kPa Vapor Pressure Deficit Research Articles

Strong differences in microclimate among the habitats of a forest-steppe ecosystem

Microclimate has a substantial impact on plant composition, survival, and growth, as well as ecosystem processes. Although microclimatic conditions in anthropogenically fragmented ecosystems have received considerable scientific attention, they are understudied in naturally fragmented ecosystems, including forest-steppes. In addition, earlier investigations in these mosaics only measured microclimate parameters for a very short period (i.e., 24 hours on a single summer day). In the present study, the long-term microclimatic conditions were described in multiple habitat types, both woody and non-woody, in a sandy forest-steppe ecosystem in the Kiskunság, central Hungary. The aim of this study was to answer (1) how air humidity and temperature conditions differ among the studied habitats during the growing season and (2) which habitats are more stressed to vegetation growth regarding vapor pressure deficit (VPD). Wireless sensors recording air temperature and humidity values were used to monitor microclimatic parameters. VPD values were calculated based on the obtained air temperature and humidity, and two thresholds at 1.2 and 3.0 kPa were defined. To compare mean air temperature and humidity variables, as well as above-threshold VPD rate among habitat types, general linear models were used. Our results indicated that open grasslands were the warmest and driest habitats. Among woody habitats, south-facing edges had the harshest microclimatic conditions. The current work also found that small forest patches and larger forest patches had similar air temperature and humidity variables. Regarding VPD, open grasslands were the most stressed for vegetation growth from May to October. During the summer season, forest patches had a small moderating effect at the limiting threshold of 1.2 kPa VPD, but a stronger moderating effect at the 3.0 kPa threshold. With ongoing climate change, this role of forest patches is expected to become increasingly important in forest-steppes. Therefore, it is suggested that the remaining near-natural forest stands in sandy forest-grassland ecosystems should be prioritized for protection, and that scattered trees or groups of trees of native species should be established in extensive treeless grasslands.

Transpiration Dynamics in Co-Located Maize, Sorghum, and Soybean Closed Canopies and Their Environmental Controls

Highlights Hourly sap flow measured in co-located and identically managed maize, sorghum, and soybean closed canopies. PAR, VPD, and ETr were strongly correlated to transpiration (T) normalized by LAI. Negative response of T to high VPD (3-4 kPa) was observed for maize and sorghum. Counterclockwise hysteresis observed for diurnal T-VPD and T-PAR. MLR models were developed to estimate T using VPD and PAR. Abstract. Transpiration (T) dominates terrestrial hydrological fluxes and is strongly coupled with vegetation productivity and water use efficiency across different biomes, including agricultural systems. Studying how T in field crops responds to environmental variability has important implications to inform and predict agroecosystems’ response to a changing environment. However, comparative T rates among major field crops remain unknown in many regions where drought severity and limited freshwater availability are projected, such as the Central U.S. Plains. We address this knowledge gap by monitoring and characterizing hourly T for field-grown maize, grain sorghum, and soybean crops under the same weather, soil, and management regimes using sap flow sensors. The relationships among crop-specific T and air temperature (Tair), relative humidity (RH), wind speed (u2), vapor pressure deficit (VPD), incoming shortwave radiation (Rs), photosynthetically active radiation (PAR), net radiation (Rn), and grass- and alfalfa-reference evapotranspiration (ETo, ETr) were investigated. T normalized by leaf area index (T LAI-1) was most correlated with PAR (r=0.88), ETr (r=0.84), and VPD (r=0.81). Mean sensitivity of T LAI-1 to unit change in Tair, Rs, PAR, Rn, u2, RH, VPD, and ETr for maize and sorghum was 88% and 59% greater than that of soybean, respectively. All crops showed non-linear T LAI-1 response to increasing VPD, and a negative response of T LAI-1 to VPD was observed in the 3.0-4.0 kPa VPD range for maize and sorghum. Each crop demonstrated a counterclockwise hysteresis effect to diurnal T-VPD and T-PAR, which was 177% and 87% greater (for T-VPD) and 44% and 17% greater (for T-PAR) in maize and sorghum, respectively, than soybean. Transpiration has rarely been measured in row crops, especially in a comparative fashion, and thus, the concurrent T dynamics and their environmental controls characterized in this research are of critical importance. These data can be instrumental for quantitatively assessing change in true crop water use (transpiration) and thus crop suitability under projected environmental change. Keywords: Hysteresis, Photosynthetically active radiation, Reference evapotranspiration, Sapflow, Vapor pressure deficit.

