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Land-atmosphere coupling is a key factor for understanding the mean hydroclimatological conditions in South America. In this study, we analyzed how land-atmosphere coupling modulates the aridity conditions in the South American continent through experiments performed with a regional climate model. We assessed changes in precipitation (P), potential evaporation (PET), aridity index (ɸ = PET / P) and vapor pressure deficit (VPD) in experiments with and without soil moisture - atmosphere coupling. Additionally, we analyzed how air dry enthalpy (Hd), moist enthalpy (Hw) and total enthalpy (Ht) change between the coupled and uncoupled experiments, which help to identify patterns in aridity conditions. When analyzing ɸ, we found opposite responses in central-eastern Brazil and southeastern South America (SESA), the two main hotspots of land-atmosphere coupling. On the other hand, a widespread amplification of VPD is found. Both VPD and enthalpy are locally driven by changes in temperature and water vapor content and reflect wetter conditions in central-eastern Brazil and drier conditions in SESA. Changes in regional circulation, described in previous studies, increase moisture convergence in Brazil and help to explain the response of each individual metric when coupling is enabled. [ABSTRACT FROM AUTHOR]

Key message: Red cedar chlorophyll fluorescence remains high during the nongrowing season, while transpiration is reduced. Evergreen trees retain their leaves during the winter and thus can photosynthesize when conditions permit. Eastern red cedar (Juniperus virginiana) is widely distributed in the eastern USA and is known to transpire and photosynthesize outside of the growing season. However, most recent work has been done in the Great Plains, which red cedar has recently invaded, while little work has been done in its original range. I hypothesized that red cedar would behave like other conifers from summer drought-free areas and show reduced chlorophyll fluorescence and transpiration during the winter. Four red cedar trees at a site near the Ohio River in Kentucky were equipped with Granier sap flow probes, while solar irradiance, temperature, relative humidity, and soil volumetric water content (VWC) were measured at the site. Dark chlorophyll fluorescence (Fv/Fm) was measured on northern and southern aspects on an approximately weekly basis from early 2019 through mid 2021. High values of Fv/Fm were measured in both growing (April-September) and nongrowing (October-March) seasons; median values in the nongrowing season were 94% of those in the growing season, which did not support my hypothesis. Fv/Fm data were fit to a Michaelis–Menten curve that used minimum temperature of the previous two nights, aspect, and maximum irradiance on the day of measurement taken before fluorescence was measured. Sap flow was explained by maximum daily temperature, vapor pressure deficit (VPD), mean daily irradiance, and VWC. Sap flow in the nongrowing season was 74% of that seen in the growing season, due to lower values of temperature, VPD, and irradiance, supporting my hypothesis. Thus, red cedar remains physiologically active during the nongrowing season. However, it does not appear to behave like other conifers from summer drought-free areas. [ABSTRACT FROM AUTHOR]

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Increasing photosynthetic efficiency in fluctuating light is an important strategy to improve plant growth. In nature, fluctuating light often occurs concomitantly with the change in vapor pressure deficit (VPD) in the daytime, but how VPD affects dynamic photosynthesis is little known. In this study, we measured photosynthetic performance in fluctuating light under different atmospheric VPD conditions in the daytime for mature leaves of greenhouse-grown Rosa hybrida "Orange Reeva". A moderate increase of VPD significantly reduced photosynthetic efficiency in fluctuating light but hardly affected steady-state photosynthesis. With the increase of atmospheric VPD, stomatal conductance (gs) and mesophyll conductance (gm) in fluctuating light decreased simultaneously, but the maximum velocity of Rubisco carboxylation did not change. The significant effect of atmospheric VPD on gs and gm in fluctuating light was attributed to the change of relative humidity. Furthermore, photosynthesis in fluctuating light did not significantly differ between morning and afternoon under the same atmospheric VPD condition. These results indicate that atmospheric VPD is an overlooked environmental factor affecting photosynthetic efficiency under fluctuating light. [ABSTRACT FROM AUTHOR]

There has been substantial research on shade in cacao agroforestry systems, with most studies focusing on the impact of the shade cast by the associated shade trees on microclimate and yield. However, to our knowledge, no cacao agroforestry studies have explored how shade and its microclimate-modifying capacity influence the agrosystem’s pest and disease regulating service. Utilizing thermal hygrometers and hemispherical photographs, we measured temperature and relative humidity during the dry and wet seasons as well as the shade of associated trees (associated shade), combining the latter with cacao self-shade (total shade). This approach enabled us to uncover how each shade type influences microclimate, yields, and pests and diseases beneath cacao trees. Additionally, we developed a novel method to estimate attainable yield, actual yield, and yield loss due to pests and diseases. Using yield loss as a proxy of the pest and disease regulating service and structural equation modeling, we built a model depicting the interaction network between shade types and their role in cacao pest and disease regulation. Our results showed that each shade type uniquely influenced cacao agroecosystem outcomes, with the associated shade negatively impacting attainable yield and total shade having a positive effect. Associated shade also mitigated the dry season microclimate and limited pest and disease occurrence. Notably, shade alone was not the sole pest and disease-related yield loss driver; it is part of a complex interaction network. These innovative shade measurement and yield loss estimation methods have enhanced enhance our understanding of pest and disease regulation. Conclusively, different management approaches for associated shade and total shade are crucial for optimizing yields and pest and disease regulation in cacao agroforestry systems. [ABSTRACT FROM AUTHOR]

Purpose: Forest management practices, such as thinning, can significantly change surface soil properties, understory plant composition, and microclimate. Nevertheless, there remains a dearth of knowledge concerning the influence of forest thinning on soil temperature (Ts) and its connection to environmental factors in open-field. Our study aims to quantify the response of surface Ts (10 cm depth) to different thinning treatments, and to investigate the associations between Ts and various meteorological factors throughout the entire plant growing season. Materials and methods: We established 12 25 m × 25 m plots from Larix principis-rupprechtii plantations receiving four different thinning treatments (i.e., no thinning, NT; low thinning: 15% removal of trees, LT; moderate thinning: 35% removal of trees, MT; heavy thinning: 50% removal of trees, HT). These plots were established on Taiyue Mountain in North China, and the microclimate data were collected 2 years after their establishment in 2012. We obtained continuous (30 min/time) Ts data of 10 cm depth for 1 year, from April 2017 to March 2018. Additionally, other meteorological data including total solar radiation (TSR), air temperature (Ta), relative humidity (RH), air pressure (AP), precipitation, vapor pressure deficit (VPD), and wind speed (WS) were simultaneously recoded by an automatic weather station located in the open area (OA) within the forest, which is completely devoid of vegetation. Results and discussion: Ts in the MT (6.18 °C) exhibited a significantly higher value compared to other treatments in the growing season. Ts under forest cover displayed higher temperatures in the winter and lower temperatures in the growing season compared to the OA. The structural equation model (SEM) indicated that Ta exerted a positive influence as the primary environmental factor affecting Ts. In contrast, VPD exhibited a negative direct effect on Ts during the growing season, while manifesting a positive direct effect throughout the non-growing season. The direct effect of TSR on Ts was negative, and TSR indirectly influenced Ts through its direct effects on VPD and Ta. However, the overall effect of TSR on Ts was found to be positive. Nevertheless, WS generally affected Ts indirectly. Notably, the path coefficients, both direct and indirect, of each meteorological factor on MT treatment were slightly smaller compared to other treatments. Conclusion: Forest thinning of larch plantations influences the dynamics of Ts significantly, with MT leading to warmer soil conditions. Moreover, the structural composition of the plant community plays a crucial role in buffering the fluctuations in understory Ts induced by climatic changes. Our results offer valuable insights into the impacts of forest thinning on Ts and provide important guidelines for future management practices. [ABSTRACT FROM AUTHOR]

Transpiration (Tc) is a critical component of the global water cycle. Soil moisture (SM) and vapor pressure deficit (VPD) are key regulators of Tc, and exploring their contributions to changes in Tc can deepen our understanding of the mechanisms of water cycling in terrestrial ecosystems. However, the driving roles of VPD and SM in Tc changes remain debated because of the coupling of SM and VPD through land-atmosphere interactions which restrict the quantification of the independent effects of SM and VPD on Tc. By decoupling the correlations between SM and VPD using a novel binning approach, this study analyzed the dominant drivers of vegetation transpiration in subtropical China from 2003 to 2018 based on multi-source data, including meteorological reanalysis, remotely sensed soil moisture, transpiration, and land cover data. The results show that Tc first increased and then remained stable with an increase in SM across the study area but changed slightly with increasing VPD. Overall, the relative contribution of SM to the change in Tc was approximately five times that of VPD. The sensitivities of Tc to SM and VPD differed among vegetation types. Although the sensitivity of Tc to SM was greater than that of VPD for all four vegetation types, the thresholds of Tc in response to SM were different, with the lowest threshold (approximately 35%) for the other forests and the highest threshold (approximately 55%) for short wood vegetation. We infer that this is associated with the differences in ecological strategies. To verify the reliability of our conclusions, we used solar- induced chlorophyll fluorescence (SIF) data as a proxy for Tc based on the tight coupling between photosynthesis and transpiration. Consistent results were obtained by repeating the analyses. The results of this study, in which the impacts of SM and VPD on Tc were decoupled, are beneficial for further understanding the critical processes involved in water cycling in terrestrial ecosystems in response to climate change. [ABSTRACT FROM AUTHOR]

