Elevated Vapor Pressure Deficit Research Articles

Assessing temporal variability in durum wheat performance and stability through multi-trait mean performance selection in Mediterranean climate

Durum wheat, a staple crop in Italy, faces substantial challenges due to increasing droughts and rising temperatures. This study examines the grain yield, agronomic traits, and quality of 41 durum wheat varieties over ten growing seasons in Southern Italy, utilizing a randomized complete block design. Notably, most varieties were not repeated between trials and 45% of the data was missing. The results indicate that the interaction between genotype and environment (GEI) significantly impacted all traits. High temperatures, elevated vapor pressure deficit (VPD), and water deficits severely affected yield and quality during warm years, while cooler years with favorable water availability promoted better growth and higher yields. Broad-sense heritability (H²) was generally low, suggesting that environmental factors played a major role in the observed traits. However, some traits, such as grain yield, ears per square meter, plant height, bleached wheat, thousand-grain weight, and hectoliter weight exhibited moderate to high heritability of the mean genotype (h²mg), indicating their potential for effective selection in breeding programs. Correlation analyses revealed strong connections between certain traits, such as protein content, and gluten index as well as between grain yield, and spike per square meter. Using the Multi-Trait Mean Performance Selection (MTMPS) index, the study identified six top-performing varieties. Among these, Antalis (G4) and Core (G18) consistently demonstrated strong adaptability and stability across different environments, particularly in hotter, drier conditions. Furio Camillo (G31) also exhibited valuable traits. This study highlights the challenges and complexities of breeding durum wheat for improved yield and quality in the face of climate change.

Compound hot–dry events greatly prolong the recovery time of dryland ecosystems

ABSTRACT Compound hot–dry events cause more severe impacts on terrestrial ecosystems than dry events, while the differences in recovery time (ΔRT) between hot–dry and dry events and their contributing factors remain unclear. Both remote sensing observations and eddy covariance measurements reveal that hot–dry events prolong the recovery time compared with dry events, with greater prolongation of recovery time in drylands than in humid regions. Random forest regression modeling demonstrates that the difference in vapor pressure deficit between hot–dry and dry events, with an importance score of 35%, is the major factor contributing to ΔRT. The severity of stomatal restriction exceeds that of non-stomatal limitation, which restricts the vegetation productivity that is necessary for the recovery process. These results emphasize the negative effect of vapor pressure deficit on vegetation recovery during hot–dry events and project an extension of drought recovery time considering elevated vapor pressure deficit in a warming world.

Coping with extremes: Responses of Quercus robur L. and Fagus sylvatica L. to soil drought and elevated vapour pressure deficit

Climate change, particularly droughts and heat waves, significantly impacts global photosynthesis and forest ecosystem sustainability. To understand how trees respond to and recover from hydrological stress, we investigated the combined effects of soil moisture and atmospheric vapour pressure deficit (VPD) on seedlings of the two major European broadleaved tree species Fagus sylvatica (FS) and Quercus robur (QR). The experiment was conducted under natural forest gap conditions, while soil water availability was strictly manipulated. We monitored gas exchange (net photosynthesis, stomatal conductance and transpiration rates), nonstructural carbohydrates (NSC) concentration in roots and stomatal morphometry (size and density) during a drought period and recovery. Our comparative empirical study allowed us to distinguish and quantify the effects of soil drought and VPD on stomatal behavior, going beyond theoretical models. We found that QR conserved water more conservatively than FS by reducing transpiration and regulating stomatal conductance under drought. FS maintained higher stomatal conductance and transpiration at elevated VPD until soil moisture became critically low. QR showed higher intrinsic water use efficiency than FS. Stomata density and size also likely played a role in photosynthetic rate and speed of recovery, especially since QR with its seasonal adjustments in stomatal traits (smaller, more numerous stomata in summer leaves) responded and recovered faster compared to FS. Our focal species showed different responses in NSC content under drought stress and recovery, suggesting possible different evolutionary pathways in coping with stress. QR mobilized soluble sugars, while FS relied on starch mobilization to resist drought. Although our focal species often co-occur in mixed forests, our study showed that they have evolved different physiological, morphological and biochemical strategies to cope with drought stress. This suggests that ongoing climate change may alter their competitive ability and adaptive potential in favor of one of the species studied.

