acritical agricultural crop nutrient, there are implications for the potential interactionsof climate change and nutrient management of forage and grain crops.Interaction between nitrogen status in plants and grain quality in wheat showed

that low nitrogen reduced grain quality with the effect on grain quality increased byexposure to high CO2 concentrations [115]. Observations from a study of CO2

enrichment and nitrogen management on grain quality in wheat and barley (H.vulgare) showed increasing CO2 to 550 mmolmol�1 with two rates of nitrogen (i.e.,adequate and half-rate of nitrogen) affected crude protein, starch, total and solubleb-amylase, and single kernel hardiness [116]. Increasing CO2 concentrationsreduced crude protein by 4–13% in wheat and 11–13% in barley, but increasedstarch by 4% when half-rate nitrogen was applied. Their conclusions from thisstudy were that nutritional and processing quality of flour will be diminished forcereal grown under elevated CO2 and low nitrogen fertilization [116]. There hasbeen a steady decline in grain protein from 1967 to 1990 in wheat in Australia,although this change cannot be specifically linked to rising CO2 [117]. In fruit trees,leaves grown under elevated CO2 had about 15% lower nitrogen concentration onaverage [118–121]. Overall, these studies suggest nutrient status in plants and soilsinteract with changing CO2 concentrations; however, a more specific under-standing of the interactions of increasing CO2 and temperatures with nitrogenmanagement across all different plant species remains to be developed.Quality changes are not isolated to changes in grain quality, Pettigrew [122]

observed in cotton grown under high temperatures there was a decline in lint yieldwith increasing temperatures along with a change in lint quality. Climate effects on


quality has been found in a number of perennial crops. For example, in apples(Malus pumila), high temperatures in the spring can reduce cell division, resultingin small fruit, while during the summer months, high temperature may causesunburn damage, accelerate maturity, reduce fruit firmness and color develop-ment, and/or decrease the suitability of fruit for short- or long-term storage[123,124]. In strawberry (Fragaria ananassa), too much light coupled with hightemperatures leads to the development of fruit bronzing (damaged fruit that isbronze in color and may be desiccated or cracked on the surface) [125].

1.6

Grand Challenge

There are no simple solutions to this very complex problem. There are variationsacross species and within species in their response to changing CO2, decreasedwater availability, and increasing temperatures. Our challenge will be to begin toassemble research teams across a range of disciplines with the capability of deve-loping new approaches to measuring plant response to the different climate stressesand treat this information with new imaginative insights in order to advance sciencetowards new frontiers of quantifying how we can cope with climate change.

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