Elevated Vapor Pressure Deficit Research Articles

Wheat drought-tolerance to enhance food security in Tunisia, birthplace of the Arab Spring

Abstract The beginning of the ‘Arab Spring’ in 2011, a regional revolution which started in the Tunisian city of Sidi Bouzid in late 2010, occurred in part as a result of drought-triggered high wheat prices, which in the past led to ‘bread riots’ across several Middle East and North Africa (MENA) nations. Here we present, for the first time, an analysis of possible amelioration of wheat yield loss and greater stability in bread supply resulting from the incorporation of putative drought-tolerant traits into wheat cultivars grown in Tunisia. To this end, we used a simulation crop modeling approach using SSM-Wheat to evaluate yield loss or gain resulting from three types of water-saving traits that have been recently identified in wheat. These consisted in partial stomatal closure at high soil water content, overall decrease in transpiration rate (TR), and partial stomatal closure under elevated vapor pressure deficit (VPD). To capture large gradients in seasonal precipitation across wheat growing areas over a small country such as Tunisia, a grid pattern of 29 × 29 km was established as a basis for the geospatial simulation. Surprisingly, the simulation reflected opposite strategies in terms of water use (water-saving vs aggressive water use). The highest yield gain (30%) resulting from water-saving modification was found to occur in the food-insecure region of Sidi Bouzid. Traits enabling aggressive water use were found to be generally favorable across Tunisia, with one trait leading to up to 80% and 40% increases in yield and its stability in the food-challenged south of the country. However, major yield penalties were found to occur if water-saving traits were to be deployed in the ‘wrong’ region. Those findings could be used as a blueprint to navigate complex trait × environment interactions and to better inform local breeding and management programs to improve wheat yield and it stability in Tunisia and the MENA region in general.

Limited-transpiration rate under high atmospheric vapour pressure deficit: a trait for developing genotypes for water-deficit conditions.

Abstract Water deficits are the major limitation in increasing crop yields in many regions of the world. Various plant traits that might result in yield increases in water-limited environments have been discussed for decades. Conservative use of soil water is an important breeding strategy in drought-prone environments that can be achieved by traits based on partial stomatal closure under specific environmental conditions to limit transpiration rate (TR). The focus of this review is on a specific trait for conservative soil water that results in partial stomatal closure that can be supported by a plant. This limit on TR is expressed in terms of the atmospheric vapour pressure deficit (VPD) at which partial stomatal closure occurs. The review provides the physiological background of partial stomatal closure under elevated VPD. Simulation studies that analyse the potential benefits of this trait are also discussed. Finally, we provide a review of the various research that has been made in the identification of genetic variability of this trait in major crops.

Inheritance of limited-transpiration trait in peanut: an update

ABSTRACTPeanut (Arachis Hypogeae L.) is commonly grown on sandy soil and in environments with intermittent rainfall, both of which can lead to soil water-deficit. Limited transpiration (LT) under elevated vapor pressure deficit (VPD) can result in water conservation, allowing sustained physiological activity later in the season during reproductive development. The objective of this study was to expand the number of progeny lines phenotyped for expression of the LT trait from the mating of Tifrunner (LT trait not expressed) × NC 3033 (LT trait expressed) to allow a preliminary examination of inheritance. Half of the 24 phenotyped lines expressed LT with their VPD threshold ranging from 2.16 to 3.38 kPa. Six of the 12 genotypes expressing LT had a threshold at 2.65 kPa or less, which is the range likely to be relevant in most peanut environments. These results, indicating epistatic inheritance, are supportive of LT expression in progeny lines at a reasonable frequency for relevant use in cultivar development for water-deficit conditions.

In situ temperature relationships of biochemical and stomatal controls of photosynthesis in four lowland tropical tree species.