Identifying physiological and genetic determinants of faba bean transpiration response to evaporative demand.

Limiting maximum transpiration rate (TR) under high vapour pressure deficit (VPD) works as a water conservation strategy. While some breeding programmes have incorporated this trait into some crops to boost yields in water-limited environments, its underlying physiological mechanisms and genetic regulation remain unknown for faba bean (Vicia faba). Thus, we aimed to identify genetic variation in the TR response to VPD in a population of faba bean recombinant inbred lines (RILs) derived from two parental lines with contrasting water use (Mélodie/2 and ILB 938/2). Plants were grown in well-watered soil in a climate-controlled glasshouse with diurnally fluctuating VPD and light conditions. Whole plant transpiration was measured in a gas exchange chamber that tightly regulated VPD around the shoot under constant light, while whole-plant hydraulic conductance and its components (root and stem hydraulic conductance) were calculated from dividing TR by water potential gradients measured with a pressure chamber. Although TR of Mélodie/2 increased linearly with VPD, ILB 938/2 limited its TR above 2.0 kPa. Nevertheless, Mélodie/2 had a higher leaf water potential than ILB 938/2 at both low (1.0 kPa) and high (3.2 kPa) VPD. Almost 90 % of the RILs limited their TR at high VPD with a break-point (BP) range of 1.5-3.0 kPa and about 10 % had a linear TR response to VPD. Thirteen genomic regions contributing to minimum and maximum transpiration, and whole-plant and root hydraulic conductance, were identified on chromosomes 1 and 3, while one locus associated with BP transpiration was identified on chromosome 5. This study provides insight into the physiological and genetic control of transpiration in faba bean and opportunities for marker-assisted selection to improve its performance in water-limited environments.

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.

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.

Phenotyping and Quantitative Trait Locus Analysis for the Limited Transpiration Trait in an Upper-Mid South Soybean Recombinant Inbred Line Population ("Jackson" × "KS4895"): High Throughput Aquaporin Inhibitor Screening.

Soybean is most often grown under rainfed conditions and negatively impacted by drought stress in the upper mid-south of the United States. Therefore, identification of drought-tolerance traits and their corresponding genetic components are required to minimize drought impacts on productivity. Limited transpiration (TRlim) under high vapor pressure deficit (VPD) is one trait that can help conserve soybean water-use during late-season drought. The main research objective was to evaluate a recombinant inbred line (RIL) population, from crossing two mid-south soybean lines (“Jackson” × “KS4895”), using a high-throughput technique with an aquaporin inhibitor, AgNO3, for the TRlim trait. A secondary objective was to undertake a genetic marker/quantitative trait locus (QTL) genetic analysis using the AgNO3 phenotyping results. A set of 122 soybean genotypes (120-RILs and parents) were grown in controlled environments (32/25-d/n °C). The transpiration rate (TR) responses of derooted soybean shoots before and after application of AgNO3 were measured under 37°C and >3.0 kPa VPD. Then, the decrease in transpiration rate (DTR) for each genotype was determined. Based on DTR rate, a diverse group (slow, moderate, and high wilting) of 26 RILs were selected and tested for the whole plant TRs under varying levels of VPD (0.0–4.0 kPa) at 32 and 37°C. The phenotyping results showed that 88% of slow, 50% of moderate, and 11% of high wilting genotypes expressed the TRlim trait at 32°C and 43, 10, and 0% at 37°C, respectively. Genetic mapping with the phenotypic data we collected revealed three QTL across two chromosomes, two associated with TRlim traits and one associated with leaf temperature. Analysis of Gene Ontologies of genes within QTL regions identified several intriguing candidate genes, including one gene that when overexpressed had previously been shown to confer enhanced tolerance to abiotic stress. Collectively these results will inform and guide ongoing efforts to understand how to deploy genetic tolerance for drought stress.

Influence of climatic factors on dry flower, grey and green mould diseases of macadamia flowers in Australia.