Key message: Longleaf pine demonstrated general resistance to reduced soil moisture and increased VPD, but results highlight the soil and atmospheric conditions that could trigger declines in longleaf pine function and productivity. Low soil moisture and high atmospheric vapor pressure deficit (VPD) independently limit tree function and forest productivity. However, questions remain about how large, established trees respond to dry soil and high VPD over longer time periods. We carried out a 3-year throughfall reduction experiment in a young (12–14-year-old) longleaf pine plantation in west Georgia (USA). We hypothesized that throughfall reduction would reduce soil moisture, leaf-scale stomatal conductance (gs), and net photosynthesis (Pnet), but increase intrinsic water-use efficiency (iWUE). We also hypothesized that throughfall reduction would reduce canopy conductance (Gs) at a reference VPD of 1 kPa and Gs sensitivity to VPD. In addition, we used Gs data collected across both treatments to identify breakpoints in the relative control of soil moisture and VPD on Gs. Throughfall reduction decreased soil moisture and caused small reductions in gs (– 21%) and Pnet (– 13%), but no change in iWUE. As expected, reduced throughfall decreased Gs and Gs sensitivity to VPD by 20 and 8%, respectively. Despite this, throughfall reduction had very little effect on tree growth or forest productivity. Importantly, Gs sensitivity to VPD was similar at intermediate soil moisture, but highest and lowest at soil moistures above field capacity and below the permanent wilting point, respectively. Consequently, we could identify thresholds in the relative control of soil moisture and VPD over Gs. These results demonstrate the general resistance of longleaf pine plantations to reduced soil moisture and increased VPD but highlight the soil and atmospheric conditions that could trigger declines in longleaf pine function and productivity. [ABSTRACT FROM AUTHOR]

The vapor pressure deficit (VPD) is an important variable used to characterize atmospheric aridity. This paper analyses the spatial and temporal characteristics of the decadal abrupt change (DAC) in the global land VPD after 1980 using monthly scale data from the Climatic Research Unit. The results show that 60.5% of the global land area underwent a significantly increased decadal abrupt change (IDAC) in the VPD, and the persistent IDAC of the VPD was obvious in the middle and low latitudes of Eurasia, Africa and parts of South America but not in central North America or Western Siberia. From 1980 to 2020, most regions experienced no more than two persistent IDACs, while more than two significant increases occurred mainly around the Mediterranean and in eastern South America. The persistent IDAC occurred relatively early in the middle and low latitudes of Eurasia, Africa, and eastern South America and after 2000 in the high latitude regions, Eastern Europe, and near the Qinghai-Tibet Plateau. The regions where the persistent IDAC lasted longer than 10 years mainly included North Africa, West Asia, eastern South America, and parts of East Asia, indicating that the persistent increases in atmospheric aridity in these regions were obvious. In general, the persistent IDAC that began in 1993–2000 was significantly more than that occurred in other periods and lasted longer than that before 1990, suggesting that the land area experiencing an abrupt increase has an expansion after the 1990s and that the role of water limitation in this persistent IDAC in Central Asia and most of China strengthened. In addition, the VPD showed another large-scale persistent IDAC over the global land region in 2009, indicating that global atmospheric aridity intensified over the last decade. At the same time, in a few global regions, the VPD has exhibited decreased decadal abrupt changes (DDACs) with durations shorter than 2 years. [ABSTRACT FROM AUTHOR]

Urbanization-induced phenological changes have received considerable attention owing to their implications for determining urban ecosystem productivity and predicting the response of plants and ecosystem carbon cycles to future climate change. However, inconsistent rural-urban gradients in plant phenology remain, and phenological drivers other than temperature are poorly understood. In this study, we simultaneously observed the micro-climate and spring leaf phenology of seven woody plant species at 13 parks along a rural-urban gradient in Beijing, China. The minimum (Tmin) and mean (Tmean) air temperature and the minimum (VPDmin) and mean (VPDmean) vapor pressure deficit increased significantly along the rural-urban gradient, but the maximum air temperature (Tmax) and maximum vapor pressure deficit (VPDmax) did not. All observed leaf phenological phases for the seven species were significantly advanced along the rural-urban gradient by 0.20 to 1.02 days/km. Advances in the occurrence of leaf phenological events were significantly correlated with increases in Tmean (accounting for 57–59% variation), Tmin (21–26%), VPDmin (12–16%), and VPDmean (3–5%), but not with changes in Tmax or VPDmax. Advances in spring leaf phenology along the rural-urban gradient differed between non-native species and native species and between shrubs and trees. The reason may be mainly that the sensitivities of spring leaf phenology to micro-climate differ with species origin and growth form. This study highlights that urbanization-induced increases in Tmean and Tmin are the major contributors to advances in spring leaf phenology along the rural-urban gradient, exerting less influence on native species than on non-native species. [ABSTRACT FROM AUTHOR]

Extreme hydrometeorological events have an effect on the growth of vegetation worldwide. Here, we used China's Yellow River Basin to deeply explore the temporal and spatial trends of extreme hydrometeorological events and their impacts on vegetation. We identified extreme drought (Dry-PRE, Dry-VPD) and extreme wet (Wet-PRE, Wet-VPD) events based on precipitation and vapor pressure deficit (VPD). In this paper, the interannual variation of extreme hydrometeorological events in the Yellow River Basin, their temporal and spatial distribution patterns and their impacts on vegetation were analyzed by combining with normalized difference vegetation index and precipitation. The results indicated that Dry-VPD, Dry-PRE and Wet-PRE events showed an upward trend year by year, Wet-VPD events showed a downward trend year by year, and Dry-VPD and Dry-PRE events increased significantly. Overall, the environment of the Yellow River Basin continued to dry out. The sensitivity of vegetation in the Yellow River Basin to extreme drought events was higher compared with that to extreme wet events. Extreme drought events based on VPD might affect vegetation growth in the upstream western region and the downstream region of the Yellow River Basin, while extreme drought events based on PRE might affect vegetation growth in the central region of the Basin. This paper showed that, in addition to the impact of precipitation on vegetation, drought caused by VPD also had a significant impact on vegetation. Furthermore, we suggested strengthening the protection of vegetation where the trend was significant. [ABSTRACT FROM AUTHOR]

Statistical analysis of daily observation series at 1719 weather stations in Russia in the period from 1966 to 2014 was used to derive an approximate formula for calculating mean daily vapor pressure deficit by the mean daily air temperature and daily precipitation totals. The formula was proposed to reduce the number of meteorological characteristics, required for regional runoff formation models in the estimation of total evaporation, by excluding data on measured vapor pressure deficit, which are more difficult to determine. Testing the proposed formula for problems of hydrological calculations was illustrated by comparing runoff hydrographs over long periods, calculated using two versions of the hydrological model, one with the actual deficit and the other with those calculated by the proposed empirical formula, for a series of regional runoff formation models developed for largest river basins in Russia based on ECOMAG model system. Statistical criteria were used to show that the calculations made with the use of the approximate formula for the majority of compared watersheds yield about the same simulation accuracy as calculations made with the use of actual vapor pressure deficit. [ABSTRACT FROM AUTHOR]

