Vapor Pressure Deficit Conditions Research Articles

Drought-Induced Responses in Maize under Different Vapor Pressure Deficit Conditions.

Water stress in plants depends on the soil water level and the evaporative demand. In this study, the physiological, biochemical, and molecular response of maize were examined under three evaporative demand conditions (low—1.00 kPa, medium—2.2 kPa, and high—4.00 kPa Vapor pressure deficit (VPD)) at three different soil water content (SWC); well-watered, 45%, and 35% SWC. Plants grown at 35% SWC under high VPD had significant (p < 0.01) lower leaf weight, leaf area, and leaf number than low VPD. Plants under low, medium, and high VPD with drought stress (45% and 35% SWC) showed a 30 to 60% reduction in their leaf area compared to well-watered plants. Gas exchange parameters including photosynthesis, stomatal conductance, and water use efficiency exhibited significant differences (p < 0.01) between treatments, with the highest reduction occuring at 35% SWC and high VPD. Both drought and VPD significantly (p < 0.01) increased C4 enzyme levels and some transcription factors with increased stress levels. Transcription factors primarily related to Abssisic Acid (ABA) synthesis were upregulated under drought, which might be related to high ABA levels. In summary, severe drought levels coupled with high VPD had shown a significant decrease in plant development by modifying enzymes, ABA, and transcription factors.

Antecedent climatic conditions spanning several years influence multiple land-surface phenology events in semi-arid environments

Ecological processes are complex, often exhibiting non-linear, interactive, or hierarchical relationships. Furthermore, models identifying drivers of phenology are constrained by uncertainty regarding predictors, interactions across scales, and legacy impacts of prior climate conditions. Nonetheless, measuring and modeling ecosystem processes such as phenology remains critical for management of ecological systems and the social systems they support. We used random forest models to assess which combination of climate, location, edaphic, vegetation composition, and disturbance variables best predict several phenological responses in three dominant land cover types in the U.S. Northwestern Great Plains (NWP). We derived phenological measures from the 25-year series of AVHRR satellite data and characterized climatic predictors (i.e., multiple moisture and/or temperature based variables) over seasonal and annual timeframes within the current year and up to 4 years prior. We found that antecedent conditions, from seasons to years before the current, were strongly associated with phenological measures, apparently mediating the responses of communities to current-year conditions. For example, at least one measure of antecedent-moisture availability [precipitation or vapor pressure deficit (VPD)] over multiple years was a key predictor of all productivity measures. Variables including longer-term lags or prior year sums, such as multi-year-cumulative moisture conditions of maximum VPD, were top predictors for start of season. Productivity measures were also associated with contextual variables such as soil characteristics and vegetation composition. Phenology is a key process that profoundly affects organism-environment relationships, spatio-temporal patterns in ecosystem structure and function, and other ecosystem dynamics. Phenology, however, is complex, and is mediated by lagged effects, interactions, and a diversity of potential drivers; nonetheless, the incorporation of antecedent conditions and contextual variables can improve models of phenology.

Time scale and influencing factor variations of water and heat fluxes in sandy land

Abstract The transfer process and variation characteristics of water and heat fluxes play an important role in both the ecosystem function and the climatic patterns of arid and semi-arid areas. However, both water (latent heat flux, Fe) and heat (sensible heat flux, Fh) fluxes, and the factors impacting each, operate on different time scales. Therefore, the wavelet transformation was used to reconstruct water and heat flux data measured by an eddy covariance system installed in Horqin sandy land, China. The distribution and response regularity of water and heat fluxes in response to control factors were then analysed across different time scales. The results indicated that the responses of Fe and Fh to controlling factors were similar on 30 min and daily scales, to which the energy terms of net radiation (Fn) and available energy (Fa) showed an extremely significant positive contribution. Removing the interference of the energy terms, Fe and Fh are strongly correlated with the vapour pressure deficit (VPD) and air temperature (Ta) at the daily scale. The water conditions of VPD and relative humidity (RH) were the main influencing factors at the monthly scale. At the seasonal scale, Fe was positively correlated with Fa and Fn but negatively correlated with soil heat flux (Fg), RH and wind speed (WS). On the contrary, Fh was positively correlated with RH but negatively correlated with Fn. At the annual scale, Fe and Fh showed a significant positive correlation with all factors. By the increasing time scale, the cross-correlations between various hydroclimatic processes become better. Generally, at the daily and annual scales, there is a high correlation between Fe, Fh and the control factors, and a pronounced periodicity is exhibited. Another factor that is known to cause such effects is the so-called long-term persistence (or else long-range dependence or the Hurst phenomenon). These results suggest that the time scales of flux and impacting factors should be considered when undertaking water and heat flux analyses of sandy land and other ecosystems.