Stomatal CO2 responses at sub- and above-ambient CO2 levels employ different pathways in Arabidopsis.

Stomatal pores that control plant CO2 uptake and water loss affect global carbon and water cycles. In the era of increasing atmospheric CO2 levels and vapor pressure deficit (VPD), it is essential to understand how these stimuli affect stomatal behavior. Whether stomatal responses to sub-ambient and above-ambient CO2 levels are governed by the same regulators and depend on VPD remains unknown. We studied stomatal conductance responses in Arabidopsis (Arabidopsis thaliana) stomatal signaling mutants under conditions where CO2 levels were either increased from sub-ambient to ambient (400 ppm) or from ambient to above-ambient levels under normal or elevated VPD. We found that guard cell signaling components involved in CO2-induced stomatal closure have different roles in the sub-ambient and above-ambient CO2 levels. The CO2-specific regulators prominently affected sub-ambient CO2 responses, whereas the lack of guard cell slow-type anion channel SLOW ANION CHANNEL-ASSOCIATED 1 (SLAC1) more strongly affected the speed of above-ambient CO2-induced stomatal closure. Elevated VPD caused lower stomatal conductance in all studied genotypes and CO2 transitions, as well as faster CO2-responsiveness in some studied genotypes and CO2 transitions. Our results highlight the importance of experimental setups in interpreting stomatal CO2-responsiveness, as stomatal movements under different CO2 concentration ranges are controlled by distinct mechanisms. Elevated CO2 and VPD responses may also interact. Hence, multi-factor treatments are needed to understand how plants integrate different environmental signals and translate them into stomatal responses.

Enhanced observations from an optimized soil-canopy-photosynthesis and energy flux model revealed evapotranspiration-shading cooling dynamics of urban vegetation during extreme heat

Previousstudies on the cooling of urban vegetation mainly focused on its transpiration or shading effect separately, neglecting to explore the combined evapotranspiration-shading cooling. Further, accurate quantification of evapotranspiration-shading cooling remains challenging due to heterogeneity of urban landscapes, which limits understanding of its high-resolution spatiotemporal patterns. Here, we integrate high-precision remote sensing data and the Soil-Canopy-Observations of Photosynthesis and Energy Fluxes (SCOPE) model to propose an optimized quantitative approach. The approach was used to investigate changes in evapotranspiration-shade cooling during extreme heat.Taking Shanghai metropolitan as case, the results show: (1) The cooling capacity of urban vegetation in nighttime (18:00–6:00) is enhanced during extreme heat, which is attributed to accumulated effect of shading and enhanced evapotranspiration due to elevated vapor-pressure deficit. (2) In densely built-up areas with limited vegetation, there is a significant lack of thermal regulation, especially in the early morning (7:00) and late evening (17:00), thus exacerbating thermal stress. (3) At midday (11:00–13:00) there was a slight decrease in evaporative cooling, probably caused by the behaviour of the stomatal closure at high temperatures. Concurrently, high radiation causes the shading effect of vegetation to become more prominent, amplifying the cooling contrast between areas with dense and sparse vegetation cover. Moreover, the study also highlights that grassland with >50% cover can provide cooling effects similar to that of forest land. Overall, our study not only enhances the understanding of urban vegetation's cooling effects but also underscores the importance of strategic urban vegetation planning in mitigating urban heat, particularly under the escalating frequency and intensity of heat waves.

Hydraulic Response of Deciduous and Evergreen Broadleaved Shrubs, Grown on Olympus Mountain in Greece, to Vapour Pressure Deficit.