Net photosynthetic carbon uptake of Panamanian lowland tropical forest species is typically optimal at 30-32°C. The processes responsible for the decrease in photosynthesis at higher temperatures are not fully understood for tropical trees. We determined temperature responses of maximum rates of RuBP-carboxylation (VCMax ) and RuBP-regeneration (JMax ), stomatal conductance (Gs ), and respiration in the light (RLight ) in situ for 4 lowland tropical tree species in Panama. Gs had the lowest temperature optimum (TOpt ), similar to that of net photosynthesis, and photosynthesis became increasingly limited by stomatal conductance as temperature increased. JMax peaked at 34-37°C and VCMax ~2°C above that, except in the late-successional species Calophyllum longifolium, in which both peaked at ~33°C. RLight significantly increased with increasing temperature, but simulations with a photosynthesis model indicated that this had only a small effect on net photosynthesis. We found no evidence for Rubisco-activase limitation of photosynthesis. TOpt of VCMax and JMax fell within the observed in situ leaf temperature range, but our study nonetheless suggests that net photosynthesis of tropical trees is more strongly influenced by the indirect effects of high temperature-for example, through elevated vapour pressure deficit and resulting decreases in stomatal conductance-than by direct temperature effects on photosynthetic biochemistry and respiration.

Tree water dynamics in a drying and warming world.

Disentangling the relative impacts of precipitation reduction and vapour pressure deficit (VPD) on plant water dynamics and determining whether acclimation may influence these patterns in the future is an important challenge. Here, we report sap flux density (FD ), stomatal conductance (Gs ), hydraulic conductivity (KL ) and xylem anatomy in piñon pine (Pinus edulis) and juniper (Juniperus monosperma) trees subjected to five years of precipitation reduction, atmospheric warming (elevated VPD) and their combined effects. No acclimation occurred under precipitation reduction: lower Gs and FD were found for both species compared to ambient conditions. Warming reduced the sensibility of stomata to VPD for both species but resulted in the maintenance of Gs and FD to ambient levels only for piñon. For juniper, reduced soil moisture under warming negated benefits of stomatal adjustments and resulted in reduced FD , Gs and KL . Although reduced stomatal sensitivity to VPD also occurred under combined stresses, reductions in Gs , FD and KL took place to similar levels as under single stresses for both species. Our results show that stomatal conductance adjustments to high VPD could minimize but not entirely prevent additive effects of warming and drying on water use and carbon acquisition of trees in semi-arid regions.

Evaluation of Elite Southern Maturity Soybean Breeding Lines for Drought‐Tolerant Traits

Breeding efforts in soybean [Glycine max (L.) Merr.] have addressed the challenge of water‐limited yields by incorporating parental stocks which exhibit drought‐tolerant traits. Multiple cycles of empirical selection for improved yielding ability in water‐deficient field environments have produced new generations of adapted breeding lines. However, the impact of this selection process on specific putative drought‐tolerant traits is unknown. The objective of this study was to determine if breeders’ selection of 10 elite lines for high seed yield under dry conditions is associated with the presence of physiological expression of three putative drought‐tolerant traits: (i) limited transpiration rate under high vapor pressure deficit (VPD), (ii) early decrease in transpiration rate with soil drying, and (iii) drought‐tolerant N2 fixation. Greenhouse experiments were undertaken to characterize each genotypes for their phenotype of each of these three traits. Unlike most soybean cultivars, 9 of the 10 elite lines expressed a limited transpiration rate under elevated VPD. The VPD at which transpiration rate became limited was 1.9 kPa or less. There was no difference among genotypes in the threshold for decline in transpiration rate with soil drying, although all genotypes expressed high thresholds indicating an ability to conserve soil water. All lines expressed drought tolerance in their N2 fixation rates, which was superior to that commonly observed in soybean. This study demonstrated that mating of parents that expressed a drought trait and multiple rounds of progeny selection based on improved yield under water‐limited conditions resulted in the elite lines expressing improved drought traits.