A plasma membrane aquaporin is potentially involved in moderating water stress and photosynthetic depression by sustaining water transport and CO2 uptake under high VPD stress conditions in Solanum lycopersicum (tomato)

The molecular mechanism underlying the plant response to high vapor pressure deficit (VPD) stress remains unclear. In the present study, the roles of the plasma membrane aquaporin SlPIP1;2 in moderating plant water stress and photosynthetic depression in response to a series of VPD stress conditions are investigated. Plant water stress and photosynthetic depression are gradually exacerbated as VPD increases. The effects of SlPIP1;2 in mediating plant water status and photosynthesis are gradually enhanced as VPD increases. SlPIP1;2 exerts no significant effect on plant water status and photosynthesis under low VPD conditions. Overexpression of SlPIP1;2 moderates plant water stress by enhancing root water uptake under high VPD stress conditions, sustaining stomatal and mesophyll conductance for photosynthetic CO2 uptake and transport. Consequently, the relatively high CO2 availability inside chloroplasts in SlPIP1;2-overexpressing plants efficiently moderates photosynthetic depression under high VPD stress conditions. The present study demonstrates that SlPIP1;2 is potentially involved in moderating water stress and photosynthetic depression by sustaining water transport and CO2 uptake under high VPD stress conditions.

Physiological and climate controls on foliar mercury uptake by European tree species

Abstract. Despite the importance of vegetation uptake of atmospheric gaseous elemental mercury (Hg(0)) within the global Hg cycle, little knowledge exists on the physiological, climatic, and geographic factors controlling stomatal uptake of atmospheric Hg(0) by tree foliage. We investigate controls on foliar stomatal Hg(0) uptake by combining Hg measurements of 3569 foliage samples across Europe with data on tree species' traits and environmental conditions. To account for foliar Hg accumulation over time, we normalized foliar Hg concentration over the foliar life period from the simulated start of the growing season to sample harvest. The most relevant parameter impacting daily foliar stomatal Hg uptake was tree functional group (deciduous versus coniferous trees). On average, we measured 3.2 times higher daily foliar stomatal Hg uptake rates in deciduous leaves than in coniferous needles of the same age. Across tree species, for foliage of beech and fir, and at two out of three forest plots with more than 20 samples, we found a significant (p&lt;0.001) increase in foliar Hg values with respective leaf nitrogen concentrations. We therefore suggest that foliar stomatal Hg uptake is controlled by tree functional traits with uptake rates increasing from low to high nutrient content representing low to high physiological activity. For pine and spruce needles, we detected a significant linear decrease in daily foliar stomatal Hg uptake with the proportion of time during which water vapor pressure deficit (VPD) exceeded the species-specific threshold values of 1.2 and 3 kPa, respectively. The proportion of time within the growing season during which surface soil water content (ERA5-Land) in the region of forest plots was low correlated negatively with foliar Hg uptake rates of beech and pine. These findings suggest that stomatal uptake of atmospheric Hg(0) is inhibited under high VPD conditions and/or low soil water content due to the regulation of stomatal conductance to reduce water loss under dry conditions. Other parameters associated with forest sampling sites (latitude and altitude), sampled trees (average age and diameter at breast height), or regional satellite-observation-based transpiration product (Global Land Evaporation Amsterdam Model: GLEAM) did not significantly correlate with daily foliar Hg uptake rates. We conclude that tree physiological activity and stomatal response to VPD and soil water content should be implemented in a stomatal Hg model to assess future Hg cycling under different anthropogenic emission scenarios and global warming.

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.

Assessment of Canopy Conductance Responses to Vapor Pressure Deficit in Eight Hazelnut Orchards Across Continents.

