Climate Prediction and Agriculture: Advances and Challenges
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These changes in climate will increase atmospheric water demand by crops and increase the potential for limitations in soil water availability, because of the increased variation in precipitation during the growing season and even more so in soils with limited water holding capacity.
For example, Xiao et al. These types of comparisons identify the factors related to WUE of different ecosystems, and they found WUE was related to annual precipitation, gross primary productivity GPP , and growing season length Xiao et al. In their comparison, forests and coastal wetlands had a higher WUE than grasslands and croplands. Guoju et al. Projected changes in climate are expected to increase the areas subjected to drought around the world Dai, ; Feng and Fu, ; Fu and Feng, ; Huang et al.
The effect of increasing drought on net primary productivity has been seen by Zhao and Running where they found a reduction of 55 petagrams of carbon due to drought. Drought will impact productivity and throughout this paper, we will explore the mechanisms of avenues to increase WUE of agricultural systems to take advantage of a limited water supply. One way to explore the impact of a changing climate on WUE is to begin at the leaf level. The interactions of a changing CO 2 and water and temperature regimes will be most evident at the leaf level because there are not the confounding effects of canopy architecture or the interactions of the soil environment on WUE.
There have been two ways proposed to calculated leaf level WUE.
Climate Prediction and Agriculture - Advances and Challenges | Mannava V.K. Sivakumar | Springer
The instantaneous WUE is calculated as the net photosynthetic rate A n divided by transpiration rate E. Another measure is the intrinsic WUE, which is calculated as A n divided by g s. Leaf level WUE has a distinctive pattern depending on the carboxylation pathway, i. A comparison of C 3 and C 4 plants with Crassulacean acid metabolism CAM reveals a completely different pattern of stomatal response to environmental conditions.
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Males and Griffiths provide an overview of the stomatal processes in CAM plants and the advantages in arid environments. Bartlett et al. The WUE of CAM plants is quite high compared to C 3 and C 4 plants because of this unique cycle of carbon fixation and storage during the diurnal cycle. Yang et al.
Climatic changes may induce or ameliorate abiotic stress to the plant, that is 1 water-deficit stress and 2 heat stress. The combined effect of heat and water-deficit stress on plant productivity have been summarized by Hatfield et al. Hatfield et al. Water-deficit stress may be induced by changes in available water and vapor pressure deficit VPD.
Heat stress may be induced by increased ambient air temperature and in the absence of water-deficit stress will decrease productivity Hatfield, There have been numerous assessment of the effects of increased temperatures or drought on the productivity of crops Long and Ort, ; Lobell et al.
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A novel observation by Vanzo et al. When exposed to hot, dry conditions, the chloroplastidic electron transport rate of NE plants became impaired, while IE plants maintained values similar to unstressed controls. During recovery from hot, dry exposures, IE plants reached higher daily net CO 2 assimilation rates compared with NE genotypes.
Examining the changes in volatile emissions from plants coupled with observations on the enzymatic activity may begin to reveal the differences among plants in their response to high temperatures, water deficits, as well as fluctuating light regimes. Leaf level responses are complex because of the internal changes in enzymatic activity in response to the environment.
Illustrative of this is the recent observations by Slattery et al. These abiotic stresses are connected and additionally interact with increasing CO 2.
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The sole effect of increasing CO 2 on A n and WUE is generally positive because the gradient between the ambient air and the intercellular spaces is increased and in the presence of light, CO 2 within the leaf is rapidly converted to carbohydrates. If we adopt the kinetic model described by Charles-Edwards as redrawn in Figure 2 then the linkages between CO 2 uptake and water loss by a leaf become apparent.
The governing factors in this kinetic model are the diffusion coefficient, which is analogous to g s. When we compare the exchange processes of CO 2 within the leaf and H 2 O vapor then the dynamics of the exchange processes are controlled by g s for water vapor and g s and mesophyll conductance g m for photosynthesis Lawson and Blatt This is also why WUE increases under water-deficit stressed conditions — the reduction in A n is less than the reduction of E or g s.
Earl found no significant difference in A n , but lower g s in soybean genotypes with higher WUE. Bierhuizen and Slatyer were among the first to explain the relationship observed between E and A n for different species due the change in saturation deficit and CO 2 concentration. However, there is a different response to rising CO 2 among C 3 photosynthesis and C 4 photosynthesis plants.
A beneficial effect is observed in C 3 plants because CO 2 is a limiting factor owing to the functioning of the carboxylation pathway. C 4 plants show little effect on increased CO 2 under optimal soil water conditions; only under drought stress high CO 2 levels is beneficial owing to partial stomate closure thus reducing transpiration, and the ability of C 4 plants to assimilate carbon even when stomates are closed Lopes et al.
