Responses to Increased Moisture Stress and Extremes: Whole Plant Response to Drought under Climate Change
Vincent VadezJana KholováSunita ChoudharyPaul ZindyMédulline TerrierL. KrishnamurthyP. RatnakumarNeil C. Turner
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Research on adaptation of crops to drought has received considerable attention over the past century. With the current and predicted increases in temperature and likely decreases in rainfall, drought research will take on added significance and urgency. Due to an increase in temperature and vapour pressure deficit (VPD), climate change will affect plant water relations along the whole soil-plant-atmosphere continuum even when soil water is plentiful. Studying how plants regulate water use under well-watered conditions, a neglected aspect of drought research, will be as important as studying the water relations and water use during periods of drought. Research on the major areas of resistance to water flow, at the root and leaf level, will be required. In addition to these thermodynamic aspects related to an increase in VPD, the increase in temperature and reduction in rainfall will also decrease the length of the growing period and shorten the cropping cycle. In addition, higher VPD may also decrease leaf expansion in certain genotypes of crops, limiting biomass accumulation and water use. Therefore, a new equilibrium between genotype duration and soil water balance will be required to ensure maximization of light capture while optimizing the use of soil water. After decades of breeding for short-duration cultivars for drought-prone environments, breeding for medium-duration cultivars may be needed, and this could be the fastest and easiest solution to mitigate the effects of climate change. The increase in VPD will also decrease water productivity, although this will be in part counterbalanced by an increased CO2 concentration. Search for germplasm capable of maintaining high water productivity under higher VPD will be required.Keywords:
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This study examined the interaction between a reflective particle film and water use efficiency (WUE) response of irrigated and non-irrigated apple trees ( Malus × domestica ) over a wide range of environmental conditions. The objectives were to measure isotopic discrimination (Δ 13 C and δ 18 O), specific gas exchange, and WUE response of ‘Empire’ apple treated with a reflective particle film (PF), with and without supplemental irrigation, compared with an untreated control, with and without supplemental irrigation, over a range of leaf area indices (LAI), seasonal evapotranspiration (ETo), and vapor pressure deficits (VPD) to determine the mechanisms of action affecting WUE in apple. Short-term whole canopy gas exchange studies and isotope discrimination analysis were used to test the hypothesis that WUE was modified by the use of a PF. In whole canopy gas exchange studies, carbon assimilation (A) and transpiration tended to increase, and WUE and canopy conductance tended to decrease, with VPD within each LAI class from 2 to 6. For VPD > 1 kPa, the PF irrigated treatment consistently had the greatest WUE and other treatments were intermediate for LAI of 2 to 4. The PF irrigated and non-irrigated treatments had greater WUE than the control irrigated and non-irrigated treatments for VPD ≤ 2 kPa and there were no treatment effects for VPD > 2 kPa in the LAI range of 4 to 6. The PF non-irrigated was equivalent to the control non-irrigated treatment at VPD of 1 to 3 kPa, but was significantly lower at VPD of 3 to 4 kPa. PF irrigated and non-irrigated treatments had the greatest carbon isotope discrimination (Δ 13 C), the control non-irrigated treatment had the lowest Δ 13 C, and the control-irrigated treatment was intermediate. Oxygen isotope enrichment (δ 18 O) was positively correlated with the mean growing season VPD and mean growing season evapotranspiration. Δ 13 C was significantly and positively correlated with δ 18 O. Seasonal WUE was negatively correlated with Δ 13 C and there was an interaction with LAI. The seasonal water use of apple is better evaluated with stable isotope discrimination integrating seasonal variation rather that the use of whole canopy gas exchange measurements that measure WUE for brief periods of time. Δ 13 C was an accurate measurement of apple WUE and indicated that the PF irrigated treatment had the greatest Δ 13 C and so the lowest WUE compared with the control non-irrigated treatment at LAI from 4 to 6. The reduced WUE of the PF irrigated treatment compared with the control non-irrigated treatment is likely due to increased g S from lower canopy temperature and increased canopy photosynthetically active radiation diffusion that drove increased A. δ 18 O was an indicator of seasonal water use over six growing seasons due to its high correlation with ETo. In ‘Empire’ apple, A can be increased with PF and irrigation treatments, but at the cost of decreased WUE.
