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Exposure to temperatures above a critical threshold (temperature of phase transition, Tp) can damage the leaf cuticle, leading to increased leaf minimum conductance (gleaf-res). Despite the implications of increased gleaf-res for species survival under hotter-drought conditions, little is known about the dynamics of gleaf-res variation after heatwave episodes. Here, we examined the gleaf-res variation before, during, and after exposure to high temperatures (HTs) in a group of representative Cerrado tree species. Through multiple experiments, we compared gleaf-res in leaves previously exposed to different temperatures for varying durations with leaves not submitted to HT. Leaves previously exposed to temperatures above Tp and subsequently cooled had higher gleaf-res measured at 25 °C than leaves not exposed to HT, suggesting a "thermal leaky legacy" effect that negatively impacted plant survival under contrasting simulated drought scenarios. This legacy effect was induced by short periods of heat stress and increased proportionally with rising temperatures. Notably, increased gleaf-res was observed even after 24 h of leaf storage, evidencing that thermal-induced damages to the leaf cuticle cannot be fully repaired within a daily cycle. Overall, our study highlights the threats that increased gleaf-res during and after heatwaves may pose to plant performance and survival under drought conditions and emphasizes the importance of considering the dynamic nature of such water leaks to improve the predictions of drought-induced mortality events in a warmer and drier world., Competing Interests: Conflict of interest statement. None declared., (© The Author(s) 2025. Published by Oxford University Press on behalf of American Society of Plant Biologists. All rights reserved. For commercial re-use, please contact reprints@oup.com for reprints and translation rights for reprints. All other permissions can be obtained through our RightsLink service via the Permissions link on the article page on our site—for further information please contact journals.permissions@oup.com.)

Water resource management and sustainable agriculture rely heavily on accurate Reference Evapotranspiration (ETo). Efforts have been made to simplify the (ETo) estimation using machine learning models. The existing approaches are limited to a single specific area. There is a need for ETo estimations of multiple locations with diverse weather conditions. The study intends to propose ETo estimation of multiple locations with distinct weather conditions using a federated learning approach. Traditional centralized approaches require aggregating all data in one place, which can be problematic due to privacy concerns and data transfer limitations. However, federated learning trains models locally and combines the knowledge, resulting in more generalized ETo estimates across different regions. The three geographical locations of Pakistan, each with diverse weather conditions, are selected to implement the proposed model using the weather data from 2012 to 2022 of the selected three locations. At each selected location, three machine learning models named Random Forest Regressor (RFR), Support Vector Regressor (SVR), and Decision Tree Regressor (DTR), are evaluated for local Evapotranspiration (ET) estimation and the federated global model. The feature importance-based analysis is also performed to assess the impacts of weather parameters on machine learning performance at each selected local location. The evaluation reveals that Random Forest Regressor (RFR) based federated learning outperformed other models with coefficient of determination (R2) = 0.97%, Root Mean Squared Error (RMSE) = 0.44, Mean Absolute Error (MAE) = 0.33 mm day-1, and Mean Absolute Percentage Error (MAPE) = 8.18%. The Random Forest Regressor (RFR) performance yields the local machine learning models against each selected site. The analysis results suggest that maximum temperature and wind speed are the most influential factors in Evapotranspiration (ET) predictions., Competing Interests: The authors declared that no competing interests exist., (Copyright: © 2025 Tausif et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.)

Although the separate effects of water and nitrogen (N) limitations on forest growth are well known, the question of how to predict their combined effects remains a challenge for modeling of climate change impacts on forests. Here, we address this challenge by developing a new eco-physiological model that accounts for plasticity in stomatal conductance and leaf N concentration. Based on optimality principle, our model determines stomatal conductance and leaf N concentration by balancing carbon uptake maximization, hydraulic risk and cost of maintaining photosynthetic capacity. We demonstrate the accuracy of the model predictions by comparing them against gross primary production estimates from eddy covariance flux measurements and sap-flow measurement scaled canopy transpiration in a long-term fertilized and an unfertilized Scots pine (Pinus sylvestris L.) forest in northern Sweden. The model also explains the response to N fertilization as a consequence of (i) reduced carbon cost of N uptake and (ii) increased leaf area per hydraulic conductance. The results suggest that leaves optimally coordinate N concentration and stomatal conductance both on short (weekly) time scales in response to weather conditions and on longer time scales in response to soil water and N availabilities., (© The Author(s) 2025. Published by Oxford University Press.)

Identifying the physiological mechanisms by which plants are adapted to drought is critical to predict species responses to climate change. We measured the responses of leaf hydraulic and stomatal conductances (K leaf and g s , respectively) to dehydration, and their association with anatomy, in seven species of California Ceanothus grown in a common garden, including some of the most drought-tolerant species in the semi-arid flora. We tested for matching of maximum hydraulic supply and demand and quantified the role of decline of K leaf in driving stomatal closure. Across Ceanothus species, maximum K leaf and g s were negatively correlated, and both K leaf and g s showed steep declines with decreasing leaf water potential (i.e., a high sensitivity to dehydration). The leaf water potential at 50% decline in g s was linked with a low ratio of maximum hydraulic supply to demand (i.e., maximum K leaf :g s ). This sensitivity of g s , combined with low minimum epidermal conductance and water storage, could contribute to prolonged leaf survival under drought. The specialized anatomy of subg. Cerastes includes trichomous stomatal crypts and pronounced hypodermis, and was associated with higher water use efficiency and water storage. Combining our data with comparative literature of other California species, species of subg. Cerastes show traits associated with greater drought tolerance and reliance on leaf water storage relative to other California species. In addition to drought resistance mechanisms such as mechanical protection and resistance to embolism, drought avoidance mechanisms such as sensitive stomatal closure could contribute importantly to drought tolerance in dry-climate adapted species., (© 2024 John Wiley & Sons Ltd.)

In ecophysiology leaves are frequently stored for hours after sampling before measuring their leaf water potential (Ψ leaf ). Here, we address a previously unidentified source of error, that metabolic heat generation can cause continuous water loss from leaves stored in impermeable bags, leading to a Ψ leaf drop over time. We tested Ψ leaf drop rates under various conditions: two bag materials, two species, initial Ψ leaf above or below the turgor loss point (Ψ tlp ), and storage at 25°C versus 4°C. We partitioned leaf water loss due to condensation on the inner bag surface or permeation through the bag. We found that Ψ leaf dropped by up to 0.39 MPa per hour, with 41%-89% of the water leaving the leaf condensed on the inner bag surface. Plastic bags showed higher Ψ leaf drop rates than aluminium bags, and leaves above Ψ tlp experienced greater drops. Storing leaves at 4°C reduced the Ψ leaf drop rate by 60% compared to 25°C. Leaves were 0.2-0.3°C warmer than the bags, likely due to metabolic heating. Our energy balance model suggests that water loss is lower when storing leaves at cooler temperatures, using leaves with low stomatal conductance, deflated bags, and leaves with low Ψ leaf ., (© 2024 John Wiley & Sons Ltd.)

Traditional photosynthesis-driven growth models have considerable uncertainties in predicting tree growth under changing climates, partially because sink activities are directly affected by the environment but not adequately addressed in growth modelling. Therefore, we developed a semi-mechanistic model coupling stomatal optimality, temperature control of enzymatic activities and phenology of cambial growth. Parameterized using Bayesian inference and measured data on Picea abies and Pinus sylvestris in peatland and mineral soils in Finland, the coupled model simulates transpiration and assimilation rates and stem radial dimension (SRD) simultaneously at 30 min resolution. The results suggest that both the sink and phenological formulations with environmental effects are indispensable for capturing SRD dynamics across hourly to seasonal scales. Simulated using the model, growth was more sensitive than assimilation to temperature and soil water, suggesting carbon gain is not driving growth at the current temporal scale. Also, leaf-specific production was occasionally positively correlated with growth duration but not with growth onset timing or annual cambial area increment. Thus, as it is hardly explained by carbon gain, phenology itself should be included in sink-driven growth models of the trees in the boreal zone and possibly other environments where sink activities and photosynthesis are both restrained by harsh conditions., (© 2024 The Author(s). Plant, Cell & Environment published by John Wiley & Sons Ltd.)