Background and Amis: Rice production consumes more freshwater than any other crop. Rice evapotranspiration (ET) response to extreme droughts due to ongoing climate change is becoming critical in agricultural water management and national food security. Here, we aim to evaluate the impact of droughts in the subtropical zone during 2009–2011 on rainfed rice ET and to explore the underlying influencing mechanisms of dryness stress on it.We detected the impact of drought events on rainfed rice ET using a modified Surface Energy Balance Algorithm for Land model. Additionally, we applied both multiple linear regression model and extreme gradient boosting models—Shapley additive explanations framework to fully investigate the sensitivity of rainfed rice ET to vapor pressure deficit (VPD) and soil moisture (SM).Compared to normal conditions, the rainfed rice ET exhibited a 24.7% reduction after the “2009/2010 drought”, whereas it increased by 4.3% and 15% before and during the “2011 drought”. Both linear and non-linear regression model results revealed that rainfed rice ET is more sensitive to variations in VPD rather than SM, but the dominant control of VPD appears to be released and the limiting effect of SM strengthened with the intensification of droughts.The rainfed rice ET is highly susceptible to water stress and can rapidly respond to changing hydroclimatic conditions even during non-drought periods. Our results highlight the susceptibility of rainfed rice ET to dryness stress and the important role of VPD and SM in analyses of drought-related impacts.Methods: Rice production consumes more freshwater than any other crop. Rice evapotranspiration (ET) response to extreme droughts due to ongoing climate change is becoming critical in agricultural water management and national food security. Here, we aim to evaluate the impact of droughts in the subtropical zone during 2009–2011 on rainfed rice ET and to explore the underlying influencing mechanisms of dryness stress on it.We detected the impact of drought events on rainfed rice ET using a modified Surface Energy Balance Algorithm for Land model. Additionally, we applied both multiple linear regression model and extreme gradient boosting models—Shapley additive explanations framework to fully investigate the sensitivity of rainfed rice ET to vapor pressure deficit (VPD) and soil moisture (SM).Compared to normal conditions, the rainfed rice ET exhibited a 24.7% reduction after the “2009/2010 drought”, whereas it increased by 4.3% and 15% before and during the “2011 drought”. Both linear and non-linear regression model results revealed that rainfed rice ET is more sensitive to variations in VPD rather than SM, but the dominant control of VPD appears to be released and the limiting effect of SM strengthened with the intensification of droughts.The rainfed rice ET is highly susceptible to water stress and can rapidly respond to changing hydroclimatic conditions even during non-drought periods. Our results highlight the susceptibility of rainfed rice ET to dryness stress and the important role of VPD and SM in analyses of drought-related impacts.Results: Rice production consumes more freshwater than any other crop. Rice evapotranspiration (ET) response to extreme droughts due to ongoing climate change is becoming critical in agricultural water management and national food security. Here, we aim to evaluate the impact of droughts in the subtropical zone during 2009–2011 on rainfed rice ET and to explore the underlying influencing mechanisms of dryness stress on it.We detected the impact of drought events on rainfed rice ET using a modified Surface Energy Balance Algorithm for Land model. Additionally, we applied both multiple linear regression model and extreme gradient boosting models—Shapley additive explanations framework to fully investigate the sensitivity of rainfed rice ET to vapor pressure deficit (VPD) and soil moisture (SM).Compared to normal conditions, the rainfed rice ET exhibited a 24.7% reduction after the “2009/2010 drought”, whereas it increased by 4.3% and 15% before and during the “2011 drought”. Both linear and non-linear regression model results revealed that rainfed rice ET is more sensitive to variations in VPD rather than SM, but the dominant control of VPD appears to be released and the limiting effect of SM strengthened with the intensification of droughts.The rainfed rice ET is highly susceptible to water stress and can rapidly respond to changing hydroclimatic conditions even during non-drought periods. Our results highlight the susceptibility of rainfed rice ET to dryness stress and the important role of VPD and SM in analyses of drought-related impacts.Conclusion: Rice production consumes more freshwater than any other crop. Rice evapotranspiration (ET) response to extreme droughts due to ongoing climate change is becoming critical in agricultural water management and national food security. Here, we aim to evaluate the impact of droughts in the subtropical zone during 2009–2011 on rainfed rice ET and to explore the underlying influencing mechanisms of dryness stress on it.We detected the impact of drought events on rainfed rice ET using a modified Surface Energy Balance Algorithm for Land model. Additionally, we applied both multiple linear regression model and extreme gradient boosting models—Shapley additive explanations framework to fully investigate the sensitivity of rainfed rice ET to vapor pressure deficit (VPD) and soil moisture (SM).Compared to normal conditions, the rainfed rice ET exhibited a 24.7% reduction after the “2009/2010 drought”, whereas it increased by 4.3% and 15% before and during the “2011 drought”. Both linear and non-linear regression model results revealed that rainfed rice ET is more sensitive to variations in VPD rather than SM, but the dominant control of VPD appears to be released and the limiting effect of SM strengthened with the intensification of droughts.The rainfed rice ET is highly susceptible to water stress and can rapidly respond to changing hydroclimatic conditions even during non-drought periods. Our results highlight the susceptibility of rainfed rice ET to dryness stress and the important role of VPD and SM in analyses of drought-related impacts. [ABSTRACT FROM AUTHOR]

Water use efficiency (WUE), the carbon uptake per unit of water consumption, is a critical indicator of the coupling between water and carbon cycles in terrestrial ecosystems. Understanding the spatiotemporal changes in WUE and its drivers is vital for predicting ecosystem responses to environmental changes. Herein, we used a remote sensing-driven analytical model to estimate spatiotemporal variations of WUE in China during 2000–2016 and quantified its key drivers. The simulated WUE matched well with the eddy covariance observations across different ecosystems in China, with coefficients of determination of 0.86 and 0.81. The annual WUE increased significantly at a rate of 0.021 g C mm−1 H2O y−1 (P < 0.01), with the mean value of 1.62 g C mm−1 H2O during 2000–2016. Analyses based on a factorial experiment (FE) and the sensitivity method (SE) showed that the increased leaf area index was the largest contributor to the increase in WUE, with contributions of 50.1% and 42.8% determined via FE and SE, respectively. CO2 enrichment was the second largest contributor (FE: 20.7% and SE: 31.4%), followed by the decrease in the vapor pressure deficit (FE: 15.4% and SE: 13.4%). The increased ratio of water loss by canopy interaction had a negative impact on WUE due to the increased non-productive water loss. Although these attributions had some inherent uncertainties, the results highlighted that the significant greening in China, which was mainly caused by human land-use management, and increased CO2 concentrations significantly promoted WUE during the recent 17 years. [ABSTRACT FROM AUTHOR]

Purpose: Impacts of dryness stress (i.e., low soil moisture [SM] and high vapor pressure deficit [VPD]) on gross primary production (GPP) have received considerable attention in recent years, however, their biological mechanism still needs to be elucidated. Methods: We decomposed GPP into canopy stomatal conductance (gs), ratio of intercellular to atmospheric CO2 partial pressure (Ci/Ca), and species composition and investigated the interaction of dryness stress, gs, Ci/Ca, and relative abundance of C4 plants (C4/(C3 + C4)) to determine how they regulate GPP along an aridity gradient in Inner Mongolia Plateau. We used biomass-weighted leaf carbon (δ13C) and oxygen (δ18O) isotopes to calculate canopy Ci/Ca and 18O enrichment in leaf tissue above source water (△18O), respectively. Results: A positive relationship between 1/△18O and canopy gs demonstrated that 1/△18O was a reliable indicator for canopy gs. Soil moisture (SM) exhibited a positive effect on 1/△18O and Ci/Ca, while Ci/Ca decreased and C4/(C3 + C4) increased with increasing water vapor pressure (VPD). 1/△18O and C4/(C3 + C4) indirectly regulated GPP via Ci/Ca, and the effect of C4/(C3 + C4) on the variability in Ci/Ca was stronger than that of 1/△18O. Interaction between dryness stressors (SM and VPD) and vegetation properties (1/△18O, Ci/Ca, and C4/(C3 + C4)) captured 74.6% of the variability in GPP, indicating that spatial variability in GPP was determined overwhelmingly by indirect effects of dryness on biological processes. Conclusions: Increasing relative abundance of C4 plants would effectively mitigate the negative effects of dryness stress on GPP. [ABSTRACT FROM AUTHOR]

Main conclusion: The combination of image-based phenotyping with in-depth anatomical analysis allows for a thorough investigation of plant physiological plasticity in acclimation, which is driven by environmental conditions and mediated by anatomical traits. Understanding the ability of plants to respond to fluctuations in environmental conditions is critical to addressing climate change and unlocking the agricultural potential of crops both indoor and in the field. Recent studies have revealed that the degree of eco-physiological acclimation depends on leaf anatomical traits, which show stress-induced alterations during organogenesis. Indeed, it is still a matter of debate whether plant anatomy is the bottleneck for optimal plant physiology or vice versa. Here, we cultivated ‘Salanova’ lettuces in a phenotyping chamber under two different vapor pressure deficits (VPDs; low, high) and watering levels (well-watered, low-watered); then, plants underwent short-term changes in VPD. We aimed to combine high-throughput phenotyping with leaf anatomical analysis to evaluate their capability in detecting the early stress signals in lettuces and to highlight the different degrees of plants’ eco-physiological acclimation to the change in VPD, as influenced by anatomical traits. The results demonstrate that well-watered plants under low VPD developed a morpho-anatomical structure in terms of mesophyll organization, stomatal and vein density, which more efficiently guided the acclimation to sudden changes in environmental conditions and which was not detected by image-based phenotyping alone. Therefore, we emphasized the need to complement high-throughput phenotyping with anatomical trait analysis to unveil crop acclimation mechanisms and predict possible physiological behaviors after sudden environmental fluctuations due to climate changes. [ABSTRACT FROM AUTHOR]