A remarkable increase in vapor pressure deficit (VPD) has been recorded in the last decades in relation to global warming. Higher VPD generally leads to stomatal closure and limitations to leaf carbon uptake. Assessing tree conductance responses to VPD is a key step for modeling plant performances and productivity under future environmental conditions, especially when trees are cultivated well outside their native range as for hazelnut (Corylus spp.). Our main aim is to assess the stand-level surface canopy conductance (Gsurf) responses to VPD in hazelnut across different continents to provide a proxy for potential productivity. Tree sap flow (Fd) was measured by Thermal dissipation probes (TDP) probes (six per sites) in eight hazelnut orchards in France, Italy, Georgia, Australia, and Chile during three growing seasons since 2016, together with the main meteorological parameters. We extracted diurnal Fd to estimate the canopy conductance Gsurf.. In all the sites, the maximum Gsurf occurred at low values of VPD (on average 0.57 kPa) showing that hazelnut promptly avoids leaf dehydration and that maximum leaf gas exchange is limited at relatively low VPD (i.e., often less than 1 kPa). The sensitivity of the conductance vs. VPD (i.e., -dG/dlnVPD) resulted much lower (average m = −0.36) compared to other tree species, with little differences among sites. We identified a range of suboptimal VPD conditions for Gsurf maximization (Gsurf > 80% compared to maximum) in each site, named “VPD80,” which multiplied by the mean Gsurf might be used as a proxy for assessing the maximum gas exchange of the orchard with a specific management and site. Potential gas exchange appeared relatively constant in most of the sites except in France (much higher) and in the driest Australian site (much lower). This study assessed the sensitivity of hazelnut to VPD and proposed a simple proxy for predicting the potential gas exchange in different areas. Our results can be used for defining suitability maps based on average VPD conditions, thus facilitating correct identification of the potentially most productive sites.

Effect of vapor pressure deficit on the photosynthesis, growth, and nutrient absorption of tomato seedlings

Vapor pressure deficit (VPD) is the main driving force of plant water transport and affects the nutrient absorption of plants. Exploring the nutrient absorption, growth, and photosynthetic characteristics of different tomato cultivars under different VPDs is of great importance for the cultivation of tomato. This experiment featured two levels of VPD (high 2.2/low 0.9 kPa) and two tomato cultivars (Solanumlycopersicum L., cv. Jinpeng and cv. Zhongza) for a total of four treatments in two controlled environment growth chambers. After about 4 weeks, the seedlings (at the three true-leaf stage) were transplanted into pots flled with inorganic medium and watered with nutrient solution. The stomatal characteristics, leaf moisture status, net photosynthetic rate (An), and plant growth of Jinpeng were not significantly different under the two VPD treatments, but high VPD resulted in a significantly increased transpiration rate (E) and whole plant hydraulic conductance (Kplant), as well as significantly increased absorption of potassium (K) and calcium (Ca) in various organs. In contrast, Zhongza had lower biomass, smaller stem height, and lower total leaf area under high vs. low VPD conditions. At high VPD, the reduction of photosynthesis in Zhongza was mainly caused by anatomical modifications of the stomata; significant reductions in stomatal aperture and length restricted photosynthesis and biomass accumulation. Moreover, the E and Kplant of Zhongza were reduced by stomatal state changes that prevent excessive transpiration. High VPD limited plant transpiration and mass flow of water, which reduced the absorption and accumulation of nutrients. Overall, these results suggest tomato cultivars differ in their stomatal sensitivity. Specifically, increasing VPD resulted in increased levels of some nutrients in certain organs in the cultivar with lower stomatal sensitivity; however, the cultivar with higher stomatal sensitivity demonstrated opposite trends.

Physiological and Transcriptomic Analyses Revealed the Implications of Abscisic Acid in Mediating the Rate-Limiting Step for Photosynthetic Carbon Dioxide Utilisation in Response to Vapour Pressure Deficit in Solanum Lycopersicum (Tomato).