As the single link between leaves and the rest of the plant, petioles must develop conductive tissues according to the water influx and sugar outflow of the leaf lamina. A scaling relationship between leaf area and anatomical traits of xylem and phloem is expected to improve the efficiency of these tissues. However, the different constraints compromising the functionality of both tissues (e.g., risk of cavitation) must not be disregarded. Additionally, deciduous and evergreen plants may have different strategies to produce and package their petiole conduits to cope with environmental restrictions. We explored in 33 oak species the relationships between petiole anatomical traits, leaf area, stomatal conductance, and photosynthesis rate. Results showed allometric scaling between anatomical structure of xylem and phloem with leaf area. We also found correlations between photosynthesis rate, stomatal conductance, and anatomical traits in the petiole. The main novelty is how oaks present a different strategy depending on the leaf habit. Deciduous species tend to increase their diameters to achieve greater leaf-specific conductivity. By contrast, evergreen oaks develop larger xylem conductive areas for a given leaf area than deciduous ones. This trade-off between safety-efficiency in petioles has never been attributed to the leaf habit of the species., (© 2024 John Wiley & Sons Ltd.)

Measuring ecophysiological responses of Eucalyptus clones grown under reduced water availability could assist in clonal selection for climate resilience. We hypothesized that clonal variation in chlorophyll a fluorescence was more readily detected than variations in leaf-level gas exchanges when 2-year-old Eucalyptus grandis W.Hill ex Maiden × Eucalyptus urophylla S.T. Blake hybrid clones (C1, C2, C3 and C4) grown under rainfed (RF) and water-restricted (WR) conditions were evaluated during dry and rainy seasons, in the morning and midday diurnal periods. The C2 clone was the most drought tolerant as it had a similar net CO2 assimilation rate (A) considering the RF and WR conditions at midday during the dry season, while C1, C3 and C4 CO2 assimilation rates (A) decreased by 29.1%, 28.3% and 13%, respectively. This response was associated with a reduction to a lesser extent in leaf water potential, stomatal conductance (gs) and transpiration rates (E) (ca 10%, 30% and 13% under WR, respectively), when compared with the other clones during the dry season at midday. The lower leaf to air vapor pressure deficit of C2 contributed to its greater water-use efficiency (WUE), resulting in greater total dry mass gain. C1, C3 and C4 were less drought tolerant, decreasing gs, E and especially A under WR, resulting in lower WUE and total dry mass gain. Chlorophyll a fluorescence indexes were better indicators of drought tolerance compared with gas exchange parameters in definition of drought tolerance of clonal Eucalyptus. Three drought-sensitive clones showed low photochemical efficiency under WR, with the electron transport rate being impaired between photosystems II and I, indicated by the greater changes in photosynthetic performance index (PIabs). Under WR conditions, Fv/Fm, Ψ0, ΦE0 and PIabs decreased in all clones while ΦD0 and DI0/CS0 increased, with C2 showing the most stable responses suggesting that the photochemical apparatus was the less damaged by drought. Thus, C2 was the best clone for regions with water scarcity., (© The Author(s) 2024. Published by Oxford University Press. All rights reserved. For permissions, please e-mail: journals.permission@oup.com.)

Tree water status is mainly determined by the amount of water taken up from roots and lost through leaves by transpiration. Variations in transpiration and stomatal conductance are often related to atmospheric conditions and leaf water potential. Yet, few experimental datasets exist that enable to relate leaf water potential, transpiration dynamics and temporal variation of root water uptake from different depths during soil drying. Here we explored the soil-plant hydraulic system using field measurements of water potentials and fluxes in soils, roots, stems and leaves of beech (Fagus sylvatica) and spruce (Picea abies) trees. Spruce maintained less negative water potentials than beech during soil drying, reflecting a more stringent stomatal control. While root water uptake depths were similar between species, water potentials in plant tissues of spruce were rather constant and less correlated across roots and the stem, possibly because of large water storage and hydraulic capacitance in these tissues. Root water uptake from deep soil layers increased during dry periods, particularly for beech. Our data suggest that species-specific root hydraulic conductance, capacitance and water uptake strategy are linked and affect transpiration dynamics. Thus, it is important to include such species-specific hydraulics when predicting transpiration rates based on plant water status., (© The Author(s) 2024. Published by Oxford University Press.)

A common assumption of plant hydraulic physiology is that high hydraulic efficiency must come at the cost of hydraulic safety, generating a trade-off that raises doubts about the possibility of selecting both productive and drought-tolerant herbaceous crops. Wetland plants typically display high productivity, which requires high hydraulic efficiency to sustain transpiration rates coupled to CO 2 uptake. Previous studies have suggested high vulnerability to xylem embolism of different wetland plants, in line with expected trade-offs. However, some hygrophytes like Cyperus alternifolius L. can also experience prolonged periods of low water levels leading to substantial drought stress. We conducted an in-depth investigation of this species' hydraulic safety and efficiency by combining gas exchange measurements, hydraulic measurements of leaf hydraulic efficiency and safety, optical measurements of xylem vulnerability to embolism, and determination of cell turgor changes under drought. Our data confirm the high hydraulic efficiency of this wetland species, but at the same time, reveal its surprising drought tolerance in terms of turgor loss point and critical water potential values inducing xylem embolism and hydraulic failure, which were well below values inducing turgor loss and full stomatal closure. C. alternifolius emerges as a highly productive plant that is also well-equipped to tolerate drought via a combination of early stomatal closure and delayed onset of hydraulic damage. The species might represent a model plant to develop crops combining two of the most desirable traits in cultivated plants, i.e., high yield and significant drought tolerance., (© 2024 The Author(s). Physiologia Plantarum published by John Wiley & Sons Ltd on behalf of Scandinavian Plant Physiology Society.)

Polyploid varieties have been suggested as an alternative approach to promote drought tolerance in citrus crops. In this study, we compared the responses of diploid and tetraploid Sunki 'Tropical' rootstocks to water deficit when grafted onto 'Valencia' sweet orange trees and subjected to water withholding in isolation or competition experiments under potted conditions. Our results revealed that, when grown in isolation, tetraploid rootstocks took longer to show drought symptoms, but this advantage disappeared when grown in competition under the same soil moisture conditions. The differences in drought responses were mainly associated with variations in endogenous leaf levels of abscisic acid (ABA), hydrogen peroxide (H₂O₂) and carbohydrates among treatments. Overall, tetraploids were more affected by drought in individual experiments, showing higher H₂O₂ production, and in competition experiments, rapidly increasing ABA production to regulate stomatal closure and reduce water loss through transpiration. Therefore, our results highlight the crucial importance of evaluating diploid and tetraploid rootstocks under the same soil moisture conditions to better simulate field conditions, providing important insights to improve selection strategies for more resilient citrus rootstocks., (© 2024 John Wiley & Sons Ltd.)

Both mistletoes and their hosts are challenged by increasing drought, highlighting the necessity of understanding their comparative hydraulic properties. The high transpiration of mistletoes requires efficient water transport, while high xylem tensions demand strong embolism resistance, representing a hydraulic paradox. This study, conducted across four environments with different aridity indices in Yunnan, China, examined the xylem traits of 119 mistletoe-host species pairs. Mistletoes showed lower water use efficiency, indicating a more aggressive water use. They also showed lower hydraulic efficiency (lower vessel diameter and theoretical hydraulic conductivity) but higher safety (lower vulnerability index and higher conduit wall reinforcement, vessel grouping index, and wood density) compared with their hosts, supporting the trade-off between efficiency and safety. Environmental variation across sites significantly affected xylem trait comparisons between mistletoes and hosts. Additionally, the xylem traits of mistletoes were strongly influenced by host water supply efficiency. The overall xylem trait relationships in mistletoes and hosts were different. These findings stress the impact of host and site on the hydraulic traits of mistletoes, and suggest that mistletoes may achieve high transpiration by maintaining high stomatal conductance under low water potentials. This study illuminates the distinctive adaptation strategies of mistletoes due to their parasitic lifestyle., (© 2024 The Author(s). New Phytologist © 2024 New Phytologist Foundation.)