Global Atmospheric Evaporative Demand over Land from 1973 to 2008

AbstractPan evaporation (EP), an index of atmospheric evaporative demand, has been widely reported to have weakened in the past decades. However, its interpretation remains controversial because EP observations are not globally available and observations of one of its key controls, surface incident solar radiation Rs, are even less available. Using global-distributed Rs from both direct measurements (available through the Global Energy Balance Archive) and derived from sunshine duration, the authors calculated the potential evaporation from 1982 to 2008 from approximately 1300 stations. The findings herein show that the contribution of water vapor pressure deficit (VPD) to monthly variability of EP is much larger than that of other controlling factors, of Rs, wind speed (WS), and air temperature Ta. The trend of the aerodynamic component of EP, which includes contributions of VPD, WS, and Ta, accounted for 86% of the long-term trend of EP. The aerodynamic component was then calculated from 4250 globally distributed stations and showed a negligible averaged trend from 1973 to 2008 because the reduction in WS canceled out the impact of the elevated VPD. The long-term trend of WS dominates the long-term trend of the aerodynamic component of EP at the 4250 stations. Atmospheric evaporative demand increased in most arid and semiarid areas, indicating a decrease in water availability in those areas.

Parameterization of Zelkova serrata stomatal conductance model to estimate stomatal ozone uptake in Japan

To parameterize stomatal conductance or ozone uptake modeling in the Eastern Asian tree species Zelkova serrata, measurements of stomatal conductance were carried out in several Japanese sites across the growing season. The new parameterization improved the stomatal conductance model performance relative to a previously proposed model. The results were used to compare the spatial distribution of AOT40 and Z. serrata stomatal ozone uptake in Japan. Including a soil moisture function improved the model in the short periods with low precipitation. In addition, elevated vapor pressure deficit and soil moisture deficit due to high temperature, and high ozone exposure induced stomatal closure. The consequent decoupling of stomatal ozone uptake from high ozone exposure suggests caution in using AOT40 as a standard for Z. serrata protection in Japan.

Reduced irrigation and rootstock effects on vegetative growth, yield and its components, and leaf physiological responses of Shiraz

Background and Aims: The study investigated whether rootstocks can modify grapevine responses to reduced irrigation. Methods and Results: Drip-irrigated Shiraz vines on eight rootstocks were subjected to industry standard and 30% reduced irrigation regimes over four seasons. Reducing irrigation decreased pruning weights and yield, but did not consistently affect irrigation water use index (IWUI). It increased leaf Δ13C. Reduced irrigation and elevated vapour pressure deficit (VPD) were associated with decreases in leaf water potential (ψl), leaf stomatal conductance and assimilation rate. Reducing irrigation raised leaf transpiration efficiency, whereas elevated VPD lowered it. These effects of reduced irrigation were independent of rootstock. Vines grafted to 101-14 had a higher ψl and achieved the highest yield and IWUI. The yields of vines grafted to Ramsey, Schwarzmann and 140 Ruggeri were also high. Vines grafted to 101-14, Ramsey and 1103 Paulsen had the higher rates of leaf assimilation. Rootstock did not affect Δ13C. Conclusion: The gain in leaf transpiration efficiency caused by reducing irrigation was not associated with a gain in IWUI. Rootstocks 101-14, Ramsey, Schwarzmann and 140 Ruggeri achieved higher yields and IWUI under both standard and reduced irrigation regimes. Significance of the Study: Among grafted vines growing on saline soil but receiving non-saline irrigation water and subject to a 30% reduction in irrigation, the yield responses of vines grafted on rootstocks rated as having good drought tolerance were the same as those of vines grafted on rootstocks rated as having poor drought tolerance.

