The impacts of rising vapour pressure deficit in natural and managed ecosystems.

An exponential rise in the atmospheric vapour pressure deficit (VPD) is among the most consequential impacts of climate change in terrestrial ecosystems. Rising VPD has negative and cascading effects on nearly all aspects of plant function including photosynthesis, water status, growth and survival. These responses are exacerbated by land-atmosphere interactions that couple VPD to soil water and govern the evolution of drought, affecting a range of ecosystem services including carbon uptake, biodiversity, the provisioning of water resources and crop yields. However, despite the global nature of this phenomenon, research on how to incorporate these impacts into resilient management regimes is largely in its infancy, due in part to the entanglement of VPD trends with those of other co-evolving climate drivers. Here, we review the mechanistic bases of VPD impacts at a range of spatial scales, paying particular attention to the independent and interactive influence of VPD in the context of other environmental changes. We then evaluate the consequences of these impacts within key management contexts, including water resources, croplands, wildfire risk mitigation and management of natural grasslands and forests. We conclude with recommendations describing how management regimes could be altered to mitigate the otherwise highly deleterious consequences of rising VPD.

Rising vapor-pressure deficit increases nitrogen fixation in a legume crop.

Atmospheric vapor-pressure deficit (VPD) is increasing in many regions and has a large impact on plant productivity. A VPD increase leads to raising transpiration rate (TR) and soil-water demand, risking productivity penalties. Like water, nitrogen is critical to productivity, but the effect of VPD on legume nitrogen fixation is undocumented. To address this, we developed a portable system for quantifying nitrogen fixation noninvasively and at a high temporal resolution by tracking the rate of hydrogen gas evolution by root nodules. Combining field and controlled-environment experiments where we measured leaf gas exchange and H2 production by nodules, we confirmed the ability of the system to track nitrogen fixation dynamics. Raising VPD from 0.5 to 3 kPa within c. 2.5 h under well-watered conditions increased nitrogen fixation by up to 25% in addition to TR, consistent with the hypothesis that raising VPD in that range might have alleviated nitrogenase feedback inhibition. Genotypic differences were found in this response, indicating a potential for breeding. Our study provides evidence for an important environmental effect on nitrogen fixation that is not taken into account in current crop and vegetation models, pointing to untapped avenues for better understanding climate change effects on legumes and nitrogen cycling.

Transpiration response to soil drying versus increasing vapor pressure deficit in crops: physical and physiological mechanisms and key plant traits.

The water deficit experienced by crops is a function of atmospheric water demand (vapor pressure deficit) and soil water supply over the whole crop cycle. We summarize typical transpiration response patterns to soil and atmospheric drying and the sensitivity to plant hydraulic traits. We explain the transpiration response patterns using a soil-plant hydraulic framework. In both cases of drying, stomatal closure is triggered by limitations in soil-plant hydraulic conductance. However, traits impacting the transpiration response differ between the two drying processes and act at different time scales. A low plant hydraulic conductance triggers an earlier restriction in transpiration during increasing vapor pressure deficit. During soil drying, the impact of the plant hydraulic conductance is less obvious. It is rather a decrease in the belowground hydraulic conductance (related to soil hydraulic properties and root length density) that is involved in transpiration down-regulation. The transpiration response to increasing vapor pressure deficit has a daily time scale. In the case of soil drying, it acts on a seasonal scale. Varieties that are conservative in water use on a daily scale may not be conservative over longer time scales (e.g. during soil drying). This potential independence of strategies needs to be considered in environment-specific breeding for yield-based drought tolerance.

The genetic basis of transpiration sensitivity to vapor pressure deficit in wheat.

Genetic manipulation of whole-plant transpiration rate (TR) response to increasing atmospheric vapor pressure deficit (VPD) is a promising approach for crop adaptation to various drought regimes under current and future climates. Genotypes with a non-linear TR response to VPD are expected to achieve yield gains under terminal drought, thanks to a water conservation strategy, while those with a linear response exhibit a consumptive strategy that is more adequate for well-watered or transient-drought environments. In wheat, previous efforts indicated that TR has a genetic basis under naturally fluctuating conditions, but because TR is responsive to variation in temperature, photosynthetically active radiation, and evaporative demand, the genetic basis of its response VPD per se has never been isolated. To address this, we developed a controlled-environment gravimetric phenotyping approach where we imposed VPD regimes independent from other confounding environmental variables. We screened three nested association mapping populations totaling 150 lines, three times over a 3-year period. The resulting dataset, based on phenotyping nearly 1400 plants, enabled constructing 63-point response curves for each genotype, which were subjected to a genome-wide association study. The analysis revealed a hotspot for TR response to VPD on chromosome 5A, with SNPs explaining up to 17% of the phenotypic variance. The key SNPs were found in haploblocks that are enriched in membrane-associated genes, consistent with the hypothesized physiological determinants of the trait. These results indicate a promising potential for identifying new alleles and designing next-gen wheat cultivars that are better adapted to current and future drought regimes.