Flower blights (grey mould, green mould and dry flower) are important diseases of macadamia. Lack of information on pathogen biology and disease epidemiology in macadamia has hampered control options. Effects of climatic variables including temperature, relative humidity (RH) and vapour pressure deficit (VPD) on the abundance, germination and growth of conidia of four fungal pathogens that cause various flower blights in macadamia were studied. Mycelial growth, sporulation, conidial germination and germ tube growth for five isolates each of Botrytis cinerea, Cladosporium cladosporioides, Pestalotiopsis macadamiae and Neopestalotiopsis macadamiae, at eight temperatures, seven RH and the corresponding VPD regimes were determined in vitro. The optimal climatic range of each of the four pathogens was validated during macadamia flowering periods in the 2019 and 2020seasons by conidia detected and quantified using quantitative PCR. Several growth models were fitted to the data with high significance; predicted optima from these models ranged from 0.9 to 1.1kPa VPD for P. macadamiae and N. macadamiae and <0.6kPa VPD for B. cinerea and C. cladosporioides. This study showed that VPD, as a determinant of the fecundity and growth of the four fungal pathogens, was predictive of flower blight incidence in macadamia. The importance of temperature, RH and, thus, VPD for defining the conditions for infection and flower blight epidemics was established. This information provides a firm basis for the development of prediction tools for flower blights in macadamia.

Fine-Scale Microclimate Pattern in Forest-Steppe Habitat

Microclimate and vegetation architecture are interdependent. Little information is available, however, about the fine-scale spatio-temporal relationship between the microclimate and herb layer of forest-steppe mosaics. In 2018 a three-season-long vegetation sampling and measurements of air temperature and air humidity were performed along 4 transects (44 m long each) in the herb layer with 89 dataloggers in the sandy region of Central Hungary, in a poplar grove and the surrounding open grassland. In order to improve data analysis, we introduced the use of a duration curve widely used in hydrology and proved to be useful in the processing of intensive climatic data. We analysed the effect of the direction and altitude of the solar irradiation and the edge effect on the microclimatic pattern. We also surveyed, seasonally, the spatial pattern of the exceedance rate for the vapour pressure deficit (VPD) in relation to the transect direction and to the edge of the grove. The exceedance rate for the VPD indicated considerable seasonal differences. The VPD exceedance rate indicates the stress effect for the vegetation. The moderating effect of the grove was small at 1.2 kPa VPD, but at 3.0 kPa—stronger stress—it was considerable. On the warmer side of the transects, mostly exposed at the south-eastern edge, the exceedance rate rose abruptly with distance from the edge compared to the gradual increase on the colder side. The cardinal and intercardinal directions as well as the altitude of the Sun all had influences on the moderating and shading effects of the grove. The southern edge was not always consistently the warmest. The distribution of the VPD values above the 3.0 kPa threshold varied within a seemingly homogeneous grassland, which highlights the importance of fine-scale sampling and analysis. This knowledge is valuable for assessing the dynamics and spatio-temporal patterns of abiotic factors and physiognomy in this type of ecosystem.

The sap flow of six tree species and stand water use of a mangrove forest in Hainan, China

Mangrove forests grow in coastal saline environments and, little is known about the stand water use (S) and regulation mechanisms of mangrove forests, particularly in the eastern mangrove group. We measured the xylem sap flow density (SFD) of six tree species for two years in a tropical mangrove forest on Hainan Island, China, which is under a monsoon climate. Bimodal daily SFD patterns were observed in all six species in spring and summer. The sap flow of all species were suppressed in winter strongly and also in spring. The annual water use per tree was positively correlated with diameter at breast height (DBH) (R2 = 0.81–0.89, P < 0.05). By scaling up from individual trees, we found that the S values of two mangrove plots ranged from 269.19 to 357.27 mm y−1 between the two study years, accounting for 18–22% of the annual rainfall. These S values are similar to those of other mangrove and tropical forests with similar stand basal areas. The crown conductance sensitivity (mc) to ln VPD (vapor pressure deficit) was strongly correlated with reference crown conductance (GCref) at 1 kPa VPD across all species. The slope value between mc and GCref divided the six species into two groups, suggesting Bruguiera sexangula, Rhizophora apiculata and Sonneratia alba were isohydric, Excoecaria agallocha, B. s. var. rhymchopetala and Xylocarpus granatum were anisohydric. The results of this study will be useful for modeling ecosystem services of mangrove ecosystems facing climate change.

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.