The regulation of plant transpiration is a key factor affecting transpiration efficiency, growth and adaptation of Eucalyptus species to limited water availability in tropical and subtropical environments. However, few studies have related this trait to the performance of Eucalyptus seedlings and none have investigated the influence of vapor pressure deficit (VPD) on transpiration rates and growth. In this study, the transpiration and growth responses of seedlings of Eucalyptus urophylla (S.T. Blake) and Eucalyptus cloeziana (F. Muell.) to progressive soil water deficits were evaluated under semi-controlled conditions using the fraction of transpirable soil water (FTSW) method. In addition, the influence of VPD on seedling transpiration, development and growth was also investigated. The FTSW threshold ranged from 0.40 to 0.99 for the transpiration rate and from 0.32 to 0.97 for the development and growth variables. Little or no changes in the FTSW threshold were detected in response to changes in atmospheric VPD. Both Eucalyptus species presented a conservation strategy under drought stress. In addition, water-conserving mechanisms during the seedling phase were related to rapid stomatal closure, reduced leaf area, and number of leaves. [ABSTRACT FROM AUTHOR]

Peatlands perform many important ecosystem functions at both the local and global scale, including hydrologic and climatic regulation. Although peatlands often act as climatic microrefugia, they have rarely been the subject of long-term microclimatic studies. In this study, we aimed to compare the local climatic conditions of a mid-forest mire to that of an open area and examine the differences in microclimates within the mire based on plant community diversity, shading, and water table depths. The peatland studied in this work was significantly cooler than the reference site, mainly due to a higher decline in nighttime air temperatures. However, the daily maximum air temperature near the ground was often higher. We also noticed that microclimates significantly differed within the studied peatland. Wet and shaded microsites were cooler than the sites having a lower water level and receiving higher amounts of solar radiation. The results of the study suggest that peatlands have locally cooler climates, and thus can serve as climate change refugia. These findings can help us interpret reconstructed data from the peat archive, and, when combined with experiments, identify tipping points for peatland ecosystems. [ABSTRACT FROM AUTHOR]

Key message: Diffuse-porous Acer saccharum and ring-porous Carya ovata exhibit opposing isohydry and anisohydry, respectively, but physiological responses of four other species indicate a spectrum of hydraulic behaviors among co-existing mature temperate trees. Temperate forests maintain a high species abundance, yet we do not fully understand how co-existing temperate tree species differentially respond to water stress. The iso/anisohydric spectrum provides a framework for contextualizing and comparing hydraulic behavior. Using this framework, we wanted to understand how hydraulic behavior differs between several mature canopy-dominant tree species in a temperate deciduous forest. We assessed multiple hydraulic parameters across the 2019 growing season in three diffuse-porous species (Acer saccharum, Fagus grandifolia, and Platanus occidentalis) and three ring-porous species (Quercus alba, Quercus rubra, and Carya ovata). We measured sap flux and leaf water potential, along with soil moisture and vapor pressure deficit over the growing season. We also collected pre-dawn leaf water potential measurements in July in addition to the mid-day measurements. Our data reveal a range of hydraulic behaviors from isohydric Acer saccharum at one end of the continuum, to anisohydric Carya ovata at the other end, with the other four species exhibiting intermediate hydraulic behaviors between these two "extremes". The range of hydraulic behaviors exhibited among co-existing temperate tree species suggests multiple strategies for coping with water stress within a single forest. Defining species-specific strategies will be important for predicting how the structure and composition of temperate forests may respond to changes in climate, particularly water availability. [ABSTRACT FROM AUTHOR]

Recent evidence has revealed the emergence of a megadrought in southwestern North America since 2000. Megadroughts extend for at least 2 decades, making it challenging to identify such events until they are well established. Here, we examined tree-ring growth and stable isotope ratios in Pinus ponderosa at its driest niche edge to investigate whether trees growing near their aridity limit were sensitive to the megadrought climatic pre-conditions, and were capable of informing predictive efforts. During the decade before the megadrought, trees in four populations revealed increases in the cellulose δ13C content of earlywood, latewood, and false latewood, which, based on past studies are correlated with increased intrinsic water-use efficiency. However, radial growth and cellulose δ18O were not sensitive to pre-megadrought conditions. During the 2 decades preceding the megadrought, at all four sites, the changes in δ13C were caused by the high sensitivity of needle carbon and water exchange to drought trends in key winter months, and for three of the four sites during crucial summer months. Such pre-megadrought physiological sensitivity appears to be unique for trees near their arid range limit, as similar patterns were not observed in trees in ten reference sites located along a latitudinal gradient in the same megadrought domain, despite similar drying trends. Our results reveal the utility of tree-ring δ13C to reconstruct spatiotemporal patterns during the organizational phase of a megadrought, demonstrating that trees near the arid boundaries of a species' distribution might be useful in the early detection of long-lasting droughts. [ABSTRACT FROM AUTHOR]

Fires can pose a threat to forest ecosystems when those ecosystems are not fire-adapted or when forest community conditions make them vulnerable to wildfires. Thus, investigating fire-prone environmental conditions is urgently needed to create action plans that preserve these ecosystems. In this sense, climate variables can determine the environmental conditions favorable for forest fires. Our study confirms that vapor pressure deficit (VPD) is an essential climate indicator for forest fires, as it is related to maximum temperatures and low humidity, representing the stress conditions for vegetation prone to fires. This study explores the extent to which ENSO phases can modulate climatic conditions that lead to high VPD over Guanajuato, a semi-arid region in central Mexico, during the dry season (March–April-May). Using fire occurrence data from MODIS (2000–2019) and Landsat 5 (1998–1999), we developed a climatic probability model for the occurrence of forest fires using VPD estimated from ERA5 reanalysis for each ENSO phase. We found that VPD and the occurrence of forest fires were higher during El Niño than under Neutral and La Niña years, with a higher risk of forest fire occurrence in Guanajuato's southern region. This study concludes that it is necessary to implement regional and local fire management plans, especially where the largest number of natural protected areas is located. [ABSTRACT FROM AUTHOR]

The tropical arboreal species Brazilian mahogany (Swietenia macrophylla) is very important economically and ecologically, for which understanding ecophysiological variables such as sap flow will improve understanding of the species and its cultivation. This paper aims to measure uncertainties (U) involved in the application of the heat ratio method for determining sap flow in Brazilian mahogany using sets of heating probes and thermometers installed on plants of 18 months of age, cultivated in Yellow Latosol, under a weighing lysimeter and located in a protected environment. The uncertainty in sap flow was calculated as the combination of uncertainty in the thermal diffusivity (Uk), conductive section (USc) and corrected sap velocity (UVc). Uk had greater weight in determining the flow of sap in Brazilian mahogany, when compared to USc and UVc. The thermal diffusivity during the cycle, or period evaluated, must be adjusted to improve the accuracy of the heat ratio method because the sap flow overestimated transpiration by 15.0%. When soil water was optimal In addition, the vapor pressure deficit linearly and indirectly influenced the SF with a difference of 14.6%. [ABSTRACT FROM AUTHOR]

Atmospheric humidity, defined as the vapor pressure deficit (VPD), is an important factor affecting plant transpiration and nutritional status. Previous work has focused on the relationship between VPD and nitrogen metabolism; however, little is known about potassium (K) responses to VPD. Here, we investigated the effects of VPD and K supply on root morphology, potassium uptake, and plant growth with tomato (Solanum lycopersicum L., cv. Jinpeng). Experiments were conducted in greenhouses under low and high VPD conditions and featured three levels of K nutrient solution supply (2, 4, and 8 mmol/L). Characteristics of root morphology were significantly promoted by low VPD, including root length, root volume, root diameter, root surface area, number of fine roots, and proportion of fine roots, which combined were more conducive to the absorption of nutrients; low VPD reduced the transpiration rate, but the K accumulation by plants showed an increasing trend, indicating increased root morphology parameters could compensate for the impact of reduced transpiration-driven mass flow on K uptake. Meanwhile, higher biomass also promotes the accumulation of potassium. Photosynthesis is promoted due to increased potassium accumulation in the leaves, which in turn increases plant biomass, especially with moderate K supply. Furthermore, low VPD increases biomass allocation in leaves and reduces biomass allocation in stems but, in contrast, relatively more potassium is allocated to stems and roots but less to foliage. In summary, low VPD promotes K uptake by facilitating root system development in summer greenhouses. This result provides new knowledge for enhancing potassium utilization efficiency and reducing the use of chemical fertilizers. [ABSTRACT FROM AUTHOR]