In this study, leaf hydraulic functionality of co-occurring evergreen and deciduous shrubs, grown on Olympus Mountain, has been compared. Four evergreen species (Arbutus andrachne, Arbutus unedo, Quercus ilex and Quercus coccifera) and four deciduous species (Carpinus betulus, Cercis siliquastrum, Coronilla emeroides and Pistacia terebinthus) were selected for this study. Predawn and midday leaf water potential, transpiration, stomatal conductance, leaf temperature and leaf hydraulic conductance were estimated during the summer period. The results demonstrate different hydraulic tactics between the deciduous and evergreen shrubs. Higher hydraulic conductance and lower stomatal conductance were obtained in deciduous plants compared to the evergreens. Additionally, positive correlations were detected between water potential and transpiration in the deciduous shrubs. The seasonal leaf hydraulic conductance declined in both deciduous and evergreens under conditions of elevated vapor pressure deficit during the summer; however, at midday, leaf water potential reached comparable low values, but the deciduous shrubs exhibited higher hydraulic conductance compared to the evergreens. It seems likely that hydraulic traits of the coexisting evergreen and deciduous plants indicate water spending and saving tactics, respectively; this may also represent a limit to drought tolerance of these species grown in a natural environment, which is expected to be affected by global warming.

Plant waterflow restrictions among sweet corn lines related to limited‐transpiration trait

AbstractWhile partial stomata closure under elevated vapor pressure deficit (VPD), that is, a limited‐transpiration trait resulting in soil water conservation and crop drought resiliency, has been identified in a few lines in all major crop species, the basis for the trait is not resolved. Since changes in stomatal aperture are associated with hydraulic processes, properties influencing waterflow in the plant are candidates associated with expression of limited‐transpiration. In a previous study with leaves of sweet corn (Zea mays L.), limited‐transpiration was only weakly associated with the effect of silver ion, an aquaporin inhibitor, on leaf transpiration rate. The hypothesis was explored that total aquaporin activity rather than only silver‐sensitive aquaporins may be more relevant in the expression of limited transpiration. Leaves were fed potent aquaporin inhibitors of mercury and peroxide which both resulted in greater transpiration inhibition than silver, but there were no differences in response among lines. A second hypothesis was that there may be a major limitation of waterflow in the root system accounting for genotypic variation in expression of the limited‐transpiration trait. No differences in root hydraulic conductance were found among sweet corn lines differing in the limited‐transpiration trait. However, it was discovered that the root conductance decreased during the photoperiod in all tested lines. Measurements of root hydraulic conductance in response to a silver treatment showed differences among lines with an association between the VPD breakpoint for limited transpiration and transpiration sensitivity to the silver treatment.

Comparison of water flow capacity in leaves among sweet corn genotypes as basis for plant transpiration rate sensitivity to vapor pressure deficit

AbstractGenotypes in crop species have been identified that initiate partial stomata closure at elevated atmospheric vapor pressure deficit (VPD), which results in conserved soil water for crop use during subsequent water‐deficit episodes and thereby allowing for possible yield increase. In sweet corn (Zea mays L), 17 genotypes have been previously identified with the VPD‐responsive trait, although the VPD value at the initiation of stomata closure varied among genotypes. A hypothesis to explain variation in transpiration response to VPD is that water flow capacity in the leaves differs among genotypes. To gauge water flow capacity in leaves, the rate of stomata opening was observed visually after stomata closure was induced by 3 kPa VPD. The stomata opening time was rapid and varied among genotypes from 90 to 179 s. However, there was no correlation between opening time and the VPD at which partial stomata closure was initiated in intact plants. An additional set of experiments was done to examine whether genotypic differences in a subpopulation of silver‐inhibited aquaporins might contribute to differences in leaf water flow. There was a correlation among genotypes between slow opening time of the stomata and greater inhibition of transpiration rate following feeding leaves with silver ion. However, the response to the silver treatment did not correlate with the VPD at which transpiration decrease of intact plants was initiated. These results indicate that the differences observed in the water flow capacity in sweet corn leaves were not major factors accounting for the genotypic differences in whole‐plant transpiration response to elevated VPD.