Atmospheric CO2 and VPD alter the diel oscillation of leaf elongation in perennial ryegrass: compensation of hydraulic limitation by stored-growth.

We explored the effects of atmospheric CO2 concentration (Ca ) and vapor pressure deficit (VPD) on putative mechanisms controlling leaf elongation in perennial ryegrass. Plants were grown in stands at a Ca of 200, 400 or 800μmolmol-1 combined with high (1.17kPa) or low (0.59kPa) VPD during the 16h-day in well-watered conditions with reduced nitrogen supply. We measured day:night-variation of leaf elongation rate (LERday :LERnight ), final leaf length and width, epidermal cell number and length, stomatal conductance, transpiration, leaf water potential and water-soluble carbohydrates and osmotic potential in the leaf growth-and-differentiation zone (LGDZ). Daily mean LER or morphometric parameters did not differ between treatments, but LERnight strongly exceeded LERday , particularly at low Ca and high VPD. Across treatments LERday was negatively related to transpiration (R2 =0.75) and leaf water potential (R2 =0.81), while LERnight was independent of leaf water potential or turgor. Enhancement of LERnight over LERday was proportional to the turgor-change between day and night (R2 =0.93). LGDZ sugar concentration was high throughout diel cycles, providing no evidence of source limitation in any treatment. Our data indicate a mechanism of diel cycling between daytime hydraulic and night-time stored-growth controls of LER, buffering Ca and daytime VPD effects on leaf elongation.

Transpiration of Dominant Tree Species Varies in Response to Projected Changes in Climate: Implications for Composition and Water Balance of Temperate Forest Ecosystems

The climate is changing in many temperate forests with winter snowpack shrinking and an increasing frequency of growing season air temperatures exceeding long-term means. We examined the effects of these changes on growing season rates of transpiration (sap flow) in two snow removal experiments in New Hampshire and Massachusetts, USA. Snow was removed during early winter, resulting in greater depth and duration of soil freezing compared to untreated plots. We examined the dominant tree species at each site, Acer saccharum at Hubbard Brook, NH and Acer rubrum and Quercus rubra at Harvard Forest, MA. Trees responded to a smaller snowpack, but with distinct species-specific responses consistent with ecological traits. Snow removal decreased rates of sap flow per kPa vapor pressure deficit (VPD) in sugar maples in the early growing season and red maples throughout the growing season. In contrast, sap flow rates per kPa VPD increased for red oak with snow removal. Downscaled climate projections from the Coupled Model Intercomparison Project indicate increases in heat stress days at both sites by the end of the century, leading to increased rates of whole-season transpiration across all three tree species, which will be offset in red maples and increased in red oaks with a smaller snowpack and increased frequency of heat stress days. Results of this study demonstrate that the combined effects of changes in climate during the growing season and winter will impact transpiration differently among tree species, with implications for forest water balance and tree species composition in the northeastern USA

The Effect of Light Intensity on Vegetative Propagation Efficacy, Growth, and Morphology of “Albion” Strawberry Plants in a Precision Indoor Propagation System

Open-field strawberry propagation is faced with several challenges such as lack of daughter plants, low quality, and disease transmission. Propagating strawberry plants in a completely enclosed controlled environment using a precision indoor propagation (PIP) system could overcome some of the challenges seen in open-field strawberry propagation. Optimizing the light intensity in a PIP system improves plant growth and reduce propagation cost. In the present study, “Albion” strawberry plants were grown as stock plants in a PIP system to examine plant propagation efficacy under three light intensities, PPF-250 (241 ± 13), PPF-350 (337 ± 13), or PPF-450 (443 ± 17) photosynthetic photon flux density (PPFD, μmol m−2 s−1) at 12 h photoperiod. They were grown under 25.7 ± 0.05 °C temperature, 0.95 ± 0.04 kPa vapor pressure deficit, and 73% ± 5.2% relative humidity. The number of daughter plants, morphology, and growth were recorded weekly (non-destructive measurements) for two intervals (01 to 12 weeks and 12 to 21 weeks). The number, total dry mass, and total fresh mass of daughter plants per stock plant increased with the increase in light intensity. The propagation efficacy to light ranged between 0.3 and 1.9 daughter plants per mole of light, depending on light intensity and harvest time. The number of daughter plants per week was estimated to be 36.2 plants wk−1 m−2. Daughter plants were classified by size and size was not influenced by the light treatment. Stock plant crown diameter, leaf area, fresh mass, dry mass, and leaf count all increased with an increase in PPFD. The shoot dry mass percent distribution to the daughter plant was 45% to 46% and was not affected by light intensity treatment. This study demonstrates the feasibility of using PIP systems for the production of strawberry daughter plants.