Transpiration increases under high-temperature stress: Potential mechanisms, trade-offs and prospects for crop resilience in a warming world.

The frequency and intensity of high-temperature stress events are expected to increase as climate change intensifies. Concomitantly, an increase in evaporative demand, driven in part by global warming, is also taking place worldwide. Despite this, studies examining high-temperature stress impacts on plant productivity seldom consider this interaction to identify traits enhancing yield resilience towards climate change. Further, new evidence documents substantial increases in plant transpiration rate in response to high-temperature stress even under arid environments, which raise a trade-off between the need for latent cooling dictated by excessive temperatures and the need for water conservation dictated by increasing evaporative demand. However, the mechanisms behind those responses, and the potential to design the next generation of crops successfully navigating this trade-off, remain poorly investigated. Here, we review potential mechanisms underlying reported increases in transpiration rate under high-temperature stress, within the broader context of their impact on water conservation needed for crop drought tolerance. We outline three main contributors to this phenomenon, namely stomatal, cuticular and water viscosity-based mechanisms, and we outline research directions aiming at designing new varieties optimized for specific temperature and evaporative demand regimes to enhance crop productivity under a warmer and dryer climate.

Systemic effects of rising atmospheric vapor pressure deficit on plant physiology and productivity.

Earth is currently undergoing a global increase in atmospheric vapor pressure deficit (VPD), a trend which is expected to continue as climate warms. This phenomenon has been associated with productivity decreases in ecosystems and yield penalties in crops, with these losses attributed to photosynthetic limitations arising from decreased stomatal conductance. Such VPD increases, however, have occurred over decades, which raises the possibility that stomatal acclimation to VPD plays an important role in determining plant productivity under high VPD. Furthermore, evidence points to more far-ranging and complex effects of elevated VPD on plant physiology, extending to the anatomical, biochemical, and developmental levels, which could vary substantially across species. Because these complex effects are typically not considered in modeling frameworks, we conducted a quantitative literature review documenting temperature-independent VPD effects on 112 species and 59 traits and physiological variables, in order to develop an integrated and mechanistic physiological framework. We found that VPD increase reduced yield and primary productivity, an effect that was partially mediated by stomatal acclimation, and also linked with changes in leaf anatomy, nutrient, and hormonal status. The productivity decrease was also associated with negative effects on reproductive development, and changes in architecture and growth rates that could decrease the evaporative surface or minimize embolism risk. Cross-species quantitative relationships were found between levels of VPD increase and trait responses, and we found differences across plant groups, indicating that future VPD impacts will depend on community assembly and crop functional diversity. Our analysis confirms predictions arising from the hydraulic corollary to Darcy's law, outlines a systemic physiological framework of plant responses to rising VPD, and provides recommendations for future research to better understand and mitigate VPD-mediated climate change effects on ecosystems and agro-systems.

Sheathing the blade: Significant contribution of sheaths to daytime and nighttime gas exchange in a grass crop.

Despite representing a sizeable fraction of the canopy, very little is known about leaf sheath gas exchange in grasses. Specifically, estimates of sheath stomatal conductance, transpiration and photosynthesis along with their responses to light, CO2 and vapour pressure deficit (VPD) are unknown. Furthermore, the anatomical basis of these responses is poorly documented. Here, using barley as a model system, and combining leaf-level gas exchange, whole-plant gravimetric measurements, transpiration inhibitors, anatomical observations, and biophysical modelling, we found that sheath and blade stomatal conductance and transpiration were similar, especially at low light, in addition to being genotypically variable. Thanks to high abaxial stomata densities and surface areas nearly half those of the blades, sheaths accounted for up to 17% of the daily whole-plant water use, which -surprisingly- increased to 45% during the nighttime. Sheath photosynthesis was on average 17-25% that of the blade and was associated with lower water use efficiency. Finally, sheaths responded differently to the environment, exhibiting a lack of response to CO2 but a strong sensitivity to VPD. Overall, these results suggest a key involvement of sheaths in feedback loops between canopy architecture and gas exchange with potentially significant implications on adaptation to current and future climates in grasses.