The sensitivity of fuel moisture to forest structure effects on microclimate

An understanding of variation in dead fuel moisture content (FMC) is essential for accurate predictions of wildfire risk, particularly in productive wet forests where FMC is a primary control on fire activity. In these systems, forest structure and composition influence microclimate, which in turn effects FMC. However, changing disturbance regimes are altering forest structure and our understanding of the sensitivity of FMC to these variations is incomplete. To explore this, we quantified the relative importance of changes in microclimate resulting from altered forest structure to FMC variability. This was done by modelling FMC using microclimate scenarios extracted from observations at six field sites with contrasting structural profiles. The scenarios related to maximum, mean, and minimum fire-related microclimate conditions. To understand sensitivity in a fire management context, we summarised results using FMC thresholds of fuel availability (FA) across three seasons – corresponding to wildfire and prescribed burning conditions. Distinct differences in FA demonstrate the potential for altered structure to influence fire activity in wet forests. Structure effects on vapour pressure deficit (VPD) and longwave radiation (LWR) exerted the strongest control on FA, resulting in increases of 125% and 87% across all seasons respectively, while shortwave radiation (SWR) had a limited influence. However, FMC sensitivity to microclimate inputs changed with season. Critically, in summer, forest structure effects generating elevated VPD conditions resulted in 8 additional days (of 30) when FMC was less than 10%, a threshold for intense fire behaviour. Our research supports the hypothesis that FMC is sensitive to forest structure change, and that this sensitivity can be mechanistically linked to elevated VPD and LWR in the microclimate resulting from these changes. Given the potential for disturbances to alter forest structure in the future, the potential impact of this on microclimate and FMC should be considered in landscape predictions of wildfire risk.

Genotypic variation of conservative and profligate water use in the vegetative and reproductive stages of canola (

Limited water availability is a major constraint to canola (Brassica napus L.) yield in the Mediterranean-type climate region. Selecting and breeding for genotypes with conservative water use characteristics is a promising strategy to improve yield in this environment. Three experiments were conducted to investigate transpiration responses (TR) to vapour pressure deficit (VPD) and progressive soil drying with 8-20 canola genotypes. We used the linear-plateau model to describe TR to elevated VPD and decreased fraction of transpirable soil water (FTSW) and identified the VPD and FTSW thresholds for plant to limit its transpiration. Canola genotypes showed significant variations in both VPD and FTSW thresholds. The genotypes with conservative water use reduced TR at a lower VPD threshold and decreased TR at a higher FTSW threshold than the profligate ones. We found that the conservative genotypes had low VPD and high FTSW thresholds while the profligate ones had high VPD and low FTSW thresholds. This conservative and profligate water use characteristics were consistent during both vegetative and reproductive stages. Furthermore, the relative yield of genotypes under drought conditions was positively related to the FTSW thresholds during the reproductive stage, indicating the better relative yield performance of conservative genotypes in water-limited farming system. We conclude that canola genotypes with lower VPD and higher FTSW thresholds could conserve water and defer water use for reproductive growth while the profligate genotypes can be deployed to take advantage of high rainfall in the high rainfall zone of southern Australia.

Limited‐transpiration rate and plant conductance in a diverse sweet corn population

AbstractLimited‐transpiration rate at elevated vapor pressure deficit (VPD) can allow soil water conservation for use during late‐season drought, but it can also result in decreased crop yields under well‐watered conditions because of restricted crop gas exchange. Previous studies with sweet corn (Zea mays L.) have found the limited‐transpiration rate was quite common among commercial cultivars even though sweet corn is commonly grown under well‐watered conditions. This study was undertaken to identify possible genetic sources of sweet corn that were not encumbered by the limited‐transpiration trait. Additionally, data were obtained to compare the plant hydraulic conductance among the lines. Among the 26 sweet corn lines included in this study, only eight did not express the limited‐transpiration trait. Four of the lines not expressing the limited‐transpiration (IL395a, IL543c, P39, and SD245) had stomata vapor conductance values over the range of tested VPD similar to the values expressed by many of the limited‐transpiration lines only at low VPD. The eight lines not expressing the limited‐transpiration trait tended to have low plant hydraulic conductance. For those lines expressing the limited‐transpiration trait, there was a correlation between the VPD at initiation of limited transpiration and plant hydraulic conductance. Expression of the limited‐transpiration traits proved, however, to be temperature sensitive in 7 of 18 tested lines expressing the trait at 32 °C because they failed to express the trait at 38 °C. The genetic variation in expression of the limited‐transpiration trait and plant hydraulic conductance identified in this study offers specific candidate inbred lines that could be used as genetic resources for improving sweet corn growth and yield for well‐watered environments.