Evaluation of photosynthetic efficiency of yam bean (Pachyrhizus erosus L.) at saturating photon flux density under elevated carbon dioxide.

The future CO2 concentration is projected to reach 900-1000ppm levels by the end of twenty-first century, pertaining to global climatic changes. Consequences of climate change such as changes in mean climatic conditions, increasing extreme weather events, relentless increase in atmospheric CO2 concentration and increasing pest damage pose serious threats to agricultural productivity. An experiment was planned to assess the response of yam bean to elevated CO2, as it is of paramount importance to identify photosynthetically efficient climate-smart crops and varieties to meet future food demand. The net photosynthetic rate (P n ), stomatal conductance (g s ) and intercellular CO2 (C i ) of yam bean variety, Rajendra Misrikand-1 was recorded under elevated carbon dioxide (400-1000ppm) and photon flux density (PPFD; 50-2000μmolm-2 h-1) at 30 ± 2°C, 70-80% relative humidity and 0.8-1.2kPa vapour pressure deficit. The mean P n rate steadily increased at 200-1000ppm owing to enhanced intercellular CO2. The same trend was observed in the case of intercellular CO2. However, contrasting results were recorded with regard to g s , which steadily decreased at ascending carbon dioxide concentrations. Further, P n had a significant (P < 0.001) linear correlation with the PPFD (R2 = 0.973). Yam bean was found to be responsive to elevated carbon dioxide as P n rate at 1000ppm increased up to 23% relative to 400ppm.

Carbon and water dynamics in co-located winter wheat and canola fields in the U.S. Southern Great Plains

The magnitudes and seasonal dynamics of net ecosystem exchange (NEE) of carbon dioxide (CO2) and evapotranspiration (ET), measured using the eddy covariance (EC) technique from co-located, paired (conventional till, CT and no-till, NT) winter wheat (Triticum aestivum L.) and canola (Brassica napus L.) fields, were compared during a presumably favorable growing season for both crops. The magnitudes (7-day average) of NEE, gross primary production (GPP), and ET reached approximately −8 g C m−2 d−1, 16 g C m−2 d−1, and 5 mm d−1, respectively, at both CT and NT wheat fields due to uniform canopy stands. The magnitudes (7-day average) of NEE reached −5.20 ± 0.49 and −4.66 ± 0.36 g C m−2 d−1, GPP reached 12.47 ± 1.16 and 10.86 ± 0.97 g C m−2 d−1, and ET reached 4.7 ± 0.42 and 4.28 ± 0.36 mm d−1 at CT and NT canola fields, respectively. Poor recovery of canola stand in the NT field after winter dormancy resulted in smaller magnitudes of fluxes in spring 2017. Wheat had a larger potential as a carbon sink during winter than canola. Larger magnitudes of CO2 fluxes and longer periods as carbon sinks for wheat caused large differences in carbon sequestration potential between wheat and canola. Fluxes showed similar responses to climatic conditions as optimum air temperature (Ta) was ∼22 °C for NEE and GPP, and ∼25 °C for ET, and the fluxes peaked at ∼1.7 kPa vapor pressure deficit (VPD) for both crops. However, the GPP-PPFD (photosynthetic photon flux density) relationship was weaker beyond optimum Ta and VPD in canola than in wheat. Ecosystem light use efficiency (ELUE) and ecosystem water use efficiency (EWUE), determined using different metrics, were higher in wheat than in canola. The results indicate higher adaptability, and water and light use efficiencies of wheat than canola. This study provides an initial baseline on the dynamics of CO2 fluxes, ET, EWUE, and ELUE for canola, and side-by-side comparison of eddy fluxes in two major winter crops grown in the Southern Great Plains.

Transpiration Response of Cotton to Vapor Pressure Deficit and Its Relationship With Stomatal Traits.