The driving forces of transpiration are not only atmospheric evaporation but also root zone water supply and stomatal regulation among species. However, the biophysiological drivers of transpiration remain incompletely understood in heterogeneous karst habitats. This study investigated the commonly coexisting tree species Mallotus philippensis and Celtis biondii in two typical karst habitats: rock-dominated (RD) habitat and control soil-dominated (SD) habitat. Over 2 years, soil moisture, transpiration, root distribution, and leaf water potential were measured. The results showed that soil moisture in the RD habitat was significantly lower than in the SD habitat. Transpiration patterns also differed between habitats, with species-specific distinctions driven by biophysiological traits. M. philippensis showed small hydroscape areas and its root system mainly distributed in the soil zone in both habitats. The isohydric behaviour and lower root density in the RD habitat drove M. philippensis to reduce transpiration in response to soil water deficiency. Conversely, C. biondii had large hydroscape areas and roots capable of penetrating bedrock. It transpired higher relying on ample accessible water through anisohydric behaviour and having a more robust root system both in soil and bedrock zones in the RD habitat. Our study highlights the critical role of root water accessibility and leaf iso/anisohydric tendencies in driving transpiration., (© 2024 John Wiley & Sons Ltd.)

Ca is a key nutrient for fruit quality due to its role in bonding with pectin in the cell wall, providing strength through cell-to-cell adhesion, thus increasing fruit firmness and extending post-harvest life. However, Ca accumulation is mostly limited to the initial stages of fruit development due to anatomical and physiological changes that occur as fruits develop. The objective of this study was to evaluate fruit transpiration, cuticle thickness, and pedicel vessel changes during cranberry fruit development and the effect these parameters might have on Ca translocation. 'Stevens' cranberry fruits were collected weekly, starting seven days after full bloom (DAFB) until 70 DAFB. For each collection date, fruit transpiration was evaluated in the field, and samples were taken to analyze total fruit Ca content, stomata density, cuticle thickness, pedicel anatomical changes, and xylem functionality. Ca accumulation in the fruit exhibited a sigmoidal curve, beginning at 0.04 mg per berry at 7 DAFB, increasing to a maximum of 0.1 mg per berry at 28 DAFB, and remaining constant until harvest (70 DAFB). Fruit Ca accumulation was mostly explained by fruit transpiration, which exhibited a similar sigmoidal pattern. The rapid decline in fruit transpiration was largely modulated by increases in cuticle thickness, as well as anatomical changes in the pedicel xylem, thereby reducing the capacity to transport water and nutrients into the fruit. Thus, this research could help cranberry growers maximize fruit Ca content by prioritizing fertilization during the early stages of fruit development., (© 2025 The Author(s). Physiologia Plantarum published by John Wiley & Sons Ltd on behalf of Scandinavian Plant Physiology Society.)

Plants differ widely in how soil drying affects stomatal conductance (g s ) and leaf water potential (ψ leaf ), and in the underlying physiological controls. Efforts to breed crops for drought resilience would benefit from a better understanding of these mechanisms and their diversity. We grew 12 diverse genotypes of common bean (Phaseolus vulgaris L.) and four of tepary bean (P. acutifolius; a highly drought resilient species) in the field under irrigation and post-flowering drought, and quantified responses of g s and ψ leaf , and their controls (soil water potential [ψ soil ], evaporative demand [Δw] and plant hydraulic conductance [K]). We hypothesised that (i) common beans would be more "isohydric" (i.e., exhibit strong stomatal closure in drought, minimising ψ leaf decline) than tepary beans, and that genotypes with larger ψ leaf decline (more "anisohydric") would exhibit (ii) smaller increases in Δw, due to less suppression of evaporative cooling by stomatal closure and hence less canopy warming, but (iii) larger K declines due to ψ leaf decline. Contrary to our hypotheses, we found that half of the common bean genotypes were similarly anisohydric to most tepary beans; canopy temperature was cooler in isohydric genotypes leading to smaller increases in Δw in drought; and that stomatal closure and K decline were similar in isohydric and anisohydric genotypes. g s and ψ leaf were virtually insensitive to drought in one tepary genotype (G40068). Our results highlight the potential importance of non-stomatal mechanisms for leaf cooling, and the variability in drought resilience traits among closely related crop legumes., (© 2024 John Wiley & Sons Ltd.)

Compound soil drought and heat extremes are expected to occur more frequently with global warming, causing wide-ranging socio-ecological repercussions. Vegetation modulates air temperature and soil moisture through biophysical processes, thereby influencing the occurrence of such extremes. Global vegetation cover is broadly expected to increase under climate change, but it remains unclear whether vegetation greening will alleviate or aggravate future increases in compound soil drought-heat events. Here, using a suite of state-of-the-art model simulations, we show that the projected vegetation greening will increase the frequency of global compound soil drought-heat events, equivalent to 12-21% of the total increment at the end of 21st century. This increase is predominantly driven by reduced albedo and enhanced transpiration associated with increased leaf area. Although greening-induced transpiration enhancement has counteracting cooling and drying effects, the excessive water loss in the early growing season can lead to later soil moisture deficits, amplifying compound soil drought-heat extremes during the subsequent warm season. These changes are most pronounced in northern high latitudes and are dominated by the warming effect of CO 2 . Our study highlights the necessity of integrating vegetation biophysical effects into mitigation and adaptation strategies for addressing compound climate risks., Competing Interests: Competing interests: The authors declare no competing interests., (© 2024. The Author(s).)

Rising global temperatures and vapor pressure deficits (VPDs) are increasing plant water demand and becoming major drivers of large-scale plant mortality. Controlling transient leaf water loss after stomatal closure (minimum stomatal conductance [gmin]) is recognized as a key trait determining how long plants survive during soil drought. Yet, substantial uncertainty remains regarding how gmin responds to elevated temperatures and VPD and the underlying mechanisms. We measured gmin in 24 Quercus species from temperate and Mediterranean climates to determine whether gmin was sensitive to a coupled temperature and VPD increase. We also explored mechanistic links to phenology, climate, evolutionary history, and leaf anatomy. We found that gmin in all species exhibited a nonlinear negative temperature and VPD dependence. At 25 °C (VPD = 2.2 kPa), gmin varied from 1.19 to 8.09 mmol m-2 s-1 across species but converged to 0.57 ± 0.06 mmol m-2 s-1 at 45 °C (VPD = 6.6 kPa). In a subset of species, the effect of temperature and VPD on gmin was reversible and linked to the degree of stomatal closure, which was greater at 45 °C than at 25 °C. Our results show that gmin is dependent on temperature and VPD, is highly conserved in Quercus species, and is linked to leaf anatomy and stomatal behavior., Competing Interests: Conflict of interest statement. None declared., (© The Author(s) 2024. Published by Oxford University Press on behalf of American Society of Plant Biologists. All rights reserved. For commercial re-use, please contact reprints@oup.com for reprints and translation rights for reprints. All other permissions can be obtained through our RightsLink service via the Permissions link on the article page on our site—for further information please contact journals.permissions@oup.com.)

Soil waterlogging and drought correspond to contrasting water extremes resulting in plant dehydration. Dehydration in response to waterlogging occurs due to impairments to root water transport, but no previous study has addressed whether limitations to water transport occur beyond this organ or whether dehydration alone can explain shoot impairments. Using common bean (Phaseolus vulgaris) as a model species, we report that waterlogging also impairs water transport in leaves and stems. During the very first hours of waterlogging, leaves transiently dehydrated to water potentials close to the turgor loss point, possibly driving rapid stomatal closure and partially explaining the decline in leaf hydraulic conductance. The initial decline in leaf hydraulic conductance (occurring within 24 h), however, surpassed the levels predicted to occur based solely on dehydration. Constraints to leaf water transport resulted in a hydraulic disconnection between leaves and stems, furthering leaf dehydration during waterlogging and after soil drainage. As leaves dehydrated later during waterlogging, leaf embolism initiated and extensive embolism levels amplified leaf damage. The hydraulic disconnection between leaves and stems prevented stem water potentials from declining below the threshold for critical embolism levels in response to waterlogging. This allowed plants to survive waterlogging and soil drainage. In summary, leaf and stem dehydration are central in defining plant impairments in response to waterlogging, thus creating similarities between waterlogging and drought. Yet, our findings point to the existence of additional players (likely chemicals) partially controlling the early declines in leaf hydraulic conductance and contributing to leaf damage during waterlogging., Competing Interests: Conflict of interest statement. None declared., (© The Author(s) 2024. Published by Oxford University Press on behalf of American Society of Plant Biologists.)