Compound climate events increase tree drought mortality across European forests

Climate change can lead to the simultaneous occurrence of extreme droughts and heat waves increasing the frequency of compound events with unknown impacts on forests. Here we use two independent datasets, a compiled database of tree drought mortality events and the ICP-Forest level I plots, to study the impacts of the simultaneous occurrence of hot summers, with elevated vapour pressure deficit (VPD), and dry years on forest defoliation and mortality across Europe. We focused on tree drought mortality and background mortality rates, and we studied their co-occurrence with compound events of hot summers and dry years. In total, 143 out of 310 mortality events across Europe, i.e. 46% of cases, corresponded with rare compound events characterized by hot summers and dry years. Over the past decades, summer temperature increased in most sites and severe droughts resulted in compound events not observed before the 1980s. From the ICP-Forest plots we identified 291 (1718 trees) and 61 plots (128 trees) where severe defoliation and mortality, respectively, were caused by drought. The analyses of these events showed that 34% and 27% of the defoliation and mortality cases corresponded with rare compound climate events, respectively. Background mortality rates across Europe in the period 1993–2013 presented higher values in regions where summer temperature and VPD more steeply rose, where drought frequency increased. The steady increase in summer temperatures and VPD in Southern and Eastern Europe may favor the occurrence of compound events of hot summers and dry conditions. Giving that both, local and intense tree drought mortality events and background forest mortality rates, are linked to such compound events we can expect an increase in forest drought mortality in these European regions over the next decades.

Drought resilience in CIMMYT maize lines adapted to Africa resulting from transpiration sensitivity to vapor pressure deficit and soil drying

ABSTRACT Low rainfall limits crop yield, particularly for maize (Zea mays L.) in southern Africa. Consequently, there is a need to identify genetic sources of specific drought-related traits that can contribute to soil water conservation and increased yields under water-limited conditions. In this study, maize genotypes released for production in southern Africa were tested for expression of two soil water-conservation traits: limited transpiration under elevated vapor pressure deficit (VPD) and decreased transpiration rate at high soil water contents earlier in the soil drying cycle. Two genotypes, CML 590 and CML 593, were identified and confirmed to initiate expression of limited-transpiration rate at VPD above about 1.9 kPa. In the soil-drying experiment, Umbelu 8923 and Umbelu 8930 closed their stomata earliest in the soil drying cycle as compared to other tested genotypes. These four genotypes with specific physiological traits for superior response to water deficit are genetic resources for further study to improve maize drought resilience.

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.

Dissecting the combined effects of air temperature and relative humidity on water‐use efficiency of barley under drought stress

AbstractAs soil water deficit limits crop growth and yield, yet the combined effects of soil drying and elevated vapour pressure deficit (VPD) of the air on crop performance have not been fully understood. The objective of this study was to investigate the interactive effects of soil drying and elevated VPD on growth and physiology of barley seedlings grown under controlled climates. The plants were grown in four separate climate chambers with two air temperature (T, 20 and 30°C) and relative humidity (RH: 60 and 75%) levels, respectively, resulting in four VPD levels, viz. VPD1 (0.59 kPa, 20°C + 75% RH), VPD2 (0.94 kPa, 20°C + 60% RH), VPD3 (1.06 kPa, 30°C + 75% RH) and VPD4 (1.70 kPa, 30°C + 60% RH). Since the 4th leaf stage, the plants were subjected to two irrigation treatments, namely well‐watered and drought‐stressed. The results showed that soil water deficits significantly limited leaf gas exchange rates and reduced shoot dry biomass (DMshoot) and water consumption (WU), whereas increased leaf ABA concentration ([ABA]leaf) and plant water‐use efficiency (WUE) at each VPD level. Plants grown under VPD1 and VPD2 had greater net photosynthetic rate (An) and stomatal conductance (gs) while lower transpiration rate (Tr) than those grown under VPD3 and VPD4 at both irrigation treatments. Besides, elevated VPD resulted in an increased daily transpiration per leaf area (DT) and greater soil water threshold at which DT starts to decrease. Interactions between VPD and drought were significant where the effects of drought on An, relative water content (RWC), DMshoot and WUE were more severe at VPD3 and VPD4 than that at VPD1 and VPD2. When disentangling the effects of T and RH, the results showed that it was T rather than RH influenced the An, stomatal density and RWC, whereas the combined effect of T and RH, that is VPD, was significant in affecting gs, Tr, [ABA]leaf, leaf area, DMshoot, WU and WUE, where WUE was negatively correlated with VPD. Therefore, it is essential to dissect the effects of T and RH when analysing their combined effects with soil water deficits on crop performance in a future warmer and drier climate.