Water Use Efficiency
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Malus
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There is a pressing need to improve the water-use efficiency of rain-fed and irrigated crop production. Breeding crop varieties with higher water-use efficiency is seen as providing part of the solution. Three key processes can be exploited in breeding for high water-use efficiency: (i) moving more of the available water through the crop rather than it being wasted as evaporation from the soil surface or drainage beyond the root zone or being left behind in the root zone at harvest; (ii) acquiring more carbon (biomass) in exchange for the water transpired by the crop, i.e. improving crop transpiration efficiency; (iii) partitioning more of the achieved biomass into the harvested product. The relative importance of any one of these processes will vary depending on how water availability varies during the crop cycle. However, these three processes are not independent. Targeting specific traits to improve one process may have detrimental effects on the other two, but there may also be positive interactions. Progress in breeding for improved water-use efficiency of rain-fed wheat is reviewed to illustrate the nature of some of these interactions and to highlight opportunities that may be exploited in other crops as well as potential pitfalls. For C3 species, measuring carbon isotope discrimination provides a powerful means of improving water-use efficiency of leaf gas exchange, but experience has shown that improvements in leaf-level water-use efficiency may not always translate into higher crop water-use efficiency or yield. In fact, the reverse has frequently been observed. Reasons for this are explored in some detail. Crop simulation modelling can be used to assess the likely impact on water-use efficiency and yield of changing the expression of traits of interest. Results of such simulations indicate that greater progress may be achieved by pyramiding traits so that potential negative effects of individual traits are neutralized. DNA-based selection techniques may assist in such a strategy.
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In eastern Australia, latitudinal gradients in vapour pressure deficit (VPD), mean temperature (T), photosynthetically active radiation (PAR), and fraction of diffuse radiation (FDR) around the critical stage for yield formation affect wheat yield and crop water-use efficiency (WUE = yield per unit evapotranspiration). In this paper we combine our current understanding of these climate factors aggregated in a normalised phototermal coefficient, NPq = (PAR· FDR)/(T · VPD), with a shire-level dynamic model of crop yield and water use to quantify WUE of wheat in 245 shires across Australia. Three measures of WUE were compared: WUE, the ratio of measured yield and modelled evapotranspiration; WUEVPD, i.e. WUE corrected by VPD; and WUENPq, i.e. WUE corrected by NPq. Our aim is to test the hypothesis that WUENPq suits regional comparisons better than WUE or WUEVPD. Actual median yield at the shire level (1975–2000) varied from 0.5 to 2.8 t/ha and the coefficient of variation ranged from 18 to 92%. Modelled median evapotranspiration varied from 106 to 620 mm and it accounted for 42% of the variation in yield among regions. The relationship was non-linear, and yield stabilised at ~2 t/ha for evapotranspiration above 343 mm. There were no associations between WUE and rainfall. The associations were weak (R2 = 0.09) but in the expected direction for WUEVPD, i.e. inverse with seasonal rainfall and direct with off-season rainfall, and strongest for WUENPq (R2 = 0.40).We suggest that the effects of VPD, PAR, FDR, and T, can be integrated to improve the regional quantification of WUE defined in terms of grain yield and seasonal water use.
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In Argentina, wheat ( Triticum aestivum L.) is cropped over a wide range of climatic conditions. Considerable variability in the ratio of dry weight produced per unit of transpired water, usually referred to as water‐use efficiency (WUE), is expected as variation in climatic factors affects photosynthesis and transpiration in different ways. Also, previous studies have shown that water supply limitations may affect WUE in wheat. The objective of this study was to quantify the effects of climatic environment and water availability on WUE in wheat crops. Six experiments were conducted at different locations of the Argentine wheat belt and crop dry weight and water use were measured in periods when water use was dominated by transpiration. Three of the experiments included both irrigated and rainfed treatments. Mean daily values of (i) pan evaporation, (ii) relative humidity, (iii) potential water use, and (iv) vapor pressure deficit, were used to find a general relationship that explained effects of the climatic environment on WUE. For experiments with high water availability, daytime vapor pressure deficit was better related to WUE than the other climatic factors. WUE was greater for experiments with water limitation, probably because stomatal closure to restrict transpiration rate occurred around midday when vapor pressure deficit was highest. As a consequence, relative dry weight under water limitation was not linearly related to relative water use as proposed in previous studies. A quadratic relationship that better represented this response was derived.