The objective of this work is to test the strength of relationships between maximum trunk daily shrinkage (MDS), soil matric potential (SMP) for various soil depths, and some weather variables in grown plum trees and medium-textured soils under continental-temperate climate conditions. The obtained relationships might be used in identification of water stress for irrigation scheduling. The study was carried out in a 7–9-year-old plum tree orchard plot with the 'Stanley' cultivar grafted on Saint Julien A semi-dwarf rootstock. Micrometric trunk diameter fluctuations of three trees were automatically measured by phytomonitors, and SMP was monitored continuously with resistance blocks installed in three profiles at four depths: 0.2, 0.4, 0.6 and 0.8 m during many wetting-drying cycles during two growing seasons. Strong and highly significant regression equations were obtained between MDS and air temperature (T) and daily vapor pressure deficit (VPD) for three growing seasons of observations. There were also weaker but significant relationships between MDS and SMP for all the four depths studied. Multiple linear regression equations between MDS, SMP for all the four depths, and either maximum T or VPD generally improved R2 from the simple regression equations obtained with each of the same variables. The MDS threshold of 0.08–0.12 mm approximately corresponds to a SMP range of about −60 to −40 kPa; however, due to the high MDS variability in time and the weight of the weather variables in the relationships between MDS, SMP and weather variables like T or VPD, the usefulness of these relationships is limited. [ABSTRACT FROM AUTHOR]

Aims: As soil dries, the loss of soil hydraulic conductivity limits water supply to the leaves, which is expected to generate a nonlinear relationship between leaf water potential (ψleaf) and transpiration (E). The effect of soil drying and root properties on ψleaf and E remains elusive. Methods: We measured E and ψleaf of pearl millet for varying E and soil moisture using a root pressure chamber. A model of water flow in soil and plant was used to fit the ψleaf (E) relationship. Results: The relation between ψleaf and E was linear at all soil moistures. The slope of ψleaf (E) increased with decreasing soil moisture due to the decreasing soil-root conductance. The fact that the relation remained linear also in dry soils and high E is surprising. Indeed, it indicates that the gradients in soil water potential (ψsoil) were small, probably because of the large root surface (13.5 cm cm−3) active in water uptake. ψleaf at E = 0 was less negative than ψsoil, indicating a more negative osmotic potential in the xylem than in the soil. Conclusions: We propose that the linearity between ψleaf and E and the high ψleaf (E = 0) compared to ψsoil support transpiration in drying soils. [ABSTRACT FROM AUTHOR]

Recent analyses on the length of drought recovery in forests have shown multi-year legacies, particularly in semi-arid, coniferous ecosystems. Such legacies are usually attributed to ecophysiological memory, although drought frequency itself, and its effect on overlapping recovery times, could also contribute. Here, we describe a multi-decadal study of drought legacies using tree-ring carbon-isotope ratios (δ13C) and ring-width index (RWI) in Pinus ponderosa at 13 montane sites traversing a winter–summer precipitation gradient in the Southwestern U.S. Sites and trees were selected to avoid collection biases that exist in archived tree-ring databanks. The spatial hydroclimate gradient and winter–summer seasonal patterns were well predicted by seasonal and inter-annual correlations between δ13C and atmospheric vapor pressure deficit (VPD). Using VPD, we found that the probability of extreme drought has increased up to 70% in this region during the past two decades. When the recent increase in drought frequency was not considered, multi-year legacies in both δ13C and RWI were observed at most sites. When the increase in drought frequency was detrended from tree-ring chronologies, some sites exhibited short legacies (1–2 years) in both δ13C and RWI, and there was a sight trend for longer legacies in RWI. However, when considered broadly across the region and multiple decades, no significant legacies were observed, which contrasts with past studies. Our results reveal that a contribution to observed multi-year legacies is related to shifts in the climate system itself, an exogenous factor, that must be considered along with physiological memory. [ABSTRACT FROM AUTHOR]

Transpiration in humid tropical forests modulates the global water cycle and is a key driver of climate regulation. Yet, our understanding of how tropical trees regulate sap flux in response to climate variability remains elusive. With a progressively warming climate, atmospheric evaporative demand [i.e., vapor pressure deficit (VPD)] will be increasingly important for plant functioning, becoming the major control of plant water use in the twenty-first century. Using measurements in 34 tree species at seven sites across a precipitation gradient in the neotropics, we determined how the maximum sap flux velocity (vmax) and the VPD threshold at which vmax is reached (VPDmax) vary with precipitation regime [mean annual precipitation (MAP); seasonal drought intensity (PDRY)] and two functional traits related to foliar and wood economics spectra [leaf mass per area (LMA); wood specific gravity (WSG)]. We show that, even though vmax is highly variable within sites, it follows a negative trend in response to increasing MAP and PDRY across sites. LMA and WSG exerted little effect on vmax and VPDmax, suggesting that these widely used functional traits provide limited explanatory power of dynamic plant responses to environmental variation within hyper-diverse forests. This study demonstrates that long-term precipitation plays an important role in the sap flux response of humid tropical forests to VPD. Our findings suggest that under higher evaporative demand, trees growing in wetter environments in humid tropical regions may be subjected to reduced water exchange with the atmosphere relative to trees growing in drier climates. [ABSTRACT FROM AUTHOR]

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

Environmental humidity is thought to be a major determinant of evaporative cooling capacity at high air temperatures (Ta), but the technical challenges of experimentally manipulating humidity in respirometry chambers have resulted in far less being known about the effects of humidity compared to those of Ta. We tested the prediction that at Ta approaching and exceeding normothermic body temperature (Tb), high humidity would result in higher Tb, lower evaporative water loss (EWL) and/or higher resting metabolic rate (RMR) in a passerine bird, the white-browed sparrow-weaver (Plocepasser mahali). We used open-system flow-through respirometry to measure EWL, RMR and Tb in sparrow-weavers experiencing Ta = 36-44 °C and chamber humidities of 6, 13, 19 or 25 g m− 3. Increasing humidity was associated with significantly higher Tb. The strongest effect of humidity, however, involved significant increases in RMR; at Ta = 40 °C, RMR at a humidity of 25 g m− 3 was ~ 40% higher compared to a humidity of 6 g m− 3. Moreover, the interaction between Ta and humidity exerted a significant effect on the ratio of evaporative heat loss (EHL) to metabolic heat production (MHP), evident as an increasing effect of humidity with increasing Ta. Our results, when compared with those of the limited number of previous studies that involved similar ranges of Ta and humidity, reveal that the relative effects of humidity on EWL and RMR vary among avian taxa, and support the notion that the overall effect of high humidity is a reduction in maximum EHL/MHP. [ABSTRACT FROM AUTHOR]

Lack of data on water use of key species of drylands constitutes an obstacle to understanding their role in hydrological processes in this environment. To elucidate seasonal variation in water consumption by Vitellaria paradoxa, the dominant species of parklands of the semi-arid areas of West Africa, we’ve measured its transpiration using heat ratio method (HRM) and seven potential explanatory variables. Sap flux was found to be significantly different among years with 0.64, 0.59 and 0.67 L h−1 dm−2 in 2008, 2009 and 2010, respectively. Sap flux was significantly higher in the dry (0.73 L h−1 dm−2) than in the wet season (0.53 L h−1 dm−2). Nighttime sap flux during dry season (0.48 L h−1 dm−2) was significantly higher than that of the wet season (0.20 L h−1 dm−2) and it contributes on average to 26% of daily sap flow with a maximum reaching 49%. The mean transpiration rate per tree was 151 L day−1 and all measured variables except rainfall and soil water content were significantly correlated with sap flux. These correlations were stronger (higher R value) during the rainy than in the dry season. Vapor Pressure Deficit (VPD) explained the highest proportion of sap flux variation and their curve was of parabolic type (R2 = 0.54) indicating that V. paradoxa can probably down-regulate its canopy conductance beyond a certain threshold of VPD, which is about 3 kPa in the present study. Future studies should investigate such hypothesis as well as the impacts of the variation of V. paradoxa transpiration due to climatic variables on hydrological cycles. [ABSTRACT FROM AUTHOR]

Background: The environmental stresses caused by climate change have become more severe in recent decades, affecting tree growth and physiology. Tropical forests have great potential for global carbon sequestration. However, they suffer from heavy rainfall and prolonged dry periods due to climate change. Swietenia macrophylla King and Melia azedarach L. are economically valuable trees that are widely planted in southern Taiwan. Plantations are exposed to either prolonged dry periods or heavy rainfall within the seasons of tropical monsoon areas. Photo-physiological comparisons may provide information that can improve management of S. macrophylla and M. azedarach plantations in tropical regions. Results: Both species exhibited a midday depression in leaf photosynthesis regardless of the season. The net photosynthetic rate ( P ), stomatal conductance ( g ), and transpiration rate ( E) in the dry season all significantly decreased in both tree species. In addition, M. azedarach used water more efficiently than did S. macrophylla during the dry season, but S. macrophylla had higher P compared with that in M. azedarach during the wet season. Temperature and vapor pressure deficit influenced P variation in S. macrophylla and M. azedarach, respectively. Our data suggested that the P and g of M. azedarach, but not of S. macrophylla, were linearly correlated during the dry season. The reduction of the leaf area was more sever in M. azedarach than in S. macrophylla, thus preventing water loss more efficiently. Conclusions: M. azedarach adapted to drought by reducing total leaf area and maintaining higher P , g , E, and WUE compared with those measured in S. macrophylla during the dry season. M. azedarach is more drought adaptation and more suitable for both humid and semi-humid areas than S. macrophylla, whereas the latter should be limited to more humid areas. [ABSTRACT FROM AUTHOR]

