Climate Change and Plant Abiotic Stress Tolerance

By Narendra Tuteja and Sarvajeet S. Gill

In this ready reference, a global team of experts comprehensively cover molecular and cell biology-based approaches to the impact of increasing global temperatures on crop productivity.
The work is divided into four parts. Following an introduction to the general challenges for agriculture around the globe due to climate change, part two discusses how the resulting increase of abiotic stress factors can be dealt with. The third part then outlines the different strategies and approaches to address the challenge of climate change, and the whole is rounded off by a number of specific examples of improvements to crop productivity.
With its forward-looking focus on solutions, this book is an indispensable help for the agro-industry, policy makers and academia.

• Language: English
• Publisher: Wiley
• Release date: Oct 30, 2013
• ISBN: 9783527675258

Book preview

Part One

Climate Change

1

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 to higher temperatures throughout their life cycle and also increase the atmospheric demand for water vapor, adding to the stress because of the increased rate of crop water use. Coupling the effect of temperature with a more variable precipitation pattern creates a combination of temperature and moisture stress on 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. One positive aspect of climate change is that rising carbon dioxide increases the rate of photosynthesis and also decreases the rate of transpiration, leading to increased WUE. Our challenge will be to determine how to extrapolate these effects to whole canopies and into management systems that take advantage of this effect. The changing climate will not only affect growth and development of plants, but also the quality of the product. In evaluating the effect of climate on plants we need to include the direct effects of perennial plants because adaptation strategies for these production systems will be more complex than in annual crops. To ensure an adequate food and feed supply required to meet the needs of 9 billion people requires a transdisciplinary approach to develop innovative strategies to manage our crop production systems to reduce or eliminate the impact of climate change.

1.1 Introduction

Climate has always impacted agriculture throughout the world. The quest for a stable food supply and the ability to feed the family has prompted many innovations in terms of cropping systems, crop selection, and cultivation practices. Our modern world is confronted with two unique challenges during the twenty-first century: an increasing global population with an ever-increasing demand for food and increasing climate variability. Climate change is occurring at rates never experienced before by modern agriculture and will place constraints on our capabilities to continue to observe the increasing trend in grain production around the world. Hatfield [1] showed that the increasing trends in grain production across the United States were offset by climate stresses (variation in precipitation) during the growing season, while Lobell et al. [2] observed that grain production levels around the world were already being affected by warming temperatures. These variations in climate are already impacting crop production, and the important question is what will be the future impact of our changing climate and what adjustments will crops make to cope with these changes?

The challenge for agriculturalists will be to adjust crop production to the changing climate to cope with increasing temperatures, more extreme events in temperature, more variable precipitation, reduced solar radiation through increased cloudiness, increased evaporative demand, and increased carbon dioxide (CO2). In this chapter, we will explore the potential adjustments crops will have to make to cope 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 μmol mol−1 by 2050 [3] throughout the world 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 of CO2 on plants. A future challenge for crop production caused by increasing CO2 is the observation that fast-growing species (e.g., weeds) are responding with greater growth 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 under estimates of reduced emissions of greenhouse gases. These changes will not be uniform throughout the world, with some regions showing increased warming more than others. General statements on the changes in climate expected have been provided by Meehl et al. [5], in which they state that heat waves are projected to become more intense, more frequent, and last longer than what is being experienced in the world today. These heat waves would have short-term durations of a few days with temperature increases of over 5 °C above the normal temperatures. It would be the summer period in which these heat waves would have the most dramatic impact. Their projections revealed that daily minimum temperatures will increase more rapidly than daily maximum temperatures, leading to a increase in the daily mean temperatures. Plants will be subjected to conditions in which the nighttime temperatures will be warmer and this will affect respiration rates more than photosynthetic rates.