The atmospheric vapour pressure deficit (VPD) has been demonstrated to be a significant environmental factor inducing plant water stress and affecting plant photosynthetic productivity. Despite this, the rate-limiting step for photosynthesis under varying VPD is still unclear. In the present study, tomato plants were cultivated under two contrasting VPD levels: high VPD (3–5 kPa) and low VPD (0.5–1.5 kPa). The effect of long-term acclimation on the short-term rapid VPD response was examined across VPD ranging from 0.5 to 4.5 kPa. Quantitative photosynthetic limitation analysis across the VPD range was performed by combining gas exchange and chlorophyll fluorescence. The potential role of abscisic acid (ABA) in mediating photosynthetic carbon dioxide (CO2) uptake across a series of VPD was evaluated by physiological and transcriptomic analyses. The rate-limiting step for photosynthetic CO2 utilisation varied with VPD elevation in tomato plants. Under low VPD conditions, stomatal and mesophyll conductance was sufficiently high for CO2 transport. With VPD elevation, plant water stress was gradually pronounced and triggered rapid ABA biosynthesis. The contribution of stomatal and mesophyll limitation to photosynthesis gradually increased with an increase in the VPD. Consequently, the low CO2 availability inside chloroplasts substantially constrained photosynthesis under high VPD conditions. The foliar ABA content was negatively correlated with stomatal and mesophyll conductance for CO2 diffusion. Transcriptomic and physiological analyses revealed that ABA was potentially involved in mediating water transport and photosynthetic CO2 uptake in response to VPD variation. The present study provided new insights into the underlying mechanism of photosynthetic depression under high VPD stress.

Genome-wide miRNAs profiles of pearl millet under contrasting high vapor pressure deficit reveal their functional roles in drought stress adaptations.

Pearl millet (Pennisetum glaucum [L.] R. Br.) is an important crop capable of growing in harsh and marginal environments, with the highest degree of tolerance to drought and heat stresses among cereals. Diverse germplasm of pearl millet shows a significant phenotypic variation in response to abiotic stresses, making it a unique model to study the mechanisms responsible for stress mitigation. The present study focuses on identifying the physiological response of two pearl millet high-resolution cross (HRC) genotypes, ICMR 1122 and ICMR 1152, in response to low and high vapor pressure deficit (VPD). Under high VPD conditions, ICMR 1152 exhibited a lower transpiration rate (Tr), higher transpiration efficiency, and lower root sap exudation than ICMR 1122. Further, Pg-miRNAs expressed in the contrasting genotypes under low and high VPD conditions were identified by deep sequencing analysis. A total of 116 known and 61 novel Pg-miRNAs were identified from ICMR 1152, while 26 known and six novel Pg-miRNAs were identified from ICMR 1122 genotypes, respectively. While Pg-miR165, 168, 170, and 319 families exhibited significant differential expression under low and high VPD conditions in both genotypes, ICMR 1152 showed abundant expression of Pg-miR167, Pg-miR172, Pg-miR396 Pg-miR399, Pg-miR862, Pg-miR868, Pg-miR950, Pg-miR5054, and Pg-miR7527 indicating their direct and indirect role in root physiology and abiotic stress responses. Drought responsive Pg-miRNA targets showed upregulation in response to high VPD stress, further narrowing down the miRNAs involved in regulation of drought tolerance in pearl millet.

Leaf gas exchange of lowland rice in response to nitrogen source and vapor pressure deficit

AbstractBackgroundIn anaerobic lowland fields, ammonium (NH4+) is the dominant form of nitrogen (N) taken up by rice plants, however, with the large expansion of water‐saving irrigation practices, nitrification is favored during drained periods, leading to an increased availability of nitrate (NO3−).AimSince the uptake and assimilation of the two N‐sources differ in their demand of photosynthates, leaf gas exchange may be subject to adjustments in response to N‐sources, particularly at high evaporative demand, when stomatal conductance (gs) is very sensitive.MethodsThree experiments were carried out to study leaf gas exchange of various lowland rice varieties in response to N‐source at low and high vapor pressure deficit (VPD). In the first experiment, seedlings of 12 rice varieties were grown at high VPD for 3 weeks. From this, four rice varieties differing in gs and CO2 assimilation rate (A) were selected and grown for 2 weeks at low VPD, and after that, they were shifted to high VPD for 1 week, whereas in the third experiment, the same varieties were grown separately at low and high VPD conditions for 2 weeks. In all three experiments, plants were grown hydroponically in nutrient solution with N‐sources as sole NH4+ or NO3−.ResultsAt high VPD, NO3− nutrition led to a higher gs and A in four out of 12 varieties (IR64, BT7, NU838, and Nipponbare) relative to NH4+ nutrition, while no effect was observed at low VPD or after a short‐term exposure to high VPD. Further, varieties with a high intrinsic water‐use efficiency (WUEi; IR64 and BT7) showed the strongest response to N‐source. Higher gs was partially supported by increased root/shoot ratio, but could not be fully explained by the measured parameters. However, higher A in NO3−‐fed plants did not always result in increased plant dry matter, which is probably related to the higher energy demand for NO3− assimilation. Our results suggest that at high VPD, NO3− nutrition can improve leaf gas exchange in varieties having a high WUEi, provided a sufficient water supply.ConclusionTherefore, intensified nitrification under water‐saving irrigation measures may improve leaf gas exchange and the growth of rice plants under high transpirational demand. However, choice of variety seems crucial since large varietal differences were observed in response to N‐source. Further, breeding strategies for genotypes adapted to aerobic soil conditions should consider responses to NO3−, potentially using gas exchange measurements as a screening tool.