Lentil ( Lens culinaris Medikus) is one of the most important annual food legumes that plays an important role in the food and nutritional security of millions in the world. Lentil is mainly grown under rainfed environments, where drought is one of the most challenging abiotic stresses that negatively impacts lentil production in the arid and semi-arid areas. Therefore, development of drought-adapted cultivars is one of the major objectives of national and international lentil breeding programs. The goal of this review is to provide a report on the current status of traits of lentil that might result in yield increases in water-limited environments and identify opportunities for research on other traits. Lately, traits that are either related to developmental plasticity and/or altered rooting and shoot characteristics have received considerable attention in the efforts to increase lentil yield in water-limited environments. However, two traits that have recently been proven to be especially useful in other legumes are still missing in lentil drought research: early partial stomatal closure under soil drying, and limited-transpiration under high atmospheric vapor pressure deficit. This review provides suggestions for further exploitation of these two soil-water-conservation traits in lentil. [ABSTRACT FROM AUTHOR]

Vapor pressure deficit (VPD) is a widely used measure of atmospheric water demand. It is closely related to crop evapotranspiration and consequently has major impacts on crop growth and yields. Most previous studies have focused on the impacts of temperature, precipitation, and solar radiation on crop yields, but the impact of VPD is poorly understood. Here, we investigated the spatial and temporal changes in VPD and their impacts on yields of major crops in China from 1980 to 2008. The results showed that VPD during the growing period of rice, maize, and soybean increased by more than 0.10 kPa (10 yr) in northeastern and southeastern China, although it increased the least during the wheat growing period. Increases in VPD had different impacts on yields for different crops and in different regions. Crop yields generally decreased due to increased VPD, except for wheat in southeastern China. Maize yield was sensitive to VPD in more counties than other crops. Soybean was the most sensitive and rice was the least sensitive to VPD among the major crops. In the past three decades, due to the rising trend in VPD, wheat, maize, and soybean yields declined by more than 10.0% in parts of northeastern China and the North China Plain, while rice yields were little affected. For China as a whole, the trend in VPD during 1980-2008 increased rice yields by 1.32%, but reduced wheat, maize, and soybean yields by 6.02%, 3.19%, and 7.07%, respectively. Maize and soybean in the arid and semi-arid regions in northern China were more sensitive to the increase in VPD. These findings highlight that climate change can affect crop growth and yield through increasing VPD, and water-saving technologies and agronomic management need to be strongly encouraged to adapt to ongoing climate change. [ABSTRACT FROM AUTHOR]

We determined the longevity (i.e., potential vase life) of cut roses across seasons and identified key factors during export that affect vase life. We obtained cut Rosa hybrida L. 'Lovely Lydia' roses harvested at an identical stage of maturity from commercial rose growers in Jeonju and Jangsu, Korea, in December 2014 and in April, July, and December 2015. We randomly divided the roses into two groups, which were transported to Seoul or Japan, and assessed their vase life in a controlled environment room (standard conditions) at Sejong University or in test rooms (export conditions) in flower markets in Japan. We regarded vase life under standard conditions as potential vase life and vase life under export conditions as consumer-level vase life. Overall, export-environmental conditions and vase life varied greatly across seasons. A seasonal variation in vase life was observed in both groups of flowers, and the difference between potential vase life and vase life increased as the potential vase life increased. Multiple regression analysis indicated that vase life is negatively correlated with flower auction temperature but positively correlated with potential vase life, RH during loading, and RH in the warehouse. Furthermore, the vase life of cut roses depends primarily on potential vase life, indicating that potential vase life, which is determined by the time of harvest, is modified by environmental conditions during export. [ABSTRACT FROM AUTHOR]

Aims: To evaluate the effects of changes in water stress and variable evaporative demand on water potential (Ψ), relative water content (RWC) and gas exchange during two consecutive years (2005 and 2006) providing insight into the mechanisms of stomatal control in the little-studied cultivar 'Cobrançosa'. Methods: Measurements of water potential, gas exchange in olive trees subjected to three irrigation treatments: rainfed (T0), continuous deficit irrigation (T1) and well irrigated (T2) that received 0, 30 and 100 % of estimated crop evapotranspiration. Results: Both the differences in irrigation amounts and the prolonged summer drought conditions characteristic of the study area (Northeast Portugal) resulted in considerable divergences among treatments, with T2 showing the highest values of both Ψ, RWC and gas exchange rates and T0 the lowest. Stomatal conductance exhibited a strong down-regulation under increasing vapor pressure deficit (VPD), with water stress leading to parallel reductions in stomatal conductance values at any given VPD and sensitivity to changes in VPD. These effects might be associated to the deduced drought-induced declines in leaf specific hydraulic conductance ( K). Conclusions: The results indicated that Cobrançosa olive trees have a near-isohydric behavior with a tendency to maintain a constant root-to-leaf water gradient (∆Ψ). [ABSTRACT FROM AUTHOR]

The paper describes the temperature profiles determined by thermal imagery in apricot tree canopies under the semi-arid conditions of the Black Sea Coast in a chernozem of Dobrogea Region, Romania. The study analyzes the thermal vertical profile of apricot orchards for three representative cultivars during summertime. Measurements were done when the soil water content (SWC) was at field capacity (FC) within the rooting depth, after intense sprinkler irrigation applications. Canopy temperature was measured during clear sky days at three heights for both sides of the apricot trees, sunlit (south), and shaded (north). For the SWC studied, i.e., FC, canopy height did not induce a significant difference between the temperature of apricot tree leaves (Tc) and the ambient air temperature (Ta) within the entire vertical tree profile, and temperature measurements by thermal imagery can therefore be taken at any height on the tree crown leaves. Differences between sunlit and shaded sides of the canopy were significant. Because of these differences for Tc-Ta among the apricot tree cultivars studied, lower base lines (LBLs) should be determined for each cultivar separately. The use of thermal imagery technique under the conditions of semi-arid coastal areas with low range of vapor pressure deficit could be useful in irrigation scheduling of apricot trees. The paper discusses the implications of the data obtained in the experiment under the conditions of the coastal area of the Black Sea, Romania, and neighboring countries with similar climate, such as Bulgaria and Turkey. [ABSTRACT FROM AUTHOR]

Main conclusion: First observation that chemical spray can induce limited-transpiration rate under high vapor pressure deficit. It appears that acibenzolar may be key in inducing this water conservation trait. Irrigation and water use have become major issues in management of turfgrasses. Plant health products that have been introduced into the turfgrass market have been observed to improve plant performance in water stress conditions. In this study, we evaluated whether a selection of common plant health products alter the ability of creeping bentgrass ( Agrostis stolonifera L.) to control transpiration under high vapor pressure deficit (VPD). The plant health treatments-Daconil Action, Insignia, and Signature-were applied to plots on golf course putting greens located in Raleigh NC and in Scottsdale, AZ. Using intact cores removed from the putting greens, transpiration rates were measured over a range of VPDs in controlled conditions. In all cases stretching over a 3-year period, bentgrass cores from field plots treated with Daconil-Action limited transpiration under high VPD conditions, while check treatments with water, and others treated with Insignia or Signature did not. Transpiration control became engaged when VPDs reached values ranging from 1.39 to 2.50 kPa, and was not strongly influenced by the field temperature at which the bentgrass was growing. Because all plots in NC had been treated with chlorothalonil-the key ingredient in Daconil Action to control diseases-it was concluded that the likely chemical ingredient in Daconil Action triggering the transpiration control response was acibenzolar. This is the first evidence that the limited-transpiration trait can be induced by a chemical application, and it implies significant potential for ameliorating drought vulnerability in cool-season turfgrasses, and likely other plant species. [ABSTRACT FROM AUTHOR]

High temperature-induced spikelet sterility is expected to become a major factor reducing rice yield under the future climate conditions. To examine the responses of spikelet sterility to air temperature, humidity, and temperatures of panicle and spikelet, two japonica rice cultivars in different maturity groups were exposed to four sets of different temperature conditions from initial heading stage in 2013 and 2014. The temperature conditions included ambient temperature (AT), AT + 1.5°C, AT + 3.0°C, and AT + 5.0°C. Spikelet fertility showed a wide range of variation from 100 to 4.6% depending on temperature treatments. The ridge regression revealed that not only air temperature but also vapor pressure deficit (VPD) was negatively associated with spikelet fertility. The spikelet fertility was well fitted to logistic equations not only of air temperature, spikelet internal temperature, and panicle surface temperature but also of VPD. No clear difference in the accuracy of sterility estimation was observed for models that use air temperature, panicle or spikelet temperature as inputs. In contrast, the logistic equation model that uses both air temperature and VPD as independent variables had better accuracy in predicting spikelet sterility. These results suggested that spikelet or panicle temperature would be no better predictor for high temperature-induced spikelet sterility than air temperature. Therefore, further study is merited to verify the VPD effects on spikelet sterility under high temperature conditions. [ABSTRACT FROM AUTHOR]

Key message: We demonstrate the role of DREB1A transcription factor in better root and shoot partitioning and higher transpiration efficiency in transgenic chickpea under drought stress Abstract: Chickpea ( Cicer arietinum L.) is mostly exposed to terminal drought stress which adversely influences its yield. Development of cultivars for suitable drought environments can offer sustainable solutions. We genetically engineered a desi-type chickpea variety to ectopically overexpress AtDREB1A, a transcription factor known to be involved in abiotic stress response, driven by the stress-inducible Atrd29A promoter. From several transgenic events of chickpea developed by Agrobacterium-mediated genetic transformation, four single copy events (RD2, RD7, RD9 and RD10) were characterized for DREB1A gene overexpression and evaluated under water stress in a biosafety greenhouse at T6 generation. Under progressive water stress, all transgenic events showed increased DREB1A gene expression before 50 % of soil moisture was lost (50 % FTSW or fraction of transpirable soil water), with a faster DREB1A transcript accumulation in RD2 at 85 % FTSW. Compared to the untransformed control, RD2 reduced its transpiration in drier soil and higher vapor pressure deficit (VPD) range (2.0-3.4 kPa). The assessment of terminal water stress response using lysimetric system that closely mimics the soil conditions in the field, showed that transgenic events RD7 and RD10 had increased biomass partitioning into shoot, denser rooting in deeper layers of soil profile and higher transpiration efficiency than the untransformed control. Also, RD9 with deeper roots and RD10 with higher root diameter showed that the transgenic events had altered rooting pattern compared to the untransformed control. These results indicate the implicit influence of rd29A::DREB1A on mechanisms underlying water uptake, stomatal response, transpiration efficiency and rooting architecture in water-stressed plants. [ABSTRACT FROM AUTHOR]

In the eastern United States, winter temperature has been increasing nearly twice as fast as summer temperature, but studies of warming effects on plants have focused on species that are photosynthetically active in summer. The terrestrial orchid Tipularia discolor is leafless in summer and acquires C primarily in winter. The optimum temperature for photosynthesis in T. discolor is higher than the maximum temperature throughout most of its growing season, and therefore growth can be expected to increase with warming. Contrary to this hypothesis, experimental warming negatively affected reproductive fitness (number of flowering stalks, flowers, fruits) and growth (change in leaf area from 2010 to 2012) in T. discolor. Temperature in June-July was critical for flowering, and mean July temperature greater than 29 °C (i.e., 2.5 °C above ambient) eliminated reproduction. Warming of 1.2 °C delayed flowering by an average of 10 days and fruiting by an average of 5 days. Warming of 4.4 °C reduced relative growth rates by about 60 %, which may have been partially caused by the direct effects of temperature on photosynthesis and respiration. Warming indirectly increased vapor pressure deficit (VPD) by 0.2-0.5 kPa, and leaf-to-air VPD over 1.3 kPa restricted stomatal conductance of T. discolor to 10-40 % of maximum conductance. These results highlight the need to account for changes in VPD when estimating temperature responses of plant species under future warming scenarios. Increasing temperature in the future will likely be an important limiting factor to the distribution of T. discolor, especially along the southern edge of its range. [ABSTRACT FROM AUTHOR]

Low air temperature accompanied with high humidity is effective for long-term cold storage of the predatory mite Neoseiulus californicus (McGregor) (Acari: Phytoseiidae). To further improve this storage method, we investigated the effect of pre-storage nutrition on survival during storage and on post-storage quality in terms of survival, oviposition, and progeny viability. The predatory mite was fed from the egg to adult stage on the diapausing two-spotted spider mite, Tetranychus urticae Koch (Acari: Tetranychidae), non-diapausing spider mites, or Japanese pear pollen, Pyrus pyrifolia Nakai. Newly emerged N. californicus adult females and males were mated, and then both were stored at 7.5 °C and a vapor pressure deficit of 0.0 kPa for up to 75 days. Survival during storage and post-storage quality was significantly better with the diapausing spider mite diet than with the other diets. No effects on the survival or sex ratio of the progeny of the stored adults were observed, regardless of diet or storage duration. Providing diapausing spider mites as a pre-storage diet therefore significantly improves the long-term storage of N. californicus. We discuss the possibility that ingestion of the cryoprotectants, antioxidants, and energy reserves that are present in rich amounts in diapausing spider mites mitigates chilling injury. [ABSTRACT FROM AUTHOR]

This study investigated the effects of atmospheric moisture on conidia development in cucurbit powdery mildew fungus ( Podosphaera xanthii) through host-plant responses. Cucumber ( Cucumis sativus L.) seedlings were grown under a high vapour pressure deficit (VPD) (3.8 kPa) or a low VPD (0.4 kPa). When the cotyledons had expanded, spores of P. xanthii were inoculated onto the adaxial surface. Inoculated seedlings for both treatments were then placed in a growth chamber maintained at a VPD of 2.1 kPa. The density of visible P. xanthii colonies on the high-VPD-acclimated cotyledons was 0.46 to 0.85 times that of the low-VPD-acclimated cotyledons 7 days after inoculation. It is likely the post-germination behaviour of P. xanthii such as the infection and consequent hyphal development was affected because spore germination did not differ between the treatments. The percentage of adaxial epidermal leaf cells with haustoria was also lower in the seedlings which had acclimated to a higher VPD. The high-VPD-acclimated cotyledons were thicker and had greater dry mass per area. The water potential of cotyledons did not differ between the treatments, although the stomatal conductance of high-VPD-acclimated cotyledons was lower than that of low-VPD-acclimated cotyledons. From these results, we conclude that the inhibition of P. xanthii conidia development on high-VPD-acclimated cotyledons was mainly caused by changes in leaf morphological properties. [ABSTRACT FROM AUTHOR]

Mountain peatlands in the temperate regions represent a small proportion of the global peatland resource, but play a unique, although less recognized role in the regional water and biogeochemical cycles. We conducted leaf and ecosystem CO exchange measurements during the growing period of 2008 to investigate the effects of vapor pressure deficit (VPD) and ground water level (WL) on leaf gas exchange of two dominant herbaceous species, Molinia caerulea (L.) and Carex nigra (L.) in a mountain peatland. Implications of the leaf-level responses for net ecosystem CO exchange (NEE) were examined. Fluctuations in WL influenced soil water content (SWC) within the top 5 cm of the peat profile. Although midday leaf water potential during the day (Ψ) was positively correlated ( P < 0.001) with SWC in the 5 cm peat profile, there was no direct relationship between WL and plant water relations parameters. VPD and stomatal conductance (gs) were negatively correlated at VPD >1 kPa regardless of the current WL status. Stomatal conductance was positively correlated with leaf assimilation (A) and root-shoot hydraulic conductance (K). A decline in NEE during summer was attributed to stomatal closure due to high VPD and low light utilization efficiency (α) of the vegetation. Ecosystem respiration (R) was positively correlated with peat temperature at 10 cm depth. Our results suggest that increases in VPD, as a possible scenario of climate change, would lower the photosynthetic capacity of the herbaceous peatland vegetation, despite the high WL, and hence lower the potential of such peatlands as sinks for atmospheric CO. [ABSTRACT FROM AUTHOR]

To determine the optimum air temperature and water vapor pressure deficit (VPD) for the storage of the predatory mite, Neoseiulus californicus, 3-day-old mated females were stored at air temperatures of 0, 5, 10, or 15 °C and VPDs of 0.1, 0.3, or 0.5 kPa for 10, 20, or 30 days. At 10 °C and 0.1 kPa, 83 % of females survived after 30 days of storage; this percentage was the highest among all conditions. VPDs of 0.3 and 0.5 kPa regardless of air temperature, and an air temperature of 0 °C regardless of VPD were detrimental to the survival of the females during storage. Since the highest survival was observed at 10 °C and 0.1 kPa, the effect of the storage duration on the post-storage quality of the stored females and their progeny was investigated at 25 °C to evaluate the effectiveness of the storage condition. The oviposition ability of the stored females, hatchability, and sex ratio of their progeny were not affected even when the storage duration was extended to 30 days. Although a slight decrease in the survival during the immature stages of progeny was observed when the storage duration was ≥20 days, the population growth of N. californicus may not be affected when individuals stored in these conditions are applied to greenhouses and agricultural fields. The results indicate that mated N. californicus females can be stored at 10 °C and 0.1 kPa VPD for at least 30 days. [ABSTRACT FROM AUTHOR]