Variability in precipitation is expected to increase over the next decades [5]. Using the current ensemble of climate models, the prediction is for precipitation to generally increase in the areas of regional tropical precipitation maxima (e.g., the monsoon regimes) and over the tropical Pacific in particular, with general decreases in the subtropics and increases at high latitudes as a consequence of a general intensification of the global hydrological cycle. This will lead to increases in the global average mean water vapor, evaporation, and precipitation. Coupled with these increases in precipitation is increased variation among seasons and among years. The intricate feedbacks between the land surface and precipitation will create conditions in which convective storms may decrease because of lack of evaporation from the surface. Agriculture will have to contend with this increased variation as part of the production system. Precipitation changes will be the most difficult to predict in long-term climate scenarios; however, the expectation of increased variation in precipitation among years, shifts in precipitation totals, and increased intensity in precipitation events creates a general statement that precipitation will become an increasing unknown in terms of agricultural systems; since agricultural production is dependent upon adequate and timely water supplies, small changes could 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 for use by crops. Soil water-holding capacity varies among soils from sandy soils with 1 millimeter of available water per centimeter of soil to clay soils with 2 millimeters of available water per centimeter of soil. This is further complicated by the seasonality of the precipitation patterns and the crop being grown at a given site. As an example, a Mediterranean climate with precipitation during the winter months would not be able to supply adequate soil water for a summer crop because of the inability of the soil to supply all of the water required 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 there would a decrease in incoming solar radiation. Observations from a global study by Stanhill and Cohen [6] showed there has been a 2.7% reduction per decade during the past 50 years, with the current solar radiation totals reduced by 20 W m−2, which these authors refer to as global dimming. In a more recent study across the United States, Stanhill and Cohen [7] found that after 1950 there has been a decrease in sunshine duration, with more sites in the Northeast, West, and Southwest showing decreases. They suggested that more detailed solar radiation records will be required to quantify the temporal changes in solar radiation related to cloudiness and aerosols. Reduction in solar radiation in agricultural areas in the last 60 years as revealed by models [8] is projected to continue [9] due to increased concentrations of atmospheric greenhouse gases and the feedbacks from atmospheric scattering. A recent study on solar radiation by Medvigy and Beaulieu [10] examined the variability in solar radiation around the world, and concluded there was an increase in solar radiation variability that was correlated with increases in precipitation variability and deep convective cloud amount. There will be changes in the solar radiation resources 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 a direct function of air temperature, then as the temperature increases, atmospheric demand will increase. One of the major components in the atmospheric demand for water vapor is the saturation vapor pressure which can be estimated as follows as proposed by Buck [11]:

(1.1) equation

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

(1.2) equation

where λE ta is the latent heat flux from the canopy (W m−2), λ is the latent heat of vaporization (J kg−1), Δ is the slope of saturation deficit curve (kPa C−1), γ is the psychrometric constant (kPa C−1), R n is the net radiation (W m−2), G is the soil heat flux (W m−2), mρCp m is the molecular weight of air (kg mol−1), ρ is the density of air (kg m−3), Cp is the specific heat of air J kg−1 C e *(z) is the saturation vapor pressure at height z, e (z) is the actual vapor pressure at height z, r ah is the aerodynamic conductance for sensible heat transfer (m s−1), r av is the aerodynamic conductance for water vapor transfer (m s−1), and r c is the canopy conductance to water vapor transfer (m s−1). This approach was originally described by Monteith [12] and is one of the most applied equations for crop water use today. Changing atmospheric demand as part of the change in climate patterns will have a direct impact on a plant's ability to withstand 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 of annual yields as demonstrated by a number of previous analyses. Interannual variations in crop yields have been related to precipitation patterns [13] and temperature [2]. The trends in crop yields demonstrate the capacity of technology to continue to develop crop varieties; however, the primary question is how well 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. To continue to increase productivity there will have to be adjustments in crops to cope with these changes in the environment and these will be discussed throughout 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 to climate change. However, the challenge will remain as to how we begin to understand the dynamics of crop response to climate parameters both in the short and long term. In this chapter we will assess how different climate parameters will have 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 the temperature ranges of the specific crop being studied. The temperature ranges for different crops have been summarized [1]. The general consensus observed for annual crops is that rising temperatures will increase the rate of development, causing smaller plants. Since the harvest index (grain yield/total biomass) is relatively constant within a given species, this will lead to reduced grain yield. This is the essence of the conclusion arrived at by Lobell et al. [2]. The projected increases in air temperature throughout the remainder of the twenty-first century shows that grain yields will continue to decrease for the major crops because of the increased temperature stress on all major grain crops [1]. Beyond a certain point, higher air temperatures adversely affect plant growth, pollination, and reproductive processes [14,15]. However, as air temperatures rise beyond the optimum, instead of falling at a rate commensurate with the temperature increase, crop yield losses accelerate. For example, 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 and then 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 species throughout the twenty-first century [1]. Other evaluations of temperature on crop yield have produced varying outcomes. Lobell et al. [2] showed estimates of yield decline between 3.8% and 5%, and Schlenker and Roberts [16] used a statistical approach 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 high emission scenarios. A limitation in their approach was the lack of incorporation of the effects of rising atmospheric CO2 on crop growth, variation among crop genetics, 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 temperature changes may be of greater importance for their effect on growth and phenology [1]. Minimum temperatures are more likely to be increased by climate change over broad geographic scales [17], while maximum temperatures are affected by local conditions, especially soil water content and evaporative heat loss as soil water evaporates [18]. Hence, in areas where changing climate is expected to cause increased rainfall or where irrigation is predominant, large increases of maximum temperatures are less likely to occur than will be the case in regions where drought is prevalent. Minimum air temperatures affect the nighttime plant respiration rate, and can reduce biomass accumulation and crop yield [1]. Even as climate warms and minimum average temperatures increase, years with low maximum temperatures may more frequently be closer to achieving the temperature optimum, which will result in higher yields than is the case today during years when average temperatures are below the optimum. Welch et al. [19] found this to be the case for a historical analysis of rice in Asia – higher minimum temperatures reduced yields, while higher maximum temperature raised yields; notably, the maximum temperature seldom reached the critical optimum temperature for rice. As future temperatures increase, they found maximum temperatures could decrease yields if 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 to temperatures above 35 °C [20–22]. There is an interaction of temperatures and vapor pressure deficit during the duration of pollen viability (prior to silk reception) because pollen viability is a function of pollen moisture content, which is strongly dependent on vapor pressure deficit [23]. Temperatures of 35 °C compared to 30 °C during 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 in maize 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; for example, in rice (Oryza sativa L.) pollen viability and production declines as the daytime maximum temperature (T max) exceeds 33 °C and becomes zero when exposed to T max above 40 °C [26]. Current cultivars of rice flower near mid-day, which makes T max a good indicator of heat stress on spikelet sterility. These exposure times occur very quickly after anthesis and exposure to temperatures above 33 °C within 1–3 h after anthesis (dehiscence of the anther, shedding of pollen, germination of pollen grains on stigma, and elongation of pollen tubes) causes negative impacts on reproduction [27]. Current observations in rice reveal that anthesis occurs between about 9 to 11 a.m. in rice [28], and exposure of rice to high temperatures may already be occurring and increase in the future. There is emerging evidence that differences exist among rice cultivars for flowering time during the day [29]. Given the negative impacts of high temperatures on pollen viability, there are recent observations from Shah et al. [30] suggesting flowering at cooler 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 a valuable phenotypic marker for high-temperature tolerance. A recent study on soybean (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 °C regardless of the night temperatures and concluded that daytime temperatures were the primary influence on pod set [32]. These studies have implications for the development of hybrid soybean, but provide even more understanding of the role that 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 specific stresses, such as periods of hot days, overall growing season climate, minimum and maximum daily temperatures, and timing of stress in relationship to developmental stages, having the greatest effect [33–37]. When plants are subjected to mild heat stress (1–4 °C above optimal growth temperature), there was moderately reduced yield [38–41]. In these plants, there was an increased sensitivity to heat stress 7–15 days before anthesis, coincident with pollen development. Subjecting plants to a more intense heat stress (generally greater than 4 °C increase over optimum) often resulted in severe yield loss up to and including complete failure of marketable produce [33,41–44]. There is evidence that temperature effects on yield loss vary among crops and among cultivars within crops. Tomatoes under heat 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 same stressed plants are cooled to normal temperatures for 10 days before pollination and 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 to successfully 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 than the air temperature. This was first observed by Tanner [46] who noticed that the leaf temperature differed from the air temperature, and further expanded by Wiegand and Namken [47] to relate leaf temperature to plant water status. Throughout the last 50 years there have been continuing refinements in the use of leaf or canopy temperatures 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 the evaporative cooling induced by transpiration from the leaf surface. However, as availability of soil water decreases the leaf temperature more closely tracks the air temperature and under extreme water stress can exceed the air temperature. The dynamics of this process were reviewed in Hatfield et al. [48] and approaches to quantifying 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 thermal stability of metabolic enzymes [49]. It was proposed that plant leaves have an optimum temperature and shown that in cotton (Gossypium hirsutum L.) carbon assimilation as measured with leaf photosynthetic chambers peaked at a canopy temperature, T c, of 29 °C while the fluorescence assay was optimal between 28 °C and 30 °C, and yield was maximum at 26 °C [50]. When the T c values exceeded 28 °C there was a decrease in yield. These findings have implications under climate change due to the increasing air temperatures and more variability in soil water availability. These linkages were more fully developed in [51] where the authors described both the energy balance responses at the leaf level and canopy level to changing air temperatures. Under well-watered conditions plants would be able to maintain leaf temperatures near their optimal range; however, as soil water becomes limiting then leaf temperatures would no longer be maintained within a plant's optimal range. The concept of the thermal kinetic window would also provide an explanation for why changing maximum air temperatures have less of an impact on plant growth and development compared to minimum air temperatures. During the day, increasing maximum temperatures would be moderated by increasing crop water use, which in turn would maintain leaf temperatures within a given range; however, at night with no transpiration occurring leaf temperatures would be in equilibrium with the air temperature because the only energy exchange will be long-wave radiation. If we develop this further in mathematical form as shown in the following equation then these relationships demonstrate the linkages between canopy and air temperature:

(1.3)

equation

where S t is the incoming solar radiation (W m−2), α l is the albedo of the leaf or canopy, L d is the incoming long-wave radiation (W m−2), is the emissivity of the leaf or canopy, σ is the Stefan–Boltzmann constant, T l is the leaf or canopy temperature, r a is the aerodynamic conductance (m s−1), and r s is the canopy conductance resistance (m s−1). During the night and with stomata closed, the right-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 they observed differences in canopy versus air temperatures during the day induced by stomatal closure, but no difference at night. During the night, leaf respiration will continue and will be a direct function of the air temperature; as temperature increases there will be an increase in the respiration rate. This is an aspect of plant response 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 be exposed to a number of hours below some threshold before flowering can occur. For example, chilling hours for apple (Malus domestica Borkh.) ranges from 400 to 2900 h (5–7 °C base) [53], while cherry trees (Prunus avium) require 900–1500 h with the same base temperature [54]. Grapes (Vitis vinifera L.) have a lower chilling threshold that other perennial plants with some varieties being as low at 90 h [55]. Increasing winter temperatures may prevent chilling hours from being obtained and projections of warmer winters in California revealed that by mid-twenty-first century, plants requiring more than 800 h may not be exposed to sufficient cooling except in very small areas of the Central Valley [56]. Climate change will impact the chilling 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 are dependent upon the species grown. Exposure to high temperatures, above 22 °C, for apples during reproduction increases the fruit size and soluble solids, but decreases firmness as a quality parameter [57]. In cherries, increasing the temperature 3 °C above the mean decreased fruit set, when the optimum temperature 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 optimum temperature range is 22–27 °C [59]. During fruit development when the temperatures exceed the optimum range of 13–27 °C with temperatures over 33 °C there is a reduction in Brix, acid content, and reduced fruit size in citrus [60]. Temperature stresses on annual and perennial crops have an impact on all phases of plant growth 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 increasing temperatures around the globe. As an example of the potential impacts of changing temperatures, 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 impacts of increasing temperatures on crop production and potentially viable adaptation strategies that could be implemented to avoid crop production declines. Projecting the impacts of increasing temperatures will identify areas and cropping systems with potential vulnerabilities to changing climate. This is an exercise that would provide an overview of anticipated problems; however, it does not derive a very detailed view of the potential physiological and genetic adjustments required to develop more robust 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 of the first is the differences among germplasm in their temperature responses during the vegetative and reproductive phases of development. One aspect that has not been examined very clearly is the recovery of plant physiological responses to temperature stresses, and the relationship between air temperature and canopy temperature relative to the physiological responses. Understanding these basic processes will advance our understanding of how plants react to temperature changes. Development of efforts to link physiologists with molecular biologists and agricultural meteorologists would be valuable to help quantify temperature effects on plant growth and development.