Many studies have demonstrated that the cotton in warm environments is vulnerable to water-limitations thus reducing the yield. A number of plant traits have been recommended to ameliorate the effects of water deficits on plant growth and yield. Limitation on maximum transpiration rate (TR) under high vapor pressure deficit (VPD), usually occurs during midday, is often considered as a water conservation trait. The genotypes with this trait are desirable in high VPD environments where water deficits commonly develop in the later part of the growing season. Our objective of the study was to find the genotypic variation for the trait limited TR under high VPD and also to study leaf temperature, water potential, photosynthesis, and stomatal conductance responses. Also, our objective was also to study the structural changes in the stomatal traits when exposed to long term high VPD conditions and involvement in such responses. In the present study, 17 cotton genotypes were studied for their (TR) response to various VPD environments under well irrigated conditions. Out of 17, eight genotypes limited TR after approximately 2 kPa VPD and rest of them increased their TR with increased VPD. Five selected genotypes with different TR response to increasing VPD were further studied for gas exchange and stomatal properties. All genotypes, irrespective of exhibiting limited TR at high VPD, reduced stomatal conductance, photosynthesis and water potential at high VPD of 3.3 kPa. The genotypes with limited TR modified their stomatal traits mostly on the adaxial surface with frequent and small stomata under high VPD. The genotypes with limited TR also exhibited an increase in epidermal cell expansion and stomatal index at contrasting VPD gradients to effectively balance the liquid and vapor phase conductance to limit TR at high VPD.

Rainfall seasonality and drought performance shape the distribution of tropical tree species in Ghana.

Tree species distribution in lowland tropical forests is strongly associated with rainfall amount and distribution. Not only plant water availability, but also irradiance, soil fertility, and pest pressure covary along rainfall gradients. To assess the role of water availability in shaping species distribution, we carried out a reciprocal transplanting experiment in gaps in a dry and a wet forest site in Ghana, using 2,670 seedlings of 23 tree species belonging to three contrasting rainfall distributions groups (dry species, ubiquitous species, and wet species). We evaluated seasonal patterns in climatic conditions, seedling physiology and performance (survival and growth) over a 2‐year period and related seedling performance to species distribution along Ghana's rainfall gradient. The dry forest site had, compared to the wet forest, higher irradiance, and soil nutrient availability and experienced stronger atmospheric drought (2.0 vs. 0.6 kPa vapor pressure deficit) and reduced soil water potential (−5.0 vs. −0.6 MPa soil water potential) during the dry season. In both forests, dry species showed significantly higher stomatal conductance and lower leaf water potential, than wet species, and in the dry forest, dry species also realized higher drought survival and growth rate than wet species. Dry species are therefore more drought tolerant, and unlike the wet forest species, they achieve a home advantage. Species drought performance in the dry forest relative to the wet forest significantly predicted species position on the rainfall gradient in Ghana, indicating that the ability to grow and survive better in dry forests and during dry seasons may allow species to occur in low rainfall areas. Drought is therefore an important environmental filter that influences forest composition and dynamics. Currently, many tropical forests experience increase in frequency and intensity of droughts, and our results suggest that this may lead to reduction in tree productivity and shifts in species distribution.

Impact of heat-wave at high and low VPD on photosynthetic components of wheat and their recovery

Indirect effects of high temperature through increased vapor pressure deficit (VPD) are vital but often ignored in climate impact studies. We investigated the direct (via heat) and indirect (via VPD) effects of a post-anthesis applied high temperature episode on biochemical and diffusional components of photosynthesis in two wheat cultivars. Plants were exposed to a five-day episode of hot humid (HH: 36 °C; 1.96 kPa) or hot dry (HD: 36 °C; 3.92 kPa) climate, including normal climate (NC: 24 °C; 1.49 kPa VPD) as control. The capacity and sensitivity of different photosynthetic components that included diffusional and biochemical processes was investigated both during the stress and after six-days of recovery under NC. The results showed that, regardless of VPD, photosynthetic capacity under high temperature was largely impaired by biochemical limitations while the diffusional limitations were relatively insignificant. The processes involved in CO2-use (i.e. in vivo carboxylation efficiency and Vcmax) presented higher sensitivity than the processes involved in light-use (PSII efficiency, quantum yield and chlorophyll content index). Maximum photosynthetic capacity under high temperature was primarily restricted by limitations to RuBP-regeneration. Effects of higher VPD through diffusional limitations (stomatal and mesophyll conductance) were potentially insignificant. The effects of VPD through changes in transpiration were observed under HH, where significantly higher gs and increase in gs (and transpiration) over time due to acclimation led to drought, because the irrigation amounts per day remained same over the stress period. Resultantly, the photosynthesis under HH was co-limited by both RuBP-carboxylation and RuBP-regeneration. Irrespective of VPD, the after-effects of heat-wave were either non-existent or potentially insignificant, as most of the photosynthetic and related parameters recovered to their respective control. The results indicated that the effect of VPD through changes in soil moisture dynamics is an important confounding variable to consider in climate-impact studies. Higher sensitivity of CO2-use suggested that even moderately high temperature-episodes might limit photosynthetic capacity and hence crop productivity, thus reiterating the need to develop crop cultivars with greater tolerance to high temperatures.