Differential sensitivity of C 3 and C 4 turfgrass species to increasing atmospheric vapor pressure deficit

Temperature and vapor pressure deficit (VPD) effects on turfgrass growth are almost always confounded in experiments because VPD commonly is substantially increased in elevated-temperature treatments. The objective of this study as to examine specifically the influence of VPD on transpiration response of four ‘warm-season’ (C 4) and four ‘cool-season’ (C 3) turfgrasses to increasing VPD at a stable temperature (29.3 ± 1.5 °C). Although transpiration rates were noticeably lower in C 4 grasses, transpiration rates increased linearly in response to increasing VPD across the range of 0.8–3.0 kPa. In contrast, transpiration rates of C 3 increased sharply with increasing VPD across the range of low VPDs, but became constrained at higher VPDs (>1.35 kPa). Restricted transpiration rate at elevated VPD was most evident in Agrostis palustris and Lolium perenne. Assuming restricted transpiration rates reflect a limitation on leaf CO 2 uptake, these results indicate that the commonly observed decline in growth of C 3 (and success of C 4) grasses at elevated temperature may include a sensitivity to elevated VPD.

Growth of European beech ( Fagus sylvatica L.) saplings is limited by elevated atmospheric vapour pressure deficits

There is an ongoing debate about how European beech might be affected by a future drier climate. While numerous studies have examined the effects of soil drought on beech growth and development, studies investigating the effects of elevated atmospheric water vapour pressure deficit (VPD) are lacking. By increasing VPD in climate and open-top chamber experiments, with moisture in the rooting medium near optimum, we tested the hypothesis that increased VPD negatively affects the growth and development of European beech saplings. In the climate chambers, a reduction in relative air humidity by 40% resulted in a 68% reduction in productivity. Similarly, in the open-top chamber experiment conducted on the forest floor, biomass declined by 30% when relative air humidity was 15% lower. The reduction in biomass was mainly a consequence of a dramatically reduced leaf growth of beech in the elevated VPD treatments. Our results show that growth and development of beech saplings strongly depend not only on soil moisture but also on the prevailing VPD level. We conclude that the vapour pressure deficit is a widely ignored factor which influences the growth and vitality, and possibly also the distribution of European beech. Future forest management schemes under an altered climate should take this factor into account.

Crop yield reduction in the tropics under climate change: Processes and uncertainties

Many modelling studies examine the impacts of climate change on crop yield, but few explore either the underlying bio-physical processes, or the uncertainty inherent in the parameterisation of crop growth and development. We used a perturbed-parameter crop modelling method together with a regional climate model (PRECIS) driven by the 2071–2100 SRES A2 emissions scenario in order to examine processes and uncertainties in yield simulation. Crop simulations used the groundnut (i.e. peanut; Arachis hypogaea L.) version of the General Large-Area Model for annual crops (GLAM). Two sets of GLAM simulations were carried out: control simulations and fixed-duration simulations, where the impact of mean temperature on crop development rate was removed. Model results were compared to sensitivity tests using two other crop models of differing levels of complexity: CROPGRO, and the groundnut model of Hammer et al. [Hammer, G.L., Sinclair, T.R., Boote, K.J., Wright, G.C., Meinke, H., and Bell, M.J., 1995, A peanut simulation model: I. Model development and testing. Agron. J. 87, 1085–1093]. GLAM simulations were particularly sensitive to two processes. First, elevated vapour pressure deficit (VPD) consistently reduced yield. The same result was seen in some simulations using both other crop models. Second, GLAM crop duration was longer, and yield greater, when the optimal temperature for the rate of development was exceeded. Yield increases were also seen in one other crop model. Overall, the models differed in their response to super-optimal temperatures, and that difference increased with mean temperature; percentage changes in yield between current and future climates were as diverse as − 50 % and over + 30 % for the same input data. The first process has been observed in many crop experiments, whilst the second has not. Thus, we conclude that there is a need for: (i) more process-based modelling studies of the impact of VPD on assimilation, and (ii) more experimental studies at super-optimal temperatures. Using the GLAM results, central values and uncertainty ranges were projected for mean 2071–2100 crop yields in India. In the fixed-duration simulations, ensemble mean yields mostly rose by 10–30%. The full ensemble range was greater than this mean change (20–60% over most of India). In the control simulations, yield stimulation by elevated CO 2 was more than offset by other processes – principally accelerated crop development rates at elevated, but sub-optimal, mean temperatures. Hence, the quantification of uncertainty can facilitate relatively robust indications of the likely sign of crop yield changes in future climates.