1.4 Water Responses

Water availability to plants is critical to physiological functions within plants, and the linkage between precipitation patterns and soil water-holding capacity governs the potential response to climate change. One of the biggest challenges facing crop production emerges when more variable precipitation throughout the growing season is coupled with a degraded soil with limited water-holding capacity. While precipitation changes and the uncertainty in precipitation amounts are a major focus in climate change, the critical role soil plays in the infiltration, storage, and release of water to growing plants is generally overlooked. The overall challenge confronting agriculture is how to increase water-use efficiency (WUE) in production 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 insights into 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 on WUE and an earlier review by Tanner and Sinclair [63] examined the principles underlying WUE. The basic equation for WUE (kg ha−1 mm−1) is:

(1.4) equation

where Y is the crop yield (kg ha−1), and ET is the crop water use as a combination of transpiration from the leaves and evaporation from the soil surface (mm); there can be a change in WUE either through changes in Y or ET. This approach describes the canopy level process in which there are techniques available to measure ET. At the leaf level, the dynamics of the system become more insightful in terms of explaining the linkages between the physiological reactions and the physical environment because we can relate physiological parameters to CO2 uptake and leaf transpiration as a simple expression of leaf CO2 exchange relative to transpiration. Other methods include measurement of differences in RuBisCO activity or rate of electron transport [64]. These physiological studies provide some insights into plant responses to environmental changes. If we examine WUE at the leaf level then we can express these relationships as:

(1.5) equation

where WUE l is the WUE at the leaf level, P l is the photosynthetic rate (mg CO2 m−2 s−1), LE l is the evaporation rate from a leaf (mg H2O m−2 s−1), f is the conversion of CO2 from ppm to g cm−3 (1.67 × 10−9), [CO2] is the ambient CO2 concentration, r a is the aerodynamic conductance for an individual leaf (m s−1), r s is the stomatal conductance (m s−1), r m is the mesophyll conductance (m s−1), ρ a is the density of air (g m−3), L is the latent heat of vaporization, is the ratio of molecular weights of water vapor and air, P a is the atmospheric pressure (kPa), e l is the vapor pressure of the leaf at leaf temperature, and e a is the actual vapor pressure of the air surrounding the leaf. The gradient of water vapor between a leaf and the atmosphere is affected by the internal leaf water vapor pressure (e; kPa) which is tightly coupled to leaf temperature (T; °C) and can be calculated from Teten's equation, e = 0.610 78 × exp(17.269 × T /(T + 237.3)). Factors affecting energy balance and leaf or canopy temperature 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 of these approaches is to be able to more completely understand the relationships between productivity and water use, and how these may be affected by climate change.