An oxygen isotope chronometer for cellulose deposition: the successive leaves formed by tillers of a C4 perennial grass.

Multiannual time series of (palaeo)hydrological information can be reconstructed from the oxygen isotope composition of cellulose (δ18 OCel ) in biological archives, for example, tree rings, but our ability to temporally resolve information at subannual scale is limited. We capitalized on the short and predictable leaf appearance interval (2.4d) of a perennial C4 grass (Cleistogenes squarrosa), to assess its potential for providing highly time-resolved δ18 OCel records of vapour pressure deficit (VPD). Plants grown at low (0.63kPa) or high (1.58kPa) VPD were swapped between VPD environments and exposed to the new environment for 7d with simultaneous 13 CO2 labelling. Then, leaves were sampled by age/position along individual tillers. Five leaves at different developmental stages were growing simultaneously. The period of most-active leaf elongation, from 10 to 90% of final length, lasted 6.6d, and ~80% of all carbon and oxygen incorporation in whole-leaf cellulose occurred within 7d. Cellulose deposition stopped at (or shortly after) full leaf expansion. The direction of change, low-to-high or high-to-low VPD, had no differential effect on new oxygen and carbon incorporation in cellulose. Successive leaves produced by tillers of C.squarrosa provide a δ18 OCel record useful for reconstructions of short-term hydrological dynamics.

A trait‐mediated, neighbourhood approach to quantify climate impacts on successional dynamics of tropical rainforests

Summary Second‐growth forests account for 40% of the terrestrial forest carbon sink and represent the dominant forest cover in tropical regions. Uncertainties in predicting responses of these ecosystems to climate change arise from high tree species diversity, complex links between eco‐physiology and demography, and the role of ontogeny and competition in mediating individual tree responses to climate. The dynamic nature of second‐growth forests adds further uncertainty to our ability to quantify the relative importance of climate in mediating successional trajectories. To address these uncertainties, we develop a hierarchical Bayesian neighbourhood modelling approach that quantifies how the joint response of two key functional axes, wood density and specific leaf area (SLA), modulate impacts of inter‐annual variation in seasonal water stress (number of days during dry season &gt; 1 kPa vapour pressure deficit) and night‐time temperature on growth and survival of small (5–10 cm dbh) and large (≥ 10 cm dbh) trees for 171 rainforest species in 6 s‐growth and 2 old‐growth 1‐ha mapped stands. We use model results to examine potential climate impacts on the successional trajectories of these stands. High water stress reduced large tree growth but favoured growth of small trees. Drought also reduced tree survival for both large and small trees. Tree species with high wood density suffered lower growth reductions and had higher survival under water stress. High SLA magnified the negative effects of water stress on tree growth and survival. Across all tree sizes, high night‐time temperatures did not influence growth or survival. Simulated successional trajectories under different climate scenarios using these results suggest that multi‐annual droughts will have substantial impacts of the successional trajectories of tropical forests, leading to lower stem numbers, basal area and biomass. Sustained drought will also shift functional composition of second‐growth forest by favouring species with low SLA which tend to dominate in late stages of succession. By incorporating trait‐mediated effects on key drivers of tree demography and successional dynamics, our approach provides an integrated perspective on interspecific variation in vulnerability to drought and consequences for successional trajectories in tropical rainforests. Our results suggest that multi‐annual drought stress will significantly alter structure, composition and dynamics of second‐growth forests and, from a functional perspective, accelerate succession. However, this effect may be hampered by dispersal limitation of old‐growth species into second‐growth forests.