Effects of the interaction between vapor-pressure deficit and potassium on the photosynthesis system of tomato seedlings under low temperature

Both vapour pressure deficit (VPD) and potassium (K) have been widely studied in regulating leaf photosynthesis and plant growth. A close correlation between VPD and plant nutrient absorption has also been observed for diverse plant species under various environmental conditions. However, the interactive effect of VPD and K on plant photosynthesis remains unclear, especially at low temperatures. Here, we investigated the role of VPD in plant K uptake and the effects of VPD × K on plant photosynthesis system under chilling stress. Plants were subjected to different levels of K nutrient solution supply (2, 4, 8 mmol/L) with contrasting VPD levels (high vs. low). Compared to high VPD, low VPD increased plant transpiration, but there was no significant difference in leaf K content between different VPD treatments; K accumulation under low VPD increased significantly, which can be partly attributed to enhanced biomass production. Meanwhile, an increase in stomatal density and size occurred under low VPD conditions. Stomatal conductance and mesophyll conductance tended to increase with K level under low VPD condition, which was beneficial to the diffusion of carbon dioxide and thus increased photosynthesis. In terms of photosynthetic performance, low VPD and elevated K increased the utilization of light energy and reduced heat dissipation, which specifically manifested as up-regulated photosynthetic pigment content and chlorophyll fluorescence parameters and down-regulated Car/Chl ratio, NPQ, and ROS content. In summary, reducing VPD and adding K promoted photosynthetic performance and can alleviate the low-temperature stress of tomato plants.

Dynamics of CO2 fluxes and controlling environmental factors in sugarcane (C4)-wheat (C3) ecosystem of dry sub-humid region in India.

In this study, CO2 exchange over sugarcane and wheat growing season was quantified by continuous measurement of CO2 fluxes using eddy covariance (EC) system from January 2014 to June 2015. We also elaborated on the response of CO2 fluxes to environmental variables. The results show that the ecosystem has seasonal and diurnal dynamics of CO2 with a distinctive U-shaped curve in both growing seasons with maximal CO2 absorption reaching up to -8.94 g C m-2 day-1 and -6.08 g C m-2 day-1 over sugarcane and wheat crop, respectively. The ecosystem as a whole acted as a carbon sink during the active growing season while it exhibits a carbon source prior to sowing and post-harvesting of crops. The cumulative net ecosystem exchange (NEE), gross primary productivity (GPP), and ecosystem respiration (Reco) were -923.04, 3316.65, and 2433.18 g C m-2 over the sugarcane growing season while the values were -192.30, 621.47, and 488.34 g C m-2 over the wheat growing season. The sesbania (green manure) appeared to be a carbon source once it is incorporated into soil. The response of day-time NEE to photosynthetically active radiation (PAR) under two vapor pressure deficit (VPD) sections (0-20 h Pa and 20-40 h Pa) seems more effective over sugarcane (R2 = 0.41-0.61) as compared to the wheat crop (R2 = 0.25-0.40). A decrease in net CO2 uptake was observed under higher VPD conditions. Similarly, night-time NEE was exponentially related to temperature at different soil moisture conditions and showed higher response to optimum soil moisture conditions for sugarcane (R2 = 0.87, 0.33 ≤ SWC < 0.42 m3 m-3) and wheat (R2 = 0.75, 0.31 ≤ SWC < 0.37 m3 m-3) crop seasons. The response of daily averaged NEE to environmental variables through path analysis indicates that PAR was the dominant predictor with the direct path coefficient of -0.65 and -0.74 over sugarcane and wheat growing season, respectively. Satellite-based GPP products from Moderate Resolution Imaging Spectroradiometer (GPPMOD) and Vegetation Photosynthetic model (GPPVPM) were also compared with the GPP obtained from EC (GPPEC) technique. The seasonal dynamics of GPPEC and GPPVPM agreed well with each other. This study covers the broad aspects ranging from micro-meteorology to remote sensing over C4-C3 cropping system.