CO2 concentrations continue to increase with general agreement that CO2 levels will increase to near 450 μmol mol−1 (ppm) over the next 50 years [65]. Since crop water use (leaf transpiration, LE l) is determined by crop physiological and morphological characteristics [66], and is described by Equation (1.2), from which an assessment of the role changes in leaf stomatal aperture and conductance for water vapor loss, vapor pressure gradient between the ambient air and substomatal cavity, 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 growth rate and transpiration [67]. When plants begin to develop sufficient canopy size with an increase in mutual shading within a plant canopy, transpiration begins to increase at a diminishing rate with increasing leaf area index (LAI) and approaches an asymptotic plateau with LAI > 4 m² m−2, causing a decoupling of transpiration from changes in LAI [67–69]. One of the effects of a projected doubling of atmospheric CO2 from present-day levels will increase average C3 species growth on the order of 30% under optimum conditions [70–73] with the expectation that concentrations near 450 μmol mol−1 would increase C3 plant growth on the order of 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 varieties adapted to the higher temperatures and plants with an extended growing season will increase the overall crop water use with no change in any of the physiological parameters. However, a direct effect of increasing atmospheric CO2 is to induce stomatal closure, causing a decrease in the rate of water vapor transfer from the canopy. Reduced stomatal conductance affects water vapor transfer more than photosynthesis because changes in stomatal conductance are the major factor controlling transpiration. Observations from chamber-based studies evaluating the effects of elevated CO2 on stomatal conductance have shown that doubling CO2 reduces stomatal conductance by nearly 34% [74]. There have been some differences observed between C3 and C4 species. Morison [75] found an average reduction of nearly 40%, while Wand et al. [76] observed across multiple studies on wild C3 and C4 grass species, grown with no stresses, elevated CO2 reduced stomatal conductance by 39% in C3 and 29% in C4 species. Significant differences in stomatal conductance of two C3 and C4 species were found from free-air CO2-enrichment experiments where daytime CO2 concentrations were increased from present concentrations to 550–600 μmol mol−1. Ainsworth and Long [77] did not observe significant differences in stomatal conductance of two C3 and C4 species when they summarized results from free-air CO2-enrichment experiments where daytime CO2 concentrations were increased from present concentrations to 550–600 μmol mol−1 with an average reduction in stomatal conductance of 20%. In soybean, a doubling of CO2 created a reduction in conductance of 40% [78,79]. Increases in atmospheric CO2 concentration to nearly 450 μmol mol−1 as estimated [65] by 2040–2050 will likely cause reductions of approximately 10% in stomatal conductance. The magnitude of these CO2 increases with their resultant effect on stomatal conductance, when considered in terms of the energy balance in the whole canopy, should lead to decreases in transpiration and potentially positive impacts 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 or ecosystem scale [80]. Elevated CO2 has been observed to increase ET from canopies [81]. There are compensatory effects that occur as a result of increased foliage temperature caused by the reduction in conductance and the increased leaf area due to CO2 enrichment, leading to negligible to small changes in ET [82]. Observations from soybean grown in controlled environment chambers under ambient and doubled CO2 exhibited a 12% reduction in seasonal transpiration and 51% increase in WUE [83]. In controlled environment chambers there has been an increase 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 increased CO2 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 air temperature will further offset the positive impacts of increased CO2 with observations at 24–26 °C showing an increase in rice WUE of 50%, declining as air temperature increased [93]. These observations illustrate that changes in WUE are possible under a changing climate; however, what is not understood is the linkage among physical variables (e.g., air temperature, wind speed, vapor pressure deficit) and physiological variables (e.g., stomatal conductance, photosynthetic rates, respiration) to be able to understand the interactions among these variables to determine the most 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 significant differences in WUE between two wheat cultivars and also observed that WUE increased with decreased water supply under a high nitrogen treatment in their pot study. Baodi et al. [94] utilized a combination of statistical methods and path analysis to evaluate the relationship between leaf WUE and physiobiochemical traits for 19 wheat genotypes. Their measurements included photosynthesis rate, stomatal conductance, transpiration rate, intercellular concentration of CO2, leaf water potential, leaf temperature, wax content, leaf relative water content, rate of water loss from excised leaf, peroxidase, and superoxide dismutase activities. Photosynthesis rate, stomatal conductance, and transpiration rate were the most important leaf WUE variables under natural rainfall conditions. They concluded selections for high leaf WUE wheat under natural rainfall could be obtained by selecting breeding lines with a combination of high photosynthesis rate, low transpiration rate, and low stomatal conductance. One of the major challenges will be develop effective methods to compare genetic material for WUE and the physiological and genetic basis for differences in order to develop improved plant resources 