Variability in temperature-independent transpiration responses to evaporative demand correlate with nighttime water use and its circadian control across diverse wheat populations.

MAIN CONCLUSION: Nocturnal transpiration, through its circadian control, plays a role in modulating daytime transpiration response to increasing evaporative demand, to potentially enable drought tolerance in wheat. Limiting plant transpiration rate (TR) in response to increasing vapor pressure deficit (VPD) has been suggested to enable drought tolerance through water conservation. However, there is very little information on the extent of diversity of TR response curves to "true" VPD (i.e., independent from temperature). Furthermore, new evidence indicate that water-saving could operate by modulating nocturnal TR (TRN), and that this response might be coupled to daytime gas exchange. Based on 3 years of experimental data on a diverse group of 77 genotypes from 25 countries and 5 continents, a first goal of this study was to characterize the functional diversity in daytime TR responses to VPD and TRN in wheat. A second objective was to test the hypothesis that these traits could be coupled through the circadian clock. Using a new gravimetric phenotyping platform that allowed for independent temperature and VPD control, we identified three and fourfold variation in daytime and nighttime responses, respectively. In addition, TRN was found to be positively correlated with slopes of daytime TR responses to VPD, and we identified pre-dawn variation in TRN that likely mediated this relationship. Furthermore, pre-dawn increase in TRN positively correlated with the year of release among drought-tolerant Australian cultivars and with the VPD threshold at which they initiated water-saving. Overall, the study indicates a substantial diversity in TR responses to VPD that could be leveraged to enhance fitness under water-limited environments, and that TRN and its circadian control may play an important role in the expression of water-saving.

Potential involvement of root auxins in drought tolerance by modulating nocturnal and daytime water use in wheat.

BACKGROUND AND AIMS: The ability of wheat genotypes to save water by reducing their transpiration rate (TR) at times of the day with high vapour pressure deficit (VPD) has been linked to increasing yields in terminal drought environments. Further, recent evidence shows that reducing nocturnal transpiration (TRN) could amplify water saving. Previous research indicates that such traits involve a root-based hydraulic limitation, but the contribution of hormones, particularly auxin and abscisic acid (ABA), has not been explored to explain the shoot-root link. In this investigation, based on physiological, genetic and molecular evidence gathered on a mapping population, we hypothesized that root auxin accumulation regulates whole-plant water use during both times of the day. METHODS: Eight double-haploid lines were selected from a mapping population descending from two parents with contrasting water-saving strategies and root hydraulic properties. These spanned the entire range of slopes of TR responses to VPD and TRN encountered in the population. We examined daytime/night-time auxin and ABA contents in the roots and the leaves in relation to hydraulic traits that included whole-plant TR, plant hydraulic conductance (KPlant), slopes of TR responses to VPD and leaf-level anatomical traits. KEY RESULTS: Root auxin levels were consistently genotype-dependent in this group irrespective of experiments and times of the day. Daytime root auxin concentrations were found to be strongly and negatively correlated with daytime TR, KPlant and the slope of TR response to VPD. Night-time root auxin levels significantly and negatively correlated with TRN. In addition, daytime and night-time leaf auxin and ABA concentrations did not correlate with any of the examined traits. CONCLUSIONS: The above results indicate that accumulation of auxin in the root system reduces daytime and night-time water use and modulates plant hydraulic properties to enable the expression of water-saving traits that have been associated with enhanced yields under drought.

Increased contribution of wheat nocturnal transpiration to daily water use under drought.