Figure 2. Schematic of the exchange of CO 2 and H 2 O vapor across from the ambient air to the intercellular spaces of a leaf. Figure 3. Response of water use efficiency in cotton leaves as a function of changing CO 2 and incident light levels at a constant wind speed of 2. Data redrawn from Bierhuizen and Slatyer Yoo et al. It is therefore significant to observe the interactions of CO 2 , temperature and water regime to understand WUE in a changing climate. While increased CO 2 can ameliorate water-deficit stress, it cannot offset the increase in heat stress, and may even be adverse, because E decreases and leaf temperature increases Lopes et al.
Allen et al. They used a combination of air temperatures in small chambers to expose soybean leaves to a range of temperatures, VPDs, and CO 2 concentrations. Leaf conductance did not show any response to increasing CO 2 but were affected by temperature with the lower conductance evident with the higher temperatures. Water use was not affected by increasing CO 2 but was increased with the higher temperatures.
The overall result was the WUE decreased with the increasing temperatures but increased with increasing CO 2 at each temperature regime. Another complicating factor in this type of experiment is the changing VPD of the air with changing temperature and the feedback effect on leaf temperature.
The change in the air temperature surrounding the leaf will change leaf temperature and directly affect the gradient of water vapor between the leaf and the atmosphere. Atmospheric factors affecting the energy balance and leaf or canopy temperature drive internal water vapor pressure and ultimately water use. Increases in air temperature will directly increase crop canopy temperature, leaf water vapor pressure, and transpiration.
The response shown in Figure 3 would be expected at the leaf level because the uptake of CO 2 is controlled more by the concentration gradient from the leaf to the air than g s or the diffusion coefficient. The CO 2 concentration gradient is large because the internal concentration at the mesophyll is near zero creating a large gradient from the ambient air into leaf. This is in contrast to the H 2 O vapor gradient, which is at saturation just inside of the stomatal guard cell and a water vapor concentration dictated by air temperature and specific humidity. The differences in these two gradients reveal that leaves would be more efficient in the photosynthetic process than the transpiration process and would exhibit a preferential shift toward greater WUE at the leaf level because A n would be affected more than E.
If we extend this across species and climate change scenarios, then humidity of air in response to changing temperatures will have a significant impact on WUE. One has to be cautious of the older literature because the effect of a rapidly changing CO 2 was not part of the research assessments and water availability, temperature, and humidity were the main variables.
The concept of WUE, alongside with other parameters, had been proposed in plant breeding to identify water use efficient genotypes under changing climate regimes, heat and water-deficit stress, and interactions among them. Variation among genotypes for WUE has been found in a number of crop species, including barley Hubick and Farquhar, , cowpea [ Vigna unguiculata L. Hubick et al. Quisenberry and McMichael, ; Saranga et al.
In a recent meta-analysis, Gago et al. Flexas et al. Gago et al. They proposed that genetic screening of plants for characteristics directly related to photosynthetic efficiency or reduced respiration would lead to insights in the potential impacts of climate change on WUE. Peng and Krieg in comparing genotypes of grain sorghum found that differences in WUE among genotypes was related to A n and leaf area because there was little difference among genotypes in their water use.
In peanut A. In these experiments, there was an interaction between WUE and high temperatures because of the effect on specific leaf area and proposed that specific leaf area could be a parameter useful for screening among genotypes for WUE.
While Ismail and Hall proposed that carbon isotope discrimination was a good selection criteria in cowpea [ V. Ramirez Builes et al. Similar results were found by Siahpoosh et al. They proposed that total biomass produced and cultivar ET during the season was a valuable screening tool Siahpoosh et al. Hufstetler et al. Kromdijk et al. Being able to identify traits related to WUE will aid in being able to screen across genetic material for their response under a changing climate.
Comparisons among species and within species is not new, Brown and Simmons demonstrated that apparent photosynthesis and transpiration under water-deficit conditions were related to WUE and could be used as tools to assess genetic material. Gebrekirstos et al. The authors identified different drought strategies among species, which could help to draw conclusions on future climate change adaption.
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Song et al. There has also been criticism of using leaf level WUE to identify water efficient plants. One drawback is the difficulty to upscale from leaf to canopy level see also section below. Medrano et al. The microclimate surrounding an individual leaf will determine the WUE which suggests that if leaves are being used to relate to canopy level responses then a composite of leaves be used that would more closely represent the canopy. Blum dismissed the WUE-concept for plant breeding because genotypes can only increase leaf level WUE by activating plant traits responsible for reducing E , not by increasing A n.
This would eventually lead to genotypes with reduced yield and drought tolerance. Instead, the author proposed to evaluate the Effective Use of Water EUW which focuses on genotypes, which are capable to maximize soil moisture capture for transpiration. There continue to be advances in our understanding of plant response to a changing CO 2 environment, one of these responses is a change in the stomatal density as observed by Caine et al.
The use of more advanced techniques, e. Each plant species has a unique arrangement of a set of single leaves and canopies consist of an arrangement of plants according to a specific cultural practice, e. The arrangement of plants creates a diverse exposure to solar radiation, i.