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 demonstrated if increased growth translates into increased grain or fruit yield. The efforts have concentrated on annual crops and there is little information on how perennial or vegetable crops respond in terms of yield under increased CO2. This leaves some uncertainty as to the amount of change in the Y term in Equation (1.4) to assess whether WUE could be enhanced through genetic selection or crop management. This also leaves the ET term in Equation (1.4) under some uncertainty due to climate change because ET is dependent upon a combination of factors: soil water availability, 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 and soybean across central Iowa with variations due to differences in atmospheric conditions, rainfall distribution from convective storms, and soil water-holding capacity. One of the major determinants to soil water-holding capacity is the soil organic matter (SOM) content and Hudson [96] found a linear relationship between these 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 the projection for greater extremes in precipitation events [65]. Increased uncertainty in precipitation amounts throughout the growing season will create scenarios in which the available soil water may not be adequate for optimum plant growth. Coupling the variation in precipitation with the increased atmospheric demand for water from the plant will cause the likelihood of water stress in the plant to increase. One avenue for increasing WUE would be to increase the available water for transpiration (T) and decouple the evaporation (E) component out of ET. This can be achieved through the use of mulches to create a surface layer on the soil to reduce the energy impinging onto the soil. This effort was reviewed in Hatfield et al. [21] and this could have an impact early in the growing season when there is less 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 soil surface to drive the evaporation process at the surface; however, mulches still remain 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 crop production across fields has been related to soil water availability during the grain-filling period [97]. Variations of yields across fields have been associated with enhanced soil erosion and soil water availability, and high-yielding management zones 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 sloping areas [98]. The difference in soil water-holding capacity among soils was the primary factor affecting the total seasonal water patterns in maize and if we extrapolate the results 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 soil degradation 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 to agricultural sustainability and environmental quality. After a recent survey, Nyssen et al. [100] reported that nearly all of the tropical highlands (areas above 1000 m above sea level covering 4.5 million km²) are degraded due to medium to severe water erosion. Conversion of farmland from the original pasture in the Horquin sands caused significant decreases in crop yield and poorer soil properties after conversion to cropland [101]. A consequence of population increase is the demand for food and Kidron et al. [102] suggested that abandoning the traditional practice of 10–15 years of cultivation followed by 10–15 years of fallow with a continuous cropping practice increased the rate of soil degradation in Mali, West Africa. SOM content displayed the strongest relationship to soil degradation and soil management practices 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 degradation and susceptibility to climate change. Wang et al. [103] observed that differences in soil structure and saturated hydraulic conductivity were related to cropping systems and degradation of soil structure throughout the soil profile caused maize yield reductions as large as 50%. This decrease in yields was attributed to the shallow root growth and limitations in water availability to the growing plant during the growing season. 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 the ability 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]. In southern Brazil, severe soil degradation was attributed to the widespread use of wheat/soybean or barley (Hordeum vulgare L.)/soybean double-cropping systems coupled 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 1200 mm of annual rainfall, conventional tillage over a number of years has contributed to poor 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 forests to cultivated lands creates the potential for soil degradation and proper management will be required to avoid further degradation. Degradation of the soil resource occurs in many different forms. In Nepal, Thapu and Paudel [108] observed watersheds severely degraded from erosion on nearly half of the land area in the upland crop terraces. This degradation was coupled with depletion of soil nutrients, which in turn affects productivity. In Ethiopia, Taddese [109] observed that severe land degradation caused by the rapid population increase, severe soil erosion, low amounts of vegetative cover, deforestation, and a lack of balance between crop and livestock production 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 reduce soil evaporation and prevent soil erosion would have the potential to maintain production for wheat systems under climate change. Improving SOM and managing soil fertility as methods to increase the capacity of the soil to store and retain water for crop use would provide an advantage to increase the efficiency of precipitation for crop production. Linking crop production, soil management, and climate change as a system to evaluate where cropping systems could become more resilient to climate change would stabilize production and also provide for future increases in production because of the enhanced WUE.