Interspecific variation in nighttime transpiration and stomatal conductance in a mixed New England deciduous forest

Transpiration is generally assumed to be insignificant at night when stomata close in response to the lack of photosynthetically active radiation. However, there is increasing evidence that the stomata of some species remain open at night, which would allow for nighttime transpiration if there were a sufficient environmental driving force. We examined nighttime water use in co-occurring species in a mixed deciduous stand at Harvard Forest, MA, using whole-tree and leaf-level measurements. Diurnal whole-tree water use was monitored continuously with Granier-style sap flux sensors in paper birch (Betula papyrifera Marsh.), red oak (Quercus rubra L.) and red maple (Acer rubrum L.). An analysis was conducted in which nighttime water flux could be partitioned between refilling of internal water stores and transpiration. Substantial nighttime sap flux was observed in all species and much of this flux was attributed to the refilling of depleted water stores. However, in paper birch, nighttime sap flux frequently exceeded recharge estimates. Over 10% of the total daily sap flux during the growing season was due to transpiration at night in paper birch. Nighttime sap flux was over 8% of the total daily flux in red oak and 2% in red maple; however, this flux was mainly associated with recharge. On nights with elevated vapor pressure deficit, sap flux continued through the night in paper birch, whereas it reached zero during the night in red oak and red maple. Measurements of leaf-level gas exchange on a night with elevated vapor pressure deficit showed stomatal conductance dropping by only 25% in paper birch, while approaching zero in red oak and red maple. The study highlighted differences in ecophysiological controls on sap flux exerted by co-occurring species. Paper birch is a fast-growing, shade-intolerant species with an earlier successional status than red oak and red maple. Risking water loss through nighttime transpiration may provide paper birch with an ecological advantage by enabling the species to maximize photosynthesis and support rapid growth. Nighttime transpiration may also be a mechanism for delivering oxygen to respiring cells in the deep sapwood of paper birch.

Effects of Water Vapor Pressure Deficit on Photochemical and Fluorescence Yields in Tobacco Leaf Tissue

The relationship between photochemical quantum yield (phi(s)) and fluorescence yield have been investigated in leaf tissue from Nicotiana tabacum using CO(2) exchange and a modulated fluorescence measuring system. The quantum yield of CO(2) fixation at 1.6% (v/v) O(2) and limiting irradiance was reduced 20% by increasing the mean H(2)O vapor pressure deficit (VPD) from 9.2 to 18.6 mbars. As [CO(2)] and irradiance were varied, the intrinsic quantum yield of open photosystem II units (phi(s)/q(Q) where q(Q) is the photochemical fluorescence quenching coefficient) declined linearly with the degree of nonphotochemical fluorescence quenching. The slope and y-intercept values for this function were significantly reduced when the mean VPD was 18.4 millibars relative to 8.9 millibars. Susceptibility of the leaf tissue to photoinhibition was unaffected by VPD. Elevated O(2) concentrations (20.5% v/v) reduced the intrinsic quantum yield of net CO(2) uptake due to the occurrence of O(2)-reducing processes. However, the relative effect of high VPD compared to low VPD on intrinsic quantum yield was not dependent on the O(2) level. This suggests that the Mehler reaction does not mediate the response of quantum yield to elevated VPD. The results are discussed with regard to the possible role of transpiration stress in regulating dissipation of excitation by electron transport pathways other than noncyclic electron flow supporting reduction of CO(2) and/or O(2).