Increasing evidence suggests that in crops, nocturnal water use could represent 30% of daytime water consumption, particularly in semi-arid and arid areas. This raises the questions of whether nocturnal transpiration rates (TRN ) are (1) less influenced by drought than daytime TR (TRD ), (2) increased by higher nocturnal vapor pressure deficit (VPDN ), which prevails in such environments and (3) involved in crop drought tolerance. In this investigation, we addressed those questions by subjecting two wheat genotypes differing in drought tolerance to progressive soil drying under two long-term VPDN regimes imposed under naturally fluctuating conditions. A first goal was to characterize the response curves of whole-plant TRN and TRN /TRD ratios to progressive soil drying. A second goal was to examine the effect of VPDN increase on TRN response to soil drying and on 13 other developmental traits. The study revealed that under drought, TRN was not responsive to progressive soil drying and - intriguingly - that TRN seemingly increased with drought under high VPDN consistently for the drought-sensitive genotype. Because TRD was concomitantly decreasing with progressive drought, this resulted in TRN representing up to 70% of TRD at the end of the drydown. In addition, under drought, VPDN increase was found not to influence traits such as leaf area or stomata density. Overall, those findings indicate that TRN contribution to daily water use under drought might be much higher than previously thought, that it is controlled by specific mechanisms and that decreasing TRN under drought might be a valuable trait for improving drought tolerance.

Pot binding as a variable confounding plant phenotype: theoretical derivation and experimental observations.

MAIN CONCLUSION: Theoretical derivation predicted growth retardation due to pot water limitations, i.e., pot binding. Experimental observations were consistent with these limitations. Combined, these results indicate a need for caution in high-throughput screening and phenotyping. Pot experiments are a mainstay in many plant studies, including the current emphasis on developing high-throughput, phenotyping systems. Pot studies can be vulnerable to decreased physiological activity of the plants particularly when pot volume is small, i.e., "pot binding". It is necessary to understand the conditions under which pot binding may exist to avoid the confounding influence of pot binding in interpreting experimental results. In this paper, a derivation is offered that gives well-defined conditions for the occurrence of pot binding based on restricted water availability. These results showed that not only are pot volume and plant size important variables, but the potting media is critical. Artificial potting mixtures used in many studies, including many high-throughput phenotyping systems, are particularly susceptible to the confounding influences of pot binding. Experimental studies for several crop species are presented that clearly show the existence of thresholds of plant leaf area at which various pot sizes and potting media result in the induction of pot binding even though there may be no immediate, visual plant symptoms. The derivation and experimental results showed that pot binding can readily occur in plant experiments if care is not given to have sufficiently large pots, suitable potting media, and maintenance of pot water status. Clear guidelines are provided for avoiding the confounding effects of water-limited pot binding in studying plant phenotype.

High resolution mapping of traits related to whole-plant transpiration under increasing evaporative demand in wheat.

Atmospheric vapor pressure deficit (VPD) is a key component of drought and has a strong influence on yields. Whole-plant transpiration rate (TR) response to increasing VPD has been linked to drought tolerance in wheat, but because of its challenging phenotyping, its genetic basis remains unexplored. Further, the genetic control of other key traits linked to daytime TR such as leaf area, stomata densities and - more recently - nocturnal transpiration remains unknown. Considering the presence of wheat phenology genes that can interfere with drought tolerance, the aim of this investigation was to identify at an enhanced resolution the genetic basis of the above traits while investigating the effects of phenology genes Ppd-D1 and Ppd-B1 Virtually all traits were highly heritable (heritabilities from 0.61 to 0.91) and a total of mostly trait-specific 68 QTL were detected. Six QTL were identified for TR response to VPD, with one QTL (QSLP.ucl-5A) individually explaining 25.4% of the genetic variance. This QTL harbored several genes previously reported to be involved in ABA signaling, interaction with DREB2A and root hydraulics. Surprisingly, nocturnal TR and stomata densities on both leaf sides were characterized by highly specific and robust QTL. In addition, negative correlations were found between TR and leaf area suggesting trade-offs between these traits. Further, Ppd-D1 had strong but opposite effects on these traits, suggesting an involvement in this trade-off. Overall, these findings revealed novel genetic resources while suggesting a more direct role of phenology genes in enhancing wheat drought tolerance.

Nighttime evaporative demand induces plasticity in leaf and root hydraulic traits.