1.5 Major Challenges

There are no simple solutions to determining the optimum pathway for genetic manipulation or agronomic management of plants to adjust to climate change. There will be continued exposure of plants to conditions beyond their range of optimum temperatures and the simple solution will be to change the geographical distribution of plants to accommodate these changing temperature regimes; however, exposure of plants to the extreme events that are more likely to occur presents a different challenge to management. The major challenges to be addressed can be divided into two categories: growth and development processes linked to WUE, and growth and development 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 aspect is the interaction of increasing CO2, increasing temperatures, and soil water availability. Current research has often focused on the primary effects of each of these variables on different species; however, there are only a few studies that have addressed the interactions among these variables [72]. One of the critical components lacking in the current studies is the comparison of genetic material exposed to different climate change parameters, and the linkage with geneticists and molecular biologists to quantify where genetic improvements to temperature and water stress can be made. There are emerging observations that reveal the complexity of the response 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 3 days in spite of having warmer canopy temperatures. The assumption has been that the warmer canopy temperatures were linked to the hastening of the phenological stages; 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. Development of plants that are resilient to stresses will require a multidisciplinary approach in order to quantify these interactions and interpret the meaning of these responses. Understanding how to enhance WUE across different species and climate stresses will provide a benefit for all of humankind in terms of being able to develop germplasm 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 of changing climate on forage and grain quality. There have been observations on the interacting effects among increasing CO2, nitrogen, and water. Luo et al. [111] developed their progressive nitrogen limitation hypothesis to demonstrate the linkage between CO2 enrichment and reduced plant-available nitrogen through the increasing plant demand for nitrogen. Morgan [112] had previously shown that there was a relationship cycling of organic matter in the soil, CO2 uptake by the plant, and the stimulation of plant growth by increased CO2 leading to a decrease in the nitrogen uptake and a potential decrease in nitrogen content in plants. Observations of 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 have resulted in a general decline in forage quality [113]. The effects of climate change and forage quality and plant composition in different rangelands was reviewed by Izaurralde [114]. In addition to production quantity, the quality of agricultural products may be altered by elevated CO2. Some non-nitrogen-fixing plants grown at elevated CO2 have shown reduced nitrogen content [77], and since nitrogen is a critical agricultural crop nutrient, there are implications for the potential interactions of 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 by exposure 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 μmol mol−1 with two rates of nitrogen (i.e., adequate and half-rate of nitrogen) affected crude protein, starch, total and soluble β-amylase, and single kernel hardiness [116]. Increasing CO2 concentrations reduced crude protein by 4–13% in wheat and 11–13% in barley, but increased starch by 4% when half-rate nitrogen was applied. Their conclusions from this study were that nutritional and processing quality of flour will be diminished for cereal grown under elevated CO2 and low nitrogen fertilization [116]. There has been 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 on average [118–121]. Overall, these studies suggest nutrient status in plants and soils interact with changing CO2 concentrations; however, a more specific understanding of the interactions of increasing CO2 and temperatures with nitrogen management 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 yield with 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, resulting in small fruit, while during the summer months, high temperature may cause sunburn damage, accelerate maturity, reduce fruit firmness and color development, and/or decrease the suitability of fruit for short- or long-term storage [123,124]. In strawberry (Fragaria ananassa), too much light coupled with high temperatures leads to the development of fruit bronzing (damaged fruit that is bronze 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 variations across species and within species in their response to changing CO2, decreased water availability, and increasing temperatures. Our challenge will be to begin to assemble research teams across a range of disciplines with the capability of developing new approaches to measuring plant response to the different climate stresses and treat this information with new imaginative insights in order to advance science towards new frontiers of quantifying how we can cope with climate change.

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