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Pan evaporation
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The ratio of carbon assimilation to water evapotranspiration (ET) of an ecosystem, referred to as ecosystem water use efficiency (WUEeco), is widely expected to increase because of the rising atmospheric carbon dioxide concentration (Ca). However, little is known about the interactive effects of rising Ca and climate change on WUEeco. On the basis of upscaled estimates from machine learning methods and global FLUXNET observations, we show that global WUEeco has not risen since 2001 because of the asymmetric effects of an increased vapor pressure deficit (VPD), which depressed photosynthesis and enhanced ET. An undiminished ET trend indicates that rising temperature and VPD may play a more important role in regulating ET than declining stomatal conductance. Projected increases in VPD are predicted to affect the future coupling of the terrestrial carbon and water cycles.
Saturation (graph theory)
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Limitations and utility of three measures of water use characteristics were evaluated: water use efficiency (WUE), intrinsic WUE and marginal water cost of carbon gain ( ∂E/∂A ) estimated, respectively, as ratios of assimilation (A) to transpiration (E), of A to stomatal conductance (gs ) and of sensitivities of E and A with variation in gs . Only the measure ∂E/∂A estimates water use strategy in a way that integrates carbon gain relative to water use under varying environmental conditions across scales from leaves to communities. This insight provides updated and simplified ways of estimating ∂E/∂A and adds depth to understanding ways that plants balance water expenditure against carbon gain, uniquely providing a mechanistic means of predicting water use characteristics under changing environmental scenarios.
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Carbon assimilation
Stomatal Conductance
Water balance
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Water cycle
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In Argentina, wheat (Triticum aestivum L.) is cropped over a wide range of climatic conditions. Considerable variability in the ratio of dry weight produced per unit of transpired water, usually referred to as water-use efficiency (WUE), is expected as variation in climatic factors affects photosynthesis and transpiration in different ways. Also, previous studies have shown that water supply limitations may affect WUE in wheat. The objective of this study was to quantify the effects of climatic environment and water availability on WUE in wheat crops. Six experiments were conducted at different locations of the Argentine wheat belt and crop dry weight and water use were measured in periods when water use was dominated by transpiration. Three of the experiments included both irrigated and rainfed treatments. Mean daily values of (i) pan evaporation, (ii) relative humidity, (iii) potential water use, and (iv) vapor pressure deficit, were used to find a general relationship that explained effects of the climatic environment on WUE. For experiments with high water availability, daytime vapor pressure deficit was better related to WUE than the other climatic factors. WUE was greater for experiments with water limitation, probably because stomatal closure to restrict transpiration rate occurred around midday when vapor pressure deficit was highest. As a consequence, relative dry weight under water limitation was not linearly related to relative water use as proposed in previous studies. A quadratic relationship that better represented this response was derived.
Water Use Efficiency
Water use
Pan evaporation
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Water Use Efficiency
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Hydrological water balance studies require knowledge of terrestrial water content within an active storage volume. While the advent of microwave remote sensing has enable frequent and global observations of surface soil moisture, by itself, surface moisture content is not directly suitable for use in water balance studies. An appropriate length-scale, Δz, is required to transform the soil moisture state to total water content. This work demonstrates the applicability of SMAP soil moisture in first-order and observation-driven water balance. First, a novel method is presented to estimate the soil water loss function. Then, by resolving the water balance equation and enforcing mass conservation, estimates of the hydrological length scale over the United States is provided. Mean precipitation is the dominant factor on Δz, such that wetter regions with higher mean precipitation have larger lengths scale. The mean soil moisture state and texture weakly influence Δz.
Water balance
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