Increasing evidence suggests that nocturnal transpiration rate (TRN ) is a non-negligible contributor to global water cycles. Short-term variation in nocturnal vapor pressure deficit (VPDN ) has been suggested to be a key environmental variable influencing TRN . However, the long-term effects of VPDN on plant growth and development remain unknown, despite recent evidence documenting long-term effects of daytime VPD on plant anatomy, growth and productivity. Here we hypothesized that plant anatomical and functional traits influencing leaf and root hydraulics could be influenced by long-term exposure to VPDN . A total of 23 leaf and root traits were examined on four wheat (Triticum aestivum) genotypes, which were subjected to two long-term (30 day long) growth experiments where daytime VPD and daytime/nighttime temperature regimes were kept identical, with variation only stemming from VPDN , imposed at two levels (0.4 and 1.4 kPa). The VPDN treatment did not influence phenology, leaf areas, dry weights, number of tillers or their dry weights, consistently with a drought and temperature-independent treatment. In contrast, vein densities, adaxial stomata densities, TRN and cuticular TR, were strongly increased following exposure to high VPDN . Simultaneously, whole-root system xylem sap exudation and seminal root endodermis thickness were decreased, hypothetically indicating a change in root hydraulic properties. Overall these results suggest that plants 'sense' and adapt to variations in VPDN conditions over developmental scales by optimizing both leaf and root hydraulics.

A common genetic determinism for sensitivities to soil water deficit and evaporative demand: meta-analysis of quantitative trait Loci and introgression lines of maize.

Evaporative demand and soil water deficit equally contribute to water stress and to its effect on plant growth. We have compared the genetic architectures of the sensitivities of maize (Zea mays) leaf elongation rate with evaporative demand and soil water deficit. The former was measured via the response to leaf-to-air vapor pressure deficit in well-watered plants, the latter via the response to soil water potential in the absence of evaporative demand. Genetic analyses of each sensitivity were performed over 21 independent experiments with (1) three mapping populations, with temperate or tropical materials, (2) one population resulting from the introgression of a tropical drought-tolerant line in a temperate line, and (3) two introgression libraries genetically independent from mapping populations. A very large genetic variability was observed for both sensitivities. Some lines maintained leaf elongation at very high evaporative demand or water deficit, while others stopped elongation in mild conditions. A complex architecture arose from analyses of mapping populations, with 19 major meta-quantitative trait loci involving strong effects and/or more than one mapping population. A total of 68% of those quantitative trait loci affected sensitivities to both evaporative demand and soil water deficit. In introgressed lines, 73% of the tested genomic regions affected both sensitivities. To our knowledge, this study is the first genetic demonstration that hydraulic processes, which drive the response to evaporative demand, also have a large contribution to the genetic variability of plant growth under water deficit in a large range of genetic material.

Influence of atmospheric vapour pressure deficit on ozone responses of snap bean (Phaseolus vulgaris L.) genotypes.

Environmental conditions influence plant responses to ozone (O(3)), but few studies have evaluated individual factors directly. In this study, the effect of O(3) at high and low atmospheric vapour pressure deficit (VPD) was evaluated in two genotypes of snap bean (Phaseolus vulgaris L.) (R123 and S156) used as O(3) bioindicator plants. Plants were grown in outdoor controlled-environment chambers in charcoal-filtered air containing 0 or 60 nl l(-1) O(3) (12 h average) at two VPDs (1.26 and 1.96 kPa) and sampled for biomass, leaf area, daily water loss, and seed yield. VPD clearly influenced O(3) effects. At low VPD, O(3) reduced biomass, leaf area, and seed yield substantially in both genotypes, while at high VPD, O(3) had no significant effect on these components. In clean air, high VPD reduced biomass and yield by similar fractions in both genotypes compared with low VPD. Data suggest that a stomatal response to VPD per se may be lacking in both genotypes and it is hypothesized that the high VPD resulted in unsustainable transpiration and water deficits that resulted in reduced growth and yield. High VPD- and water-stress-induced stomatal responses may have reduced the O(3) flux into the leaves, which contributed to a higher yield compared to the low VPD treatment in both genotypes. At low VPD, transpiration increased in the O(3) treatment relative to the clean air treatment, suggesting that whole-plant conductance was increased by O(3) exposure. Ozone-related biomass reductions at low VPD were proportionally higher in S156 than in R123, indicating that differential O(3) sensitivity of these bioindicator plants remained evident when environmental conditions were conducive for O(3) effects. Assessments of potential O(3) impacts on vegetation should incorporate interacting factors such as VPD.

Harnessing nighttime transpiration dynamics for drought tolerance in grasses.