Provenance- and life-history stage-specific responses of the dwarf shrub Calluna vulgaris to elevated vapour pressure deficit

Climate change may alter microscale-effective ecosystem properties such as atmospheric water vapour pressure, but consequences for plant growth are insufficiently understood. Within a northwest German heathland an open-top chamber experiment was established to analyse the effects of elevated vapour pressure deficit (eVPD) on growth responses of Calluna vulgaris considering both plant origin (Atlantic (AP), sub-Atlantic (SAP), sub-Continental (SCP)) and life-history stage (1-year vs. 10-year old plants). We hypothesised that the plants’ sensitivity to eVPD decreases (i) from AP to SCP and (ii) with progressing life-history stage. Elevated VPD caused a provenance-specific decrease of shoot increment whilst aboveground biomass productivity remained unaffected. AP and SAP responded with increasing belowground biomass δ13C signatures to eVPD, whereas δ13C values decreased for SCP. Moreover, eVPD increased and decreased belowground biomass δ13C signatures of 1- and 10-year old plants, respectively. These responses to eVPD were related to differences in morphological-chemical traits and the plants’ trait plasticity in response to eVPD. SCP showed the highest aboveground tissue mass density and significantly increased tissue C:N ratios under eVPD. One-year old plants had a tenfold higher shoot:root ratio than 10-year old plants, making young plants more sensitive to eVPD. Our findings demonstrate that the atmospheric water status affects the morphology and physiology of Calluna independent of the soil water status. The results have implications for the conservation of heathlands under climate change: (i) SCP may constitute an appropriate ecotype for assisted migration-approaches, and (ii) management needs to weigh different options for heathland rejuvenation.

Cotton Genotypic Variability for Transpiration Decrease with Progressive Soil Drying

Drought is a major abiotic stress factor limiting cotton yield. It is important to identify the genotypes that can conserve water under drought stress conditions and improve yield. The objective of the current study was to evaluate cotton genotypes for water conservation traits, i.e., high FTSW (Fraction of Transpirable Soil Water) threshold for transpiration. Plants utilize water slowly by declining transpiration at high FTSW and conserving soil water, which can be used by the plant later in the growing season to improve yield. Fifteen cotton varieties were selected based on their differences in transpiration response to elevated vapor pressure deficit (VPD) to study drought responses. Two pot experiments were carried out in the greenhouse to determine the FTSW threshold for the transpiration rate as the soil dried. A significant variation (p < 0.01) in the FTSW threshold values for transpiration decline was observed, ranging from 0.35 to 0.60 among cotton cultivars. Genotypes with high FTSW thresholds also displayed low transpiration under well-watered conditions. Further studies with four selected genotype contrasts in FTSW threshold values for transpiration showed differences (p < 0.05 to 0.001) in gas exchange parameters and water potentials. This study demonstrated that there are alternate traits among the cotton genotypes for enhancing soil water conservation to improve yield under water-limited conditions.