Response of leaf water potential, stomatal conductance and chlorophyll content under different levels of soil water, air vapor pressure deficit and solar radiation in chili pepper (Capsicum chinense)

Flood and drought have been increasing due to global climate changes and these extreme water-related events often affect crop productions globally. This study aims to develop a growth simulation model that correspond to sequential soil water status by revealing morph-physiological related responses on the chili pepper (Capsicum spp.). Two experiments were conducted in a greenhouse and nine treatments were established between dry and waterlogged soil conditions by using a slope field with sand. Results showed leaf water potential and stomatal conductance decreased with increasing soil water stress level. On the other hand, chili pepper increased total chlorophyll content and decreased chlorophyll a/b ratio to compensate the redaction of the leaf area as a response to soil water stress. Moreover, high air vapor pressure deficit and solar radiation condition influenced their morph-physiological traits as environmental stress coupled with soil water stress. A multiple regression model for estimating shoot dry weight (SDW) using plant height (PH) and soil plant analysis development (SPAD) was developed (SDW = 0.131 PH + 0.047SPAD − 2.408). It showed a high coefficient of determination (R2adj = 0.836). This model is certainly useful for estimating SDW under wide range soil water statuses and vapor pressure deficit conditions.

Short-Term Exposure to High Atmospheric Vapor Pressure Deficit (VPD) Severely Impacts Durum Wheat Carbon and Nitrogen Metabolism in the Absence of Edaphic Water Stress.

Low atmospheric relative humidity (RH) accompanied by elevated air temperature and decreased precipitation are environmental challenges that wheat production will face in future decades. These changes to the atmosphere are causing increases in air vapor pressure deficit (VPD) and low soil water availability during certain periods of the wheat-growing season. The main objective of this study was to analyze the physiological, metabolic, and transcriptional response of carbon (C) and nitrogen (N) metabolism of wheat (Triticum durum cv. Sula) to increases in VPD and soil water stress conditions, either alone or in combination. Plants were first grown in well-watered conditions and near-ambient temperature and RH in temperature-gradient greenhouses until anthesis, and they were then subjected to two different water regimes well-watered (WW) and water-stressed (WS), i.e., watered at 50% of the control for one week, followed by two VPD levels (low, 1.01/0.36 KPa and high, 2.27/0.62 KPa; day/night) for five additional days. Both VPD and soil water content had an important impact on water status and the plant physiological apparatus. While high VPD and water stress-induced stomatal closure affected photosynthetic rates, in the case of plants watered at 50%, high VPD also caused a direct impairment of the RuBisCO large subunit, RuBisCO activase and the electron transport rate. Regarding N metabolism, the gene expression, nitrite reductase (NIR) and transport levels detected in young leaves, as well as determinations of the δ15N and amino acid profiles (arginine, leucine, tryptophan, aspartic acid, and serine) indicated activation of N metabolism and final transport of nitrate to leaves and photosynthesizing cells. On the other hand, under low VPD conditions, a positive effect was only observed on gene expression related to the final step of nitrate supply to photosynthesizing cells, whereas the amount of 15N supplied to the roots that reached the leaves decreased. Such an effect would suggest an impaired N remobilization from other organs to young leaves under water stress conditions and low VPD.

Effect of vapor pressure deficit on growth and water status in muskmelon and cucumber

Climatic warming and water shortages have become global environmental issues affecting agricultural production. The change of morphology and anatomical structures in plant organs can greatly affect plant growth. The study combined temperature and relative humidity to regulate vapor pressure deficit (VPD) to form low and high VPD environments (LVPD and HVPD, respectively) in two climate-controlled greenhouses. The effects of different VPD conditions on gas exchange parameters, dry matter, and leaf and stem anatomical structure parameters of muskmelon and cucumber were compared and studied. The results show that the background VPD conditions give different internal structure of muskmelon and cucumber, therefore it can improve the transport capacity of water to the leaf surface under LVPD conditions. At the same time, the stomatal closure induced by atmospheric drought stress is avoided and the gas exchange capacity of the leaf stomata is enhanced, thereby maintaining high photosynthetic rate. Thus, reducing VPD is the key to achieving high yield and productivity in greenhouse muskmelon and cucumber production.