Non-negligible nighttime transpiration rates (TRN) have been identified in grasses such as wheat and barley. Evidence from the last 30 years indicate that in drought-prone environments with high evaporative demand, TRN could amount to 8-55% of daytime TR, leading several investigators to hypothesize that reducing TRN might represent a viable water-saving strategy that minimizes seemingly 'wasteful' water loss that is not traded for CO2 fixation. More recently however, evidence suggests that actual increases in TRN during pre-dawn hours, which are presumably controlled by the circadian clock, mediate drought tolerance - not through water conservation - but by enabling maximized gas exchange early in the morning before midday depression sets in. Finally, new findings point to a previously undocumented role for leaf sheaths as substantial contributors (up to 45%) of canopy TRN, although the extent of their involvement in these two strategies remains unknown. In this paper, we synthesize and reconcile key results from experimental and simulation-based modeling efforts conducted at scales ranging from the leaf tissue to the field plot on wheat and barley to show that both strategies could in fact concomitantly enable yield gains under limited water supply. We propose a simple framework highlighting the role played by TRN dynamics in drought tolerance and provide a synthesis of potential research directions, with an emphasis on the need for further examining the role played by the circadian clock and leaf sheath gas exchange.

Transpiration response of 'slow-wilting' and commercial soybean (Glycine max (L.) Merr.) genotypes to three aquaporin inhibitors.

The slow-wilting soybean [Glycine max (L.) Merr.] genotype, PI 416937, exhibits a limiting leaf hydraulic conductance for transpiration rate (TR) under high vapour pressure deficit (VPD). This genotype has a constant TR at VPD greater than 2 kPa, which may be responsible for its drought tolerance as a result of soil water conservation. However, the exact source of the hydraulic limitation between symplastic and apoplastic water flow in the leaf under high VPD conditions are not known for PI 416937. A comparison was made in the TR response to aquaporin (AQP) inhibitors between PI 416937 and N01-11136, a commercial genotype that has a linear TR response to VPD in the 1-3.5 kPa range. Three AQP inhibitors were tested: cycloheximide (CHX, a de novo synthesis inhibitor), HgCl(2), and AgNO(3). Dose-response curves for the decrease in TR following exposure to each inhibitor were developed. Decreases in TR of N01-11136 following treatment with inhibitors were up to 60% for CHX, 82% for HgCl(2), and 42% for AgNO(3). These results indicate that the symplastic pathway terminating in the guard cells of these soybean leaves may be at least as important as the apoplastic pathway for water flow in the leaf under high VPD. While the decrease in TR for PI 416937 was similar to that of N01-11136 following exposure to CHX and HgCl(2), TR of PI 416937 was insensitive to AgNO(3) exposure. These results indicate the possibility of a lack of a Ag-sensitive leaf AQP population in the slow-wilting line, PI 416937, and the presence of such a population in the commercial line, N01-11136.

Sleep tight and wake-up early: nocturnal transpiration traits to increase wheat drought tolerance in a Mediterranean environment.

In wheat, night-time transpiration rate (TRN) could amount to 14-55% of daytime transpiration rate (TR), depending on the cultivar and environment. Recent evidence suggests that TRN is much less responsive to soil drying than daytime TR, and that such 'wasteful' water losses would increase the impact of drought on yields. In contrast, other evidence indicates that pre-dawn, circadian increases in TRN may enable enhanced radiation use efficiency, resulting in increased productivity under water deficit. Until now, there have been no attempts to evaluate these seemingly conflicting hypotheses in terms of their impact on yields in any crop. Here, using the Mediterranean environment of Tunisia as a case study, we undertook a simulation modelling approach using SSM-Wheat to evaluate yield outcomes resulting from these TRN trait modifications. TRN represented 15% of daytime TR-generated yield penalties of up to 20%, and these worsened when TRN was not sensitive to soil drying TR. For the same TRN level (15%), simulating a predawn increase in TRN alleviated yield penalties, leading to yield gains of up to 25%. Overall, this work suggests that decreasing TRN but increasing pre-dawn circadian control would be a viable breeding target to increase drought tolerance in a Mediterranean environment.

The Hidden Costs of Nighttime Warming on Yields.

Nighttime warming poses a threat to global food security as it is driving yield declines worldwide, but our understanding of the physiological basis of this phenomenon remains very limited. Furthermore, it is often assumed that such declines are driven solely by increases in nighttime temperature (TNight). Here we argue that, in addition to temperature, increases in nighttime evaporative demand may 'conspire' to penalize yields and end-use quality traits. We propose an ecophysiological framework outlining the possible mechanistic basis of such declines in yield and quality. We suggest ways to use the proposed framework as a guide to future efforts aimed at alleviating productivity losses by integrating crop ecophysiology with modeling, breeding, and management.