Transpiration is a key process that couples the land-atmosphere exchange of water and carbon, and its maximum water transport ability affects plant productivity. Functional traits significantly influence the maximum transpiration rate; however, which factor plays the dominant role remains unknown. SAPFLUXNET dataset, which includes sap flux density of diverse species worldwide, provides fundamental data to test the importance of photosynthetic and hydraulic traits on maximum tree-level sap flux density (J s_max ). Here, we investigated variations in J s_max of 2194 trees across 129 species using data from the SAPFLUXNET dataset, and analysed the relationship of J s_max with photosynthetic and hydraulic traits. Our results indicated that J s_max was positively correlated with photosynthetic traits at both leaf and tree level. Regarding hydraulic traits, J s_max was positively related to xylem hydraulic conductivity (K s ), leaf-specific hydraulic conductivity (K l ), xylem pressure inducing 50 % loss of hydraulic conductivity (P 50 ), xylem vessel diameter (V dia ), and leaf-to-sapwood area ratio (A l A s ). Random forest model showed that 87 % of the variability in J s_max can be explained by functional traits, and hydraulic traits (e.g., P 50 and sapwood area, A s ) exerted larger effects on J s_max than photosynthetic traits. Moreover, trees with a lower sapwood area or depth could increase their sap flux density to compensate for the reduced whole-tree transpiration. J s_max of the angiosperms was significantly higher than that of the gymnosperms. Mean annual total precipitation (MAP) were positively related to J s_max with a weak correlation coefficient. Furthermore, J s_max showed a significant phylogenetic signal with Blomberg's K below 0.2. Overall, tree species with acquisitive resource economics or more efficient hydraulic systems show higher water transport capacity, and the efficiency of xylem hydraulic system rather than the demand for carbon uptake predominantly determines water transport capacity., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 Elsevier B.V. All rights reserved.)

Incorporating data-driven technologies into agriculture presents a promising approach to optimizing crop production, especially in regions dependent on irrigation, where escalating heat waves and droughts driven by climate change pose increasing challenges. Recent advancements in sensor technology have introduced diverse methods for assessing irrigation needs, including meteorological sensors for calculating reference evapotranspiration, belowground sensors for measuring plant available water, and plant sensors for direct water status measurements. Among these, infrared thermometry stands out as a non-destructive remote sensing method for monitoring transpiration, with significant potential for integration into drone- or satellite-based models. This study applies infrared thermometry to develop a crop water stress index (CWSI) model for European hazelnuts ( Corylus avellana ), a key crop in Oregon, the leading hazelnut-producing state in the United States. Utilizing low-cost, open-source infrared thermometers and data loggers, we aim to provide hazelnut farmers with a practical tool for improving irrigation efficiency and enhancing yields. The CWSI model was validated against plant water status metrics such as stem water potential and gas exchange measurements. Our results show that when stem water potential is below -6 bar, the CWSI remains under 0.2, indicating low plant stress, with corresponding leaf conductance rates ranging between 0.1 and 0.4 mol m 2 s -1 . Additionally, un-irrigated hazelnuts were stressed (CWSI > 0.2) from mid-July through the end of the season, while irrigated plants remained unstressed. The findings suggest that farmers can adopt a leaf conductance threshold of 0.2 mol m 2 s -1 or a water potential threshold of -6 bar for irrigation management. This research introduces a new CWSI model for hazelnuts and highlights the potential of low-cost technology to improve agricultural monitoring and decision-making.

Drought and subsequent rewatering are common in agriculture, where recovery from mild droughts is easier than from severe ones. The specific drought threshold and factors limiting recovery are under-researched. This study subjected maize plants to varying drought degrees before rewatering, and measuring plant water status, gas exchange, hydraulic conductance, hormone levels, and cellular damage throughout. We discovered that stomatal reopening in plants was inhibited with leaf water potentials below about -1.7 MPa, hindering postdrought photosynthetic recovery. Neither hydraulic loss nor abscisic acid (ABA) content was the factor inhibited stomatal reopening on the second day following moderate drought stress and rewatering. But stomatal reopening was significantly correlated to the interaction between hydraulic signals and ABA content under severe drought. Extended drought led to leaf death at about -2.8 MPa or 57% relative water content, influenced by reduced rehydration capacity, not hydraulic failure. The lethal threshold remained relatively constant across leaf stages, but the recoverable safety margin (RSM), that is, the water potential difference between stomatal closure and recovery capacity loss, significantly decreased with leaf aging due to delayed stomatal closure during drought. Our findings indicate hydraulic failure alone does not cause maize leaf death, highlighting the importance of RSM in future research., (© 2024 John Wiley & Sons Ltd.)

While dynamic regulation of photosynthesis in fluctuating light is increasingly recognized as an important driver of carbon uptake, acclimation to realistic irradiance fluctuations is still largely unexplored. We subjected Arabidopsis thaliana (L.) wild-type and jac1 mutants to irradiance fluctuations with distinct amplitudes and average irradiance. We examined how irradiance fluctuations affected leaf structure, pigments and physiology. A wider amplitude of fluctuations produced a stronger acclimation response. Large reductions of leaf mass per area under fluctuating irradiance framed our interpretation of changes in photosynthetic capacity and mesophyll conductance as measured by three separate methods, in that photosynthetic investment increased markedly on a mass basis, but only a little on an area basis. Moreover, thermal imagery showed that leaf transpiration was four times higher under fluctuating irradiance. Leaves growing under fluctuating irradiance, although thinner, maintained their photosynthetic capacity, as measured through light- and CO 2 -response curves; suggesting their photosynthesis may be more cost-efficient than those under steady light, but overall may incur increased maintenance costs. This is especially relevant for plant performance globally because naturally fluctuating irradiance creates conflicting acclimation cues for photosynthesis and transpiration that may hinder progress towards ensuring food security under climate-related extremes of water stress., (© The Author(s). Plant, Cell & Environment published by John Wiley & Sons Ltd.)

By regulating carbon uptake and water loss by plants, stomata are not only responsible for productivity but also survival during drought. The timing of the onset of stomatal closure is crucial for preventing excessive water loss during drought, but is poorly explained by plant hydraulics alone and what triggers stomatal closure remains disputed. We investigated whether the hormone abscisic acid (ABA) was this trigger in a highly embolism-resistant tree species Umbellularia californica. We tracked leaf ABA levels, determined the leaf water potential and gravimetric soil water content (gSWC) thresholds for stomatal closure and transpiration decline during a progressive drought. We found that U. californica plants have a peaking-type ABA dynamic, where ABA levels rise early in drought and then decline under prolonged drought conditions. The early increase in ABA levels correlated with the closing of stomata and reduced transpiration. Furthermore, we found that transpiration declined before any large decreases in predawn plant water status and could best be explained by transient drops in midday water potentials triggering increased ABA levels. Our results indicate that ABA-mediated stomatal regulation may be an integral mechanism for reducing transpiration during drought before major drops in bulk soil and plant water status., (© 2024 The Author(s). Plant, Cell & Environment published by John Wiley & Sons Ltd.)

Hydraulic failure due to xylem embolism has been identified as one of the main mechanisms involved in drought-induced forest decline. Trees vulnerability to hydraulic failure depends on their hydraulic safety margin (HSM). While it has been shown that HSM globally converges between tree species and biomes, there is still limited knowledge regarding how HSM can adjust locally to varying drought conditions within species. In this study, we relied on three long-term partial rainfall exclusion experiments to investigate the plasticity of hydraulic traits and HSM for three Mediterranean tree species (Quercus ilex L., Quercus pubescens Willd., and Pinus halepensis Mill.). For all species, a homeostasis of HSM in response to rainfall reduction was found, achieved through different mechanisms. For Q. ilex, the convergence in HSM is attributed to the adjustment of both the turgor loss point (Ψtlp) and the water potential at which 50% of xylem conductivity is lost due to embolism (P50). In contrast, the maintenance of HSM for P. halepensis and Q. pubescens is related to its isohydric behavior for the first and leaf area adjustment for the latter. It remains to be seen whether this HSM homeostasis can be generalized and if it will be sufficient to withstand extreme droughts expected in the Mediterranean region., (© 2024 John Wiley & Sons Ltd.)