High Vapor Pressure Deficit Decreases the Productivity and Water Use Efficiency of Rain‐Induced Pulses in Semiarid Ecosystems

AbstractIntermittent rain events drive dynamic pulses of carbon and water exchange in many arid and semiarid ecosystems. Although soil moisture is known to control these pulses, the effect of atmospheric dryness on pulses is not well documented. Here we hypothesized that vapor pressure deficit (VPD) modulates net ecosystem production (NEP) and ecosystem‐scale water use efficiency (WUE) during pulse events due to its effects on canopy stomatal conductance and evapotranspiration. We quantified relationships between VPD and carbon and water exchange during growing season rain events and tested their generality across four semiarid flux sites with varied vegetation in the southwest United States. Across grassland, shrubland, and savanna sites, we found that high VPD during pulses suppressed ecosystem photosynthesis and surface conductance to a greater degree than respiration or evapotranspiration, particularly when soil moisture was high. Thus, periods of high VPD were associated with a 13–64% reduction in NEP and an 11–25% decrease in WUE, relative to moderate VPD conditions. Sites dominated by shrubs with the C3 photosynthetic pathway were more sensitive to VPD than sites dominated by C4 grasses. We found that a 1 kPa increase in VPD reduced the average NEP of pulse events by 13–56%, which illustrates the potential for projected increases in atmospheric demand to reduce the net productivity of semiarid ecosystems.

Observed changes in vapor pressure deficit suggest a systematic drying of the atmosphere in Xinjiang of China

The vapor pressure deficit (VPD) is the difference between the atmospheric water vapor holding capacity at saturation (es) and actual amount of water vapor (ea) in the air at a given temperature. In this study, we investigated the changes in VPD and related variables (air temperature, relative humidity, es and ea), and assessed the relationship between VPD and atmospheric conditions in Xinjiang over the period 1961–2017. The annual VPD records showed increasing trends during all seasons. In terms of spatial distribution, the increasing and decreasing trends of annual VPD were 79% (57% significant) and 21% (9% significant) of the observation stations, respectively. A trend reversal from downward to upward was observed in 1993, and the VPD records indicated rapid drying of the atmosphere in Xinjiang for 1994–2017, especially in the summer months. The differences in es and ea resulted in changes in the VPD, and the ea generally increased less than the es. We suggest that the increasing VPD in Xinjiang is due to increasing es from rising temperatures and decreasing ea from decreasing relative humidity since 1994. The decreased relative humidity has a strong influence on the increased VPD and accounts for ~74% of the variance in Xinjiang. In different environments, the VPD trends followed changes in air temperature and ea, with the greatest VPD increase in the desert, followed by the oasis and mountain environments. The increased VPD observed in Xinjiang between 1994 and 2017 is consistent with the global intensification of drought and adverse ecological effects of the 21st century. Thus, VPD change can be an accurate measure of regional drought and vegetation dynamics in Xinjiang.

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.

Stomatal closure response to soil drying at different vapor pressure deficit conditions in maize

A plant transpiration rate under progressive soil drying remains constant until a threshold fraction of transpirable soil water (FTSW) is reached, and subsequently decreases linearly. The sensitivity of this function and the involvement of abscisic acid (ABA) and aquaporins in such responses have not been compared at various levels of atmospheric evaporative demand conditions. This study was conducted in controlled environment chambers with a drought-tolerant maize hybrid imposing progressive drought stress under three levels of vapor pressure deficit (VPD- 1.2, 2.3, and 3.5 kPa). A shift in threshold-FTSW from 1.2 kPa (FTSW-0.42) VPD to 3.5 kPa(FTSW-0.51) VPD was observed, showing an effect of VPD on stomatal closure response under soil drought conditions. Foliar ABA showed a substantial rise approximately at the same time as of stomatal closure initiated (FTSW-threshold), indicating ABA involvement. As the drought progressed, an increase in plasma membrane intrinsic protein and a decrease in tonoplast intrinsic protein expression levels were observed. Overall, this study suggests the influence of evaporative demand on the initiation of stomatal closure of drought-tolerant maize subjected to soil drying. The sensitivity of stomatal closure was associated with foliar ABA under drought stress but not under high evaporative demand conditions, indicating alternative water conservative mechanisms.