Aims: Climate, soil water potential (SWP), leaf relative water content (RWC), stomatal conductance (g), fruit and shoot growth, and carbohydrate levels were monitored during the 2008 and 2009 growing seasons to study the responses of 'Gala' and 'Fuji' apple trees to irrigation placement or volume. Methods: Three irrigation treatments were imposed, conventional irrigation (CI), partial root-zone drying (PRD, 50% of CI water on one side of the root-zone, which was alternated periodically), and continuous deficit irrigation (DI, 50% of CI water on both sides of the root-zone). Results: After each irrigation season, DI generated twice the soil water deficit (SWD) than PRD (average of dry and wet sides) and a greater integrated leaf water deficit (LWD) than PRD and CI. Both PRD and DI reduced g by 9 and 15% over the irrigation period. RWC of both PRD and DI was directly related to SWP and inversely related (non-linear) to vapor pressure deficit (VPD), whereas it was unrelated to g. Considering individual sampling days, g of 'Gala' leaves was inversely related to VPD mainly until early August (fruit at cell expansion phase and high VPD), while it was directly related to VPD in September (no fruit and low VPD). On the contrary, g of 'Fuji' leaves was inversely related to VPD from late August until mid October (low VPD and fruit at cell expansion phase). Fruit growth was not affected by irrigation, whereas shoot and trunk growth was reduced by DI. Irrigation induced sporadic and inconsistent changes in carbohydrate contents or partitioning, with a general tendency of DI leaves to degrade and PRD to accumulate sorbitol and sucrose in dry periods. Conclusions: 'Gala' trees exhibited a more conservative water use than 'Fuji' trees due primarily to different timing of fruit growth and crop loads. Different levels of SWD, rather than changes in stomatal control and carbohydrate partitioning, seem to play a major role in determining a better water status in PRD than in DI trees. [ABSTRACT FROM AUTHOR]

We investigated the occurrence of patchy stomatal behavior in leaves of saplings and a forest canopy tree of Quercus crispula Blume. Through a combination of leaf gas-exchange measurements and numerical simulation, we detected patterns of stomatal closure (either uniform or patchy bimodal) coupled with depression of net assimilation rate ( A). There was a clear inhibition of A associated with stomatal closure in leaves of Q. crispula during the day, but the magnitude of inhibition varied among days and growing conditions. Comparisons of observed and simulated A values for both saplings and the canopy tree identified patterns of stomatal behavior that shifted flexibly between uniform and patchy frequency distributions depending on environmental conditions. Bimodal stomatal closure explained severe depression of A in saplings under conditions of relatively high leaf temperature and vapor pressure deficit. Model simulations of A depression through bimodal stomatal closure were corroborated by direct observations of stomatal aperture distribution using Suzuki's Micro-Printing method; these demonstrated that there was a real bimodal frequency distribution of stomatal apertures. Although there was a heterogeneous distribution of stomatal apertures both within and among patches, induction of heterogeneity in intercellular CO concentration among patches, and hence severe depression of A, resulted only from bimodal stomatal closure among patches (rather than within patches). [ABSTRACT FROM AUTHOR]

Quantifying the controls on soil respiration is important for understanding ecosystem physiology and for predicting the response of soil carbon reservoirs to climate change. The majority of soil respiration is typically considered to occur in the top 20-30 cm of soils. In desert soils, where organic matter concentrations tend to be low and plants are deeply rooted, deeper respiration might be expected. However, little is known about the depth distribution of respiration in dryland soils. Here we show that the average depth of soil respiration between pulse precipitation events is almost always greater than 20 cm and is frequently greater than 50 cm in two central New Mexico desert shrublands. The average depth of soil respiration in a piñon-juniper woodland was shallower, between 5 and 40 cm. In the shrublands, 8‰ seasonal variations in the carbon isotope composition of soil-respired CO (δC) that correlate with vapor pressure deficit support root/rhizosphere respiration as the dominant source of soil CO. Such deep autotrophic respiration indicates that shrubs preferentially allocate photosynthate to deep roots when conditions near the surface are unfavorable. Therefore, respiration rates in these soils are not necessarily correlated with root biomass. The δC values provide no evidence for CO evolved from soil inorganic carbon. Our results also suggest that organic carbon cycling is rapid and efficient in these soils and that the δC value of CO respired from soils in much of the southwestern US, and perhaps in other semiarid regions, varies seasonally by at least 4‰. [ABSTRACT FROM AUTHOR]

Humanity is heading toward the major challenge of having to increase food production by about 50% by 2050 to cater for an additional three billion inhabitants, in a context of arable land shrinking and degradation, nutrient deficiencies, increased water scarcity, and uncertainty due to predicted climatic changes. Already today, water scarcity is probably the most important challenge, and the consensual prediction of a 2-4°C degree increase in temperature over the next 100 years will add new complexity to drought research and legume crop management. This will be especially true in the semi-arid tropic areas, where the evaporative demand is high and where the increased temperature may further strain plant-water relations. Hence, research on how plants manage water use, in particular, on leaf/root resistance to water flow will be increasingly important. Temperature increase will variably accelerate the onset of flowering by increasing thermal time accumulation in our varieties, depending on their relative responses to day length, ambient, and vernalizing temperature, while reducing the length of the growing period by increasing evapotranspiration. While the timeframe for these changes (>10-20 years) may be well in the realm of plant adaptation within breeding programs, there is a need for today's breeding to understand the key mechanisms underlying crop phenology at a genotype level to better balance crop duration with available soil water and maximize light capture. This will then be used to re-fit phenology to new growing seasons under climate change conditions. The low water use efficiency, i.e., the amount of biomass or grain produced per unit of water used, under high vapor pressure deficit, although partly offset by an increased atmospheric CO concentration, would also require the search of germplasm capable of maintaining high water use efficiency under such conditions. Recent research has shown an interdependence of C and N nutrition in the N performance of legumes, a balance that may be altered under climate change. Ecophysiological models will be crucial in identifying genotypes adapted to these new growing conditions. An increased frequency of heat waves, which already happen today, will require the development of varieties capable of setting and filling seeds at high temperature. Finally, increases in temperature and CO will affect the geographical distribution of pests, diseases, and weeds, presenting new challenges to crop management and breeding programs. [ABSTRACT FROM AUTHOR]

Climate change variables have rarely been analyzed specifically for the growing season of crops. In this study, changes in minimum (Tmin) and maximum temperature (Tmax), daily maximum saturated vapor pressure (VPsat) and estimated daily VPD weighted for transpiration (VPDtran) were examined at five locations in Iran over 40 years during the growing seasons of chickpea and maize. The results of this analysis showed a general trend of increasing Tmax. The increase in Tmax was especially pronounced during the period when maize is grown. Increasing Tmax resulted in the calculation of increasing VPsat at all locations. Minimum temperature also showed a clear increase at four of the five locations. The one exception in increasing Tmin was a location (Isfahan) leeward of the Zagros mountains where the air may be dried as it traverses the mountain, likely resulting in the observed stability of the dew point at this location. City growth encroached in the vicinity of the weather stations so some of the warming might be attributed to the expansion of city boundaries. At the four locations where both Tmax, and Tmin increased, no increase in VPDtran was identified. Therefore, no increasing trend for increasing water requirement by the crops was identified. On the other hand, the lack of an increase in Tmin at Isfahan resulted in an increase in atmospheric VPDtran. The increased VPDtran results in higher water requirement in growing crops. Using the genotypes with breakpoint in their transpiration rate at high VPD might alleviate the deleterious effect of high VPDtran on growth of rainfed crops at Isfahan. [ABSTRACT FROM AUTHOR]

In 2005 and 2006, air samples were collected at the base of a Douglas-fir watershed to monitor seasonal changes in the δ13CO2 of ecosystem respiration (δ13CER). The goals of this study were to determine whether variations in δ13CER correlated with environmental variables and could be used to predict expected variations in canopy-average stomatal conductance ( G s). Changes in δ13CER correlated weakly with changes in vapor pressure deficit (VPD) measured 0 and 3–7 days earlier and significantly with soil matric potential (ψm) ( P value <0.02) measured on the same day. Midday G s was estimated using sapflow measurements (heat-dissipation method) at four plots located at different elevations within the watershed. Values of midday G s from 0 and 3–7 days earlier were correlated with δ13CER, with the 5-day lag being significant ( P value <0.05). To examine direct relationships between δ13CER and recent G s, we used models relating isotope discrimination to stomatal conductance and photosynthetic capacity at the leaf level to estimate values of stomatal conductance (“ G s–I”) that would be expected if respired CO2 were derived entirely from recent photosynthate. We compared these values with estimates of G s using direct measurement of transpiration at multiple locations in the watershed. Considering that the approach based on isotopes considers only the effect of photosynthetic discrimination on δ13CER, the magnitude and range in the two values were surprisingly similar. We conclude that: (1) δ13CER is sensitive to variations in weather, and (2) δ13CER potentially could be used to directly monitor average, basin-wide variations in G s in complex terrain if further research improves understanding of how δ13CER is influenced by post-assimilation fractionation processes. [ABSTRACT FROM AUTHOR]