Basis of limited-transpiration rate under elevated vapor pressure deficit and high temperature among sweet corn cultivars

One plant trait that has been developed in several crop species to increase the effectiveness in water use through the cropping season is limited-transpiration under elevated atmospheric vapor pressure deficit (VPD). This trait allows water conservation early in the season so that there is more soil water available late in the season for sustained physiological activity during seed development. In sweet corn (Zea mays L. saccharata), where the quality of the kernels is important, this trait could prove to be especially beneficial. The background objective of this study was to explore 16 sweet corn cultivars for expression of the limited-transpiration trait. It was found at 32 °C that 13 of the 16 cultivars expressed the trait. It was found in a subset of eight of these cultivars, however, only half retained the limited-transpiration trait at 38 °C. The additional objectives were to explore the hypotheses that expression of the limited-transpiration trait was related to plant hydraulic conductance, and to the abundance of silver-sensitive aquaporins in the leaves. In cultivars that lost expression of the limited-transpiration trait at 38 °C there were large increases in plant hydraulic conductance at 38 °C as compared to 32 °C. Abundance of silver-sensitive aquaporins was related to the transpiration rate under low VPD conditions. That is, those cultivars with more abundant silver-sensitive aquaporins had greater transpiration rates as a result of greater stomatal conductance. These results showed that while expression of the limited-transpiration trait in sweet corn at 32 °C was common, differences in expression of the trait at 38 °C were observed due to differences in plant hydraulic conductance and stomatal conductance.

The Response of Water Dynamics to Long-Term High Vapor Pressure Deficit Is Mediated by Anatomical Adaptations in Plants.

Vapor pressure deficit (VPD) is the driver of water movement in plants. However, little is known about how anatomical adaptations determine the acclimation of plant water dynamics to elevated VPD, especially at the whole plant level. Here, we examined the responses of transpiration, stomatal conductance (gs), hydraulic partitioning, and anatomical traits in two tomato cultivars (Jinpeng and Zhongza) to long-term high (2.2–2.6 kPa) and low (1.1–1.5 kPa) VPD. Compared to plants growing under low VPD, no variation in gs was found for Jinpeng under high VPD conditions; however, high VPD induced an increase in whole plant hydraulic conductance (Kplant), which was responsible for the maintenance of high transpiration. In contrast, transpiration was not influenced by high VPD in Zhongza, which was primarily attributed to a coordinated decline in gs and Kplant. The changes in gs were closely related to stomatal density and size. Furthermore, high VPD altered hydraulic partitioning among the leaf, stem, and root for both cultivars via adjustments in anatomy. The increase in lumen area of vessels in veins and large roots in Jinpeng under high VPD conditions improved water transport efficiency in the leaf and root, thus resulting in a high Kplant. However, the decreased Kplant for Zhongza under high VPD was the result of a decline of water transport efficiency in the leaf that was caused by a reduction in vein density. Overall, we concluded that the tradeoff in anatomical acclimations among plant tissues results in different water relations in plants under high VPD conditions.

Maize yield under a changing climate: The hidden role of vapor pressure deficit

Temperatures over the next century are expected to rise to levels detrimental to crop growth and yield. As the atmosphere warms without additional water vapor input, vapor pressure deficit (VPD) increases as well. Increased temperatures and accompanied elevated VPD levels can both lead to negative impacts on crop yield. The independent importance of VPD, however, is often neglected or conflated with that from temperature due to a tight correlation between the two climate factors. We used a coupled process-based crop (MAIZSIM) and soil (2DSOIL) model to gain a mechanistic understanding of the independent roles temperature and VPD play in crop yield projections, as well as their interactions with rising CO2 levels and changing precipitation patterns. We found that by separating out the VPD effect from rising temperatures, VPD increases had a greater negative impact on yield (12.9 ± 1.8%, increase in VPD associated with 2 °C warming) compared to that from warming (8.5 ± 1.4%, the direct effect of 2 °C warming). The negative impact of these two factors varied with precipitation levels and influenced yield through separate mechanisms. Warmer temperatures caused yield loss mainly through shortening the growing season, while elevated VPD increased water loss and triggered several water stress responses such as reduced photosynthetic rates, lowered leaf area development, and shortened growing season length. Elevated CO2 concentrations partially alleviated yield loss under warming or increased VPD conditions through water savings, but the impact level varied with precipitation levels and was most pronounced under drier conditions. These results demonstrate the key role VPD plays in crop growth and yield, displaying a magnitude of impact comparative to temperature and CO2. A mechanistic understanding of the function of VPD and its relation with other climate factors and management practices is critical to improving crop yield projections under a changing climate.