Analyses of leaf gas exchange rely on an Ohmic analogy that arrays single stomatal, internal air space, and mesophyll conductances in series. Such models underlie inferences of mesophyll conductance and the relative humidity of leaf airspaces, reported to fall as low as 80%. An unresolved question is whether such series models are biased with respect to real leaves, whose internal air spaces are chambered at various scales by vasculature. To test whether unsaturation could emerge from modeling artifacts, we compared series model estimates with true parameter values for a chambered leaf with varying distributions and magnitudes of leaf surface conductance ('patchiness'). Distributions of surface conductance can create large biases in gas exchange calculations. Both apparent unsaturation and internal CO 2 gradient inversion can be produced by the evolution of broader distributions of stomatal apertures consistent with a decrease in surface conductance, as might occur under increasing vapor pressure deficit. In gas exchange experiments, the behaviors of derived quantities defined by simple series models are highly sensitive to the true partitioning of flux and stomatal apertures across leaf surfaces. New methods are needed to disentangle model artifacts from real biological responses., (© 2024 The Author(s). New Phytologist © 2024 New Phytologist Foundation.)

Future climate presents conflicting implications for forest biomass. We evaluate how plant hydraulic traits, elevated CO 2 levels, warming, and changes in precipitation affect forest primary productivity, evapotranspiration, and the risk of hydraulic failure. We used a dynamic vegetation model with plant hydrodynamics (FATES-HYDRO) to simulate the stand-level responses to future climate changes in a wet tropical forest in Barro Colorado Island, Panama. We calibrated the model by selecting plant trait assemblages that performed well against observations. These assemblages were run with temperature and precipitation changes for two greenhouse gas emission scenarios (2086-2100: SSP2-45, SSP5-85) and two CO 2 levels (contemporary, anticipated). The risk of hydraulic failure is projected to increase from a contemporary rate of 5.7% to 10.1-11.3% under future climate scenarios, and, crucially, elevated CO 2 provided only slight amelioration. By contrast, elevated CO 2 mitigated GPP reductions. We attribute a greater variation in hydraulic failure risk to trait assemblages than to either CO 2 or climate. Our results project forests with both faster growth (through productivity increases) and higher mortality rates (through increasing rates of hydraulic failure) in the neo-tropics accompanied by certain trait plant assemblages becoming nonviable., (© 2024 The Author(s). New Phytologist © 2024 New Phytologist Foundation.)

Mesophyll conductance ( g m ) describes the efficiency with which CO 2 moves from substomatal cavities to chloroplasts. Despite the stipulated importance of leaf architecture in affecting g m , there remains a considerable ambiguity about how and whether leaf anatomy influences g m . Here, we employed nonlinear machine-learning models to assess the relationship between 10 leaf architecture traits and g m . These models used leaf architecture traits as predictors and achieved excellent predictability of g m . Dissection of the importance of leaf architecture traits in the models indicated that cell wall thickness and chloroplast area exposed to internal airspace have a large impact on interspecific variation in g m . Additionally, other leaf architecture traits, such as leaf thickness, leaf density and chloroplast thickness, emerged as important predictors of g m . We also found significant differences in the predictability between models trained on different plant functional types. Therefore, by moving beyond simple linear and exponential models, our analyses demonstrated that a larger suite of leaf architecture traits drive differences in g m than has been previously acknowledged. These findings pave the way for modulating g m by strategies that modify its leaf architecture determinants., (© 2024 The Author(s). Plant, Cell & Environment published by John Wiley & Sons Ltd.)

During periods of stomatal closure, such as drought, plant leaves continue to lose water at a rate determined by the minimum leaf conductance, g min . Although g min varies with temperature, less is known about what drives this variation, including how the pathways of water loss (cuticle or stomata) vary with temperature. We used gas exchange and bench drying methods to measure g min and cuticular conductance, g cw , across a wide temperature range (20-50°C) in 11 broadleaf species. Vapour pressure deficit (VPD) covaried with temperature from 0.83 to 10.7 kPa. The dominant pathway of water loss for g min shifted from stomatal transpiration towards cuticular transpiration as temperature increased. Leaf traits had variable, temperature-dependent relationships with g min and g cw , with trait-conductance relationships being generally stronger at higher temperatures. Cuticular thickness varied inversely with high-temperature g cw . Simulation results showed that g cw may impact photosynthetic capacity estimates, particularly in species with low stomatal conductance. The pathways of water loss in leaves during times of stomatal closure depend strongly on temperature. This effect may have large implications for landscape-scale water balance modelling and improving gas exchange measurements. We propose variation in VPD as a potential contributing factor in g min and g cw variation among studies., (© 2024 The Author(s). New Phytologist © 2024 New Phytologist Foundation.)

Transpirational cooling is crucial for plant thermal regulation to avoid overheating; however, during prolonged and/or acute heat stress it often necessitates stomatal closure to reduce the risk of hydraulic failure due to dehydration. The intricate interplay between thermal regulation, water transport and use may govern plant performance in water-limited and simultaneously heat-stressed environments, yet this remains inadequately understood. Here, in a common garden, we evaluated the functional associations among physiological characteristics related to leaf thermoregulation, heat tolerance, xylem water transport, and stomatal regulation in eight shrub species commonly used for fixing active sand dunes in northern China. Our study showed that traits associated with heat adaptation and xylem hydraulics were closely related to stomatal regulation. More isohydric shrub species with higher water transport efficiency possessed stronger transpirational cooling capacity; whereas the more anisohydric species demonstrated greater tolerance to overheating. Moreover, leaf heat tolerance was strongly coordinated with drought tolerance reflected by leaf turgor loss point. These results underscore the importance of stomatal regulation in shaping plant thermal adaptive strategies and provide valuable insights into the coupling of water and heat-related physiological processes in plants adapted to sandy land environments prone to combined drought and heat stresses., (© 2024 John Wiley & Sons Ltd.)

Background: Accurately measured or estimated reference evapotranspiration (ET o ) data are needed to properly manage water resources and prioritise their future uses. ET o can be most accurately measured using lysimeter systems. However, high installation and operating costs, as well as difficult and time-consuming measurement processes limit the use of these systems. Therefore, the approach of estimating ET o by empirical models is more preferred and widely used. However, since those models are well in accordance with the climatic and environmental traits of the region in which they were developed, their reliability must be examined if they are utilised in distinctive regions. This study aims to test the usability of mass transfer-based Dalton, Rohwer, Penman, Romanenko, WMO and Mahringer models in Van Lake microclimate conditions and to calibrate them in compatible with local conditions., Methods: Firstly, the original equations of these models were tested using 9 years of daily climate data measured between 2012 and 2020. Then, the models were calibrated using the same data and their modified equations were created. The original and modified equations of the models were also tested with the 2021 and 2022 current climate data. Modified equations have been created using the Microsoft Excel program solver add-on, which is based on linear regression. The daily average ET o values estimated using the six mass transfer-based models were compared with the daily average ET o values calculated using the standard FAO-56 PM equation. The statistical approaches of the mean absolute error (MAE), mean absolute percentage error (MAPE), root mean square error (RMSE), Nash-Sutcliffe Efficiency (NSE), and determination coefficient (R 2 ) were used as comparison criterion., Results: The best and worst performing models in the original equations were Mahringer (MAE = 0.70 mm day -1 , MAPE = 15.86%, RMSE = 0.87 mm day -1 , NSE = 0.81, R 2 = 0.94) and Penman (MAE = 1.84 mm day -1 , MAPE = 33.68%, RMSE = 2.39 mm day -1 , NSE = -0.49, R 2 = 0.91), respectively, whereas in the modified equations Dalton (MAE = 0.29 mm day -1 , MAPE = 7.51%, RMSE = 0.33 mm day -1 , NSE = 0.97, R 2 = 0.97) and WMO (MAE = 0.36 mm day -1 , MAPE = 8.89%, RMSE = 0.43 mm day -1 , NSE = 0.95, R 2 = 0.97). The RMSE errors of the daily average ET o values estimated using the modified equations were generally below the acceptable error limit (RMSE < 0.50 mm day -1 ). It has been concluded that the modified equations of the six mass transfer-based models can be used as alternatives to the FAO-56 PM equation under the Van Lake microclimate conditions (NSE > 0.75), while the original equations-except for those of Mahringer (NSE = 0.81), WMO (NSE = 0.79), and Romanenko (NSE = 0.76)-cannot be used., Competing Interests: The author declares that he has no competing interests., (© 2024 Usta.)

Stomata regulate CO2 and water vapor exchange between leaves and the atmosphere. Stomata are a target for engineering to improve crop intrinsic water use efficiency (iWUE). One example is by expressing genes that lower stomatal density (SD) and reduce stomatal conductance (gsw). However, the quantitative relationship between reduced SD, gsw, and the mechanisms underlying it is poorly understood. We addressed this knowledge gap using low-SD sugarcane (Saccharum spp. hybrid) as a case study alongside a meta-analysis of data from 10 species. Transgenic expression of EPIDERMAL PATTERNING FACTOR 2 from Sorghum bicolor (SbEPF2) in sugarcane reduced SD by 26-38% but did not affect gsw compared with the wild type. Further, no changes occurred in stomatal complex size or proxies for photosynthetic capacity. Measurements of gas exchange at low CO2 concentrations that promote complete stomatal opening to normalize aperture size between genotypes were combined with modeling of maximum gsw from anatomical data. These data suggest that increased stomatal aperture is the only possible explanation for maintaining gsw when SD is reduced. Meta-analysis across C3 dicots, C3 monocots, and C4 monocots revealed that engineered reductions in SD are strongly correlated with lower gsw (r2=0.60-0.98), but this response is damped relative to the change in anatomy., (© The Author(s) 2024. Published by Oxford University Press on behalf of the Society for Experimental Biology.)

Background and Aims: Leaf elongation is vital for productivity of Poaceae species, influenced by atmospheric CO2 concentration ([CO2]) and climate-induced water availability changes. Although [CO2] mitigates the effects of drought on reducing transpiration per unit leaf area, it also increases total leaf area and water use. These complex interactions associated with leaf growth pose challenges in anticipating climate change effects. This study aims to assess [CO2] effects on leaf growth response to drought in perennial ryegrass (Lolium perenne), tall fescue (Festuca arundinacea) and wheat (Triticum aestivum)., Methods: Plants were cultivated in growth chambers with [CO2] at 200 or 800 ppm. At leaf six to seven unfolding, half of the plants were subjected to severe drought treatment. Leaf elongation rate (LER) was measured daily, whereas plant transpiration was continuously recorded gravimetrically. Additionally, water-soluble carbohydrate (WSC) content along with water and osmotic potentials in the leaf growing zone were measured at drought onset, mid-drought and leaf growth cessation., Key Results: Elevated [CO2] mitigated drought impacts on LER and delayed growth cessation across species. A positive correlation between LER and soil relative water content (SRWC) was observed. At the same SRWC, perennial grasses exhibited a higher LER with elevated [CO2], probably due to enhanced stomatal regulation. Despite stomatal closure and WSC accumulation, CO2 did not influence nighttime water potential or osmotic potential. The marked increase in leaf area across species resulted in similar (wheat and tall fescue) or higher (ryegrass) total water use by the end of the experiment, under both watered and unwatered conditions., Conclusions: Elevated [CO2] mitigates the adverse effects of drought on leaf elongation in three Poaceae species, due to its impact on plant transpiration. Overall, these findings provide valuable insights into CO2 and drought interactions that may help anticipate plant responses to climate change., (© The Author(s) 2024. Published by Oxford University Press on behalf of the Annals of Botany Company. All rights reserved. For commercial re-use, please contact reprints@oup.com for reprints and translation rights for reprints. All other permissions can be obtained through our RightsLink service via the Permissions link on the article page on our site—for further information please contact journals.permissions@oup.com.)

Background: Accurate leaf water potential (Ψ w ) determination is crucial in studying plant responses to water deficit. After excision, water potential decreases, even under low evaporative demand conditions, which has been recently attributed to the equilibration of pre-excision Ψ w gradients across the leaf. We assessed the influence of potential re-equilibration on water potential determination by monitoring leaf Ψ w and relative water content decline after excision using different storage methods., Results: Even though leaf Ψ w declined during storage under low evaporative demand conditions, this was strongly reduced when covering the leaf with a hydrophobic layer (vaseline) and explained by changes in relative water content. However, residual water loss was variable between species, possibly related to morpho-physiological leaf traits. Provided water loss was minimized during storage, pre-excision leaf transpiration rate did not affect to the magnitude of leaf Ψ w decline after excision, confirming that transpiration-driven Ψ w gradients have no effect on leaf Ψ w determination., Conclusions: Disequilibrium in water potentials across a transpiring leaf upon excision is dissipated very quickly, well within the elapsed time between excision and pressurization, therefore, not resulting in overestimation of leaf Ψ w measured immediately after excision. When leaf storage is required, the effectiveness of a storage under low evaporative demand varied among species. Covering with a hydrophobic layer is an acceptable alternative., (© 2024. The Author(s).)

Desert shrubs play a crucial role in controlling desertification and promoting revegetation, but drought often hinders their growth. Investigating the hydraulic strategies of desert shrubs is important in order to understand their drought adaptation and predict future dynamics under climate change. In this study, we measured the hydraulic-related characteristics of roots, stems and leaves in 19 desert shrub species from northern China. We aimed to explore the hydraulic coordination and segmentation between different plant organs. The results were as follows: (i) specific root length was positively correlated with the water potential inducing a 50% loss in stem hydraulic conductivity (P50stem) and negatively correlated with stem hydraulic safety margin. This suggested that water uptake efficiency of the fine roots was traded off with stem embolism resistance and hydraulic safety. (ii) The water potential inducing a 50% loss in leaf hydraulic conductance was significantly less negative than P50stem, and fine root turgor loss point was significantly less negative than P50stem, indicating a hydraulic segmentation between the main stem and terminal organs. (iii) The most negative leaf turgor loss point indicated that leaf wilting occurred after substantial leaf and stem embolism. The high desiccation resistance of the leaves may serve as an important physiological mechanism to increase carbon gain in a relatively brief growth period. In summary, this study elucidated the hydraulic strategies employed by desert shrubs from a whole-plant perspective., (© The Author(s) 2024. Published by Oxford University Press. All rights reserved. For permissions, please e-mail: journals.permission@oup.com.)

Unravelling the complexities of transpiration can be assisted by understanding the oxygen isotope composition of transpired water vapour (δE). It is often assumed that δE is at steady state, thereby mirroring the oxygen isotope composition of source water (δsource), but this assumption has never been tested at the whole-tree scale. This study utilized the unique infrastructure of 12 whole-tree chambers enclosing Eucalyptus parramattensis E.C.Hall trees to measure δE along with concurrent temperature and gas exchange data. Six chambers tracked ambient air temperature and six were exposed to an ambient +3 °C warming treatment. Day time means for δE were within 1.2‰ of δsource (-3.3‰) but varied considerably throughout the day. Our observations show that E. parramattensis trees are seldom transpiring at isotopic steady state over a diel period, but transpiration approaches source water isotopic composition over longer time periods., (© The Author(s) 2024. Published by Oxford University Press.)

Drought is one of the vital meteorological disasters that influence crop growth. Timely and accurately estimating the drought dynamics of crops is valuable for decision-maker to formulate scientific management measures of agricultural drought risk. In this study, the evapotranspiration and drought dynamics of winter wheat from 1981 to 2020 in the Huang-Huai-Hai (HHH) region of China were evaluated based on long-term multi-source observation data. Four key developmental stages of winter wheat were given attentions: growth before winter stage, overwintering stage, stage of greening-heading, and stage of filling-maturity. The crop water deficit index (CWDI) on a daily scale was established for quantitatively appraising the impacts of drought on winter wheat. Our results indicated that interannual variation in reference crop evapotranspiration (ET 0 ) during the growth season of winter wheat from 1981 to 2020 in the HHH region showed a slight increase trend, with an average of 602.4 mm and obvious spatial differences of decreasing from the Northeast to the Southwest. Over the past forty years, the winter wheat in the HHH region was most severely affected by severe drought, followed by moderate drought, and finally mild drought. In addition, the impacts of drought on winter wheat at different critical growth stages varied greatly. For the growth before winter stage, the winter wheat was mainly threatened by mild, moderate, and severe droughts. For the overwintering stage, the winter wheat was mainly threatened by moderate, severe, and extreme droughts. For the greening-heading stage, the winter wheat was mainly threatened by mild, moderate, severe, and extreme droughts. For the filling-maturity stage, the winter wheat was mainly threatened by mild and moderate droughts. Finally, the impacts of drought on winter wheat during 1981-2020 in the HHH region were revealed to differ extraordinarily in space. In particular, the areas of winter wheat affected by severe drought significantly decreased. However, the areas of winter wheat affected by moderate drought clearly expanded. Our findings provide new insights for further improving climate change impact studies and agricultural drought defense capabilities adapting to continuous environmental change., Competing Interests: Declaration of competing interest The authors declare they have no competing interests., (Copyright © 2024 Elsevier B.V. All rights reserved.)

Grapevine (Vitis vinifera L.) is the world's third most valuable horticultural crop, and the current environmental scenario is massively shifting the grape cultivation landscape. The increase in heatwaves and drought episodes alter fruit ripening, compromise grape yield and vine survival, intensifying the pressure on using limited water resources. ABA is a key phytohormone that reduces canopy transpiration and helps plants to cope with water deficit. However, the exogenous application of ABA is impractical because it suffers fast catabolism, and UV-induced isomerization abolishes its bioactivity. Consequently, there is an emerging field for developing molecules that act as ABA receptor agonists and modulate ABA signaling but have a longer half-life. We have explored the foliar application of the iSB09 and AMF4 agonists in the two grapevine cultivars cv. 'Bobal' and 'Tempranillo' to induce an ABA-like response to facilitate plant adaptation to drought. The results indicate that iSB09 and AMF4 act through the VviPYL1-like, VviPYL4-like, and VviPYL8-like ABA receptors to trigger stomatal closure, reduce plant transpiration, and increase water use efficiency. Structural and bioinformatic analysis of VviPYL1 in complex with ABA or these agonists revealed key structural determinants for efficient ligand binding, providing a mechanistic framework to understand receptor activation by the ligands. Physiological analyses further demonstrated that iSB09 has a more sustained effect on reducing transpiration than ABA, and agonist spraying of grapevine leaves protected PSII during drought stress. These findings offer innovative approaches to strengthen the vine's response to water stress and reduce plant consumptive water use under limited soil water conditions., (© 2024 The Author(s). Physiologia Plantarum published by John Wiley & Sons Ltd on behalf of Scandinavian Plant Physiology Society.)

Irrigation of grasses dominates domestic water use across the globe, and better understanding of water use and drought resistance in grasses is of undeniable importance for water conservation. Breeding programs have released cultivars with improved drought resistance, but the underlying mechanisms remain unknown. We sought to characterize the mechanisms driving drought resistance in four zoysiagrass cultivars (Lobo, Zeon, Empire, and Meyer) reported to exhibit contrasting levels of drought resistance. A dry-down was performed through deficit irrigation until 70% decline in evapotranspiration. All cultivars exhibited similar drought avoidance as they dehydrated similarly throughout the drought. Lobo and Zeon, however, exhibited a 70% decline in evapotranspiration two to three days after Empire and Meyer, thus experiencing lower water potentials. Regarding drought tolerance, Lobo and Zeon maintained higher normalized difference vegetation index (NDVI) and lower perceived canopy mortality at higher dehydration levels than Empire and Meyer. We use "perceived" because visual assessments of canopy mortality are influenced by drought-induced leaf rolling. During the recovery, leaves rehydrated and unrolled, so the "actual" canopy mortality could be evaluated. All cultivars exhibited similar mortality on the first recovery day despite Lobo and Zeon experiencing more severe dehydration. Throughout the recovery, Lobo and Empire exhibited faster re-growth and showed the lowest canopy mortality, and Lobo exhibited the highest NDVI. The improved drought resistance of Lobo and Zeon results from greater dehydration tolerance rather than avoidance. This study has implications for lawn owners selecting the best cultivars and for breeding programs aiming at improving drought resistance of zoysiagrasses., (© 2024 The Author(s). Physiologia Plantarum published by John Wiley & Sons Ltd on behalf of Scandinavian Plant Physiology Society.)

Succulence is a trait that describes water storage in plant organs and tissues regardless of life form. Plants use the stored water to maintain physiological function and delay desiccation. However, it is unclear whether succulence in leaves, stems and roots of woody plants delays desiccation, whether it provides 'utilizable water' to maintain physiological function, or buffers changes in water status in drying soils through capacitance. We conducted a pot dry-down experiment with nine shrub species to determine whether woody plants with greater leaf, stem, or root succulence have greater shoot utilizable water or capacitance. We also investigated whether greater succulence delays desiccation, represented by cumulative VPD, until evapotranspiration ceased or until utilizable water was exhausted. Greater leaf and stem succulence were strongly related to greater shoot utilizable water and capacitance. However, desiccation time was not delayed in plants with greater total shoot succulence, utilizable water, or capacitance. Instead, woody plants with greater root succulence had longer desiccation times. This suggests that woody plants use aboveground succulence to maintain physiological function and water status during drought, whereas root succulence extends desiccation time. Our study improves the mechanistic understanding of how woody plants use stored water to survive in dryland ecosystems., (© 2024 Scandinavian Plant Physiology Society.)

Climate change not only leads to higher air temperatures but also increases the vapour pressure deficit (VPD) of the air. Understanding the direct effect of VPD on leaf gas exchange is crucial for precise modelling of stomatal functioning. We conducted combined leaf gas exchange and online isotope discrimination measurements on four common European tree species across a VPD range of 0.8-3.6 kPa, while maintaining constant temperatures without soil water limitation. In addition to applying the standard assumption of saturated vapour pressure inside leaves (e i ), we inferred e i from oxygen isotope discrimination of CO 2 and water vapour. e i desaturated progressively with increasing VPD, consistently across species, resulting in an intercellular relative humidity as low as 0.73 ± 0.11 at the highest tested VPD. Assuming saturation of e i overestimated the extent of reductions in stomatal conductance and CO 2 mole fraction inside leaves in response to increasing VPD compared with calculations that accounted for unsaturation. In addition, a significant decrease in mesophyll conductance with increasing VPD only occurred when the unsaturation of e i was considered. We suggest that the possibility of unsaturated e i should not be overlooked in measurements related to leaf gas exchange and in stomatal models, especially at high VPD., (© 2024 The Author(s). New Phytologist © 2024 New Phytologist Foundation.)

Contradictory evidence exists regarding the relevance of Péclet-like gradients in leaf water isotopes, making it difficult to accurately predict variation in isotope composition. Here, we use H 2 18 O vapour labelling to directly test whether leaf water isotopes diffuse back into the xylem to be carried forward to more distal leaf portions. Backward diffusion has been assumed, due to observations of increasing enrichment towards the tip and outer edges of some leaves. Further complicating the selection of leaf water isotope models is the observation that some, but not all, leaves demonstrate a radial Péclet effect in bulk leaf water and that the hydraulic design of leaves may influence the development of isotope gradients in leaves. Carry-forward of H 2 18 O vapour label was detected in the two monocot species assessed (oat and corn), but not in the two dicot species (foxglove and sunflower). Further, bulk leaf water measurements at differing transpiration rates indicated that a bulk leaf water Péclet effect was relevant for foxglove only. We conclude that both leaf hydraulic design and relative velocities of water within transport pathways influence leaf water isotope composition, reconciling seemingly contradictory previous results regarding the relevance of Péclet effects to leaf water isotopes., (© 2024 The Author(s). New Phytologist © 2024 New Phytologist Foundation.)

Understanding the relationship between wind speed and gas exchange in plants is a longstanding challenge. Our aim was to investigate the impact of wind speed on maximum rates of gas exchange and the kinetics of stomatal responses. We conducted experiments in different angiosperm and fern species using an infrared gas analyzer equipped with a controlled leaf fan, enabling precise control of the boundary layer conductance. We first showed that the chamber was adequately mixed even at extremely low wind speed (<0.005 m s -1 ) and evaluated the link between fan speed, wind speed, and boundary layer conductance. We observed that higher wind speeds led to increased gas exchange of both water vapor and CO₂, primarily due to the increase in boundary layer conductance. This increase in transpiration subsequently reduced epidermal pressure, leading to stomatal opening. We documented that stomatal opening in response to light was 2.5 times faster at a wind speed of 2 m s -1 compared to minimal wind speed in Vicia faba, while epidermal peels in a buffer with no transpiration exhibited a similar opening rate. The increase in stomatal conductance under high wind was also observed in four angiosperm species under field conditions, but it was not observed in Boston fern (Nephrolepis exaltata), which lacks epidermal mechanical advantage. Our findings highlight the significant impact of boundary layer conductance on determining gas exchange rates and the kinetics of gas exchange responses to environmental changes., (© 2024 The Author(s). The Plant Journal published by Society for Experimental Biology and John Wiley & Sons Ltd.)

Background: Precision phenotyping of short-term transpiration response to environmental conditions and transpiration patterns throughout wheat development enables a better understanding of specific trait compositions that lead to improved transpiration efficiency. Transpiration and related traits were evaluated in a set of 79 winter wheat lines using the custom-built "DroughtSpotter XXL" facility. The 120 l plant growth containers implemented in this phenotyping platform enable gravimetric quantification of water use in real-time under semi-controlled, yet field-like conditions across the entire crop life cycle., Results: The resulting high-resolution data enabled identification of significant developmental stage-specific variation for genotype rankings in transpiration efficiency. In addition, for all examined genotypes we identified the genotype-specific breakpoint in transpiration in response to increasing vapour pressure deficit, with breakpoints ranging between 2.75 and 4.1 kPa., Conclusion: Continuous monitoring of transpiration efficiency and diurnal transpiration patterns enables identification of hidden, heritable genotypic variation for transpiration traits relevant for wheat under drought stress. Since the unique experimental setup mimics field-like growth conditions, the results of this study have good transferability to field conditions., (© 2024. The Author(s).)

Predicting how plants respond to drought requires an understanding of how physiological mechanisms and drought response strategies occur, as these strategies underlie rates of gas exchange and productivity. We assessed the response of 11 plant traits to repeated experimental droughts in four co-occurring species of central Australia. The main goals of this study were to: (i) compare the response to drought between species; (ii) evaluate whether plants acclimated to repeated drought; and (iii) examine the degree of recovery in leaf gas exchange after cessation of drought. Our four species of study were two tree species and two shrub species, which field studies have shown to occupy different ecohydrological niches. The two tree species (Eucalyptus camaldulensis Dehnh. and Corymbia opaca (D.J.Carr & S.G.M.Carr) K.D.Hill & L.A.S.Johnson) had large reductions in stomatal conductance (gs) values, declining by 90% in the second drought. By contrast, the shrub species (Acacia aptaneura Maslin & J.E.Reid and Hakea macrocarpa A.Cunn. ex R.Br.) had smaller reductions gs in the second drought of 52 and 65%, respectively. Only A. aptaneura showed a physiological acclimatation to drought due to small declines in gs versus ᴪpd (0.08 slope) during repeated droughts, meaning they maintained higher rates of gs compared with plants that only experienced one final drought (0.19 slope). All species in all treatments rapidly recovered leaf gas exchange and leaf mass per area following drought, displaying physiological plasticity to drought exposure. This research refines our understanding of plant physiological responses to recurrent water stress, which has implications for modelling of vegetation, carbon assimilation and water use in semi-arid environments under drought., (© The Author(s) 2024. Published by Oxford University Press.)

Changes in vapour pressure deficit can lead to the depletion and replenishment of stem water pools to buffer water potential variations in the xylem. Yet, the precise velocity at which stem water pools track environmental cues remains poorly explored. Nine eucalyptus seedlings grown in a glasshouse experienced high-frequency environmental oscillations and their stem radial variations (ΔR) were monitored at a 30-s temporal resolution in upper and lower stem locations and on the bark and xylem. The stem ΔR response to vapour pressure deficit changes was nearly instantaneous (<1 min), while temperature lagged behind stem ΔR. No temporal differences in the stem ΔR response were observed between locations. Punctual gravimetric measurements confirmed the synchrony between transpiration and stem ΔR dynamics. These results indicate (i) that stem-stored water can respond to the atmospheric evaporative demand much faster than commonly assumed and (ii) that the origin of the water released to the transpiration stream seems critical in determining time lags in stem water pool dynamics. Near-zero time lags may be explained by the high elasticity of eucalyptus woody tissues and the predominant water use from the xylem, circumventing the hydraulic radial barriers to water flow from/to the outer tissues., (© The Author(s) 2024. Published by Oxford University Press. All rights reserved. For permissions, please e-mail: journals.permissions@oup.com.)

Modern plant physiological theory stipulates that the resistance to water movement from plants to the atmosphere is overwhelmingly dominated by stomata. This conception necessitates a corollary assumption-that the air spaces in leaves must be nearly saturated with water vapour; that is, with a relative humidity that does not decline materially below unity. As this idea became progressively engrained in scientific discourse and textbooks over the last century, observations inconsistent with this corollary assumption were occasionally reported. Yet, evidence of unsaturation gained little traction, with acceptance of the prevailing framework motivated by three considerations: (1) leaf water potentials measured by either thermocouple psychrometry or the Scholander pressure chamber are largely consistent with the framework; (2) being able to assume near saturation of intercellular air spaces was transformational to leaf gas exchange analysis; and (3) there has been no obvious mechanism to explain a variable, liquid-phase resistance in the leaf mesophyll. Here, we review the evidence that refutes the assumption of universal, near saturation of air spaces in leaves. Refining the prevailing paradigm with respect to this assumption provides opportunities for identifying and developing mechanisms for increased plant productivity in the face of increasing evaporative demand imposed by global climate change., (© 2024 The Author(s). Plant, Cell & Environment published by John Wiley & Sons Ltd.)

The onset of stomatal closure reduces transpiration during drought. In seed plants, drought causes declines in plant water status which increases leaf endogenous abscisic acid (ABA) levels required for stomatal closure. There are multiple possible points of increased belowground resistance in the soil-plant atmospheric continuum that could decrease leaf water potential enough to trigger ABA production and the subsequent decreases in transpiration. We investigate the dynamic patterns of leaf ABA levels, plant hydraulic conductance and the point of failure in the soil-plant conductance in the highly embolism-resistant species Callitris tuberculata using continuous dendrometer measurements of leaf water potential during drought. We show that decreases in transpiration and ABA biosynthesis begin before any permanent decreases in predawn water potential, collapse in soil-plant hydraulic pathway and xylem embolism spread. We find that a dynamic but recoverable increases in hydraulic resistance in the soil in close proximity to the roots is the most likely driver of declines in midday leaf water potential needed for ABA biosynthesis and the onset of decreases in transpiration., (© 2024 The Author(s). New Phytologist © 2024 New Phytologist Foundation.)

The shortage of decades-long continuous measurements of ecosystem processes limits our understanding of how changing climate impacts forest ecosystems. We used continuous eddy-covariance and hydrometeorological data over 2002-2022 from a young Douglas-fir stand on Vancouver Island, Canada to assess the long-term trend and interannual variability in evapotranspiration (ET) and transpiration (T). Collectively, annual T displayed a decreasing trend over the 21 years with a rate of 1% yr -1 , which is attributed to the stomatal downregulation induced by rising atmospheric CO 2 concentration. Similarly, annual ET also showed a decreasing trend since evaporation stayed relatively constant. Variability in detrended annual ET was mostly controlled by the average soil water storage during the growing season (May-October). Though the duration and intensity of the drought did not increase, the drought-induced decreases in T and ET showed an increasing trend. This pattern may reflect the changes in forest structure, related to the decline in the deciduous understory cover during the stand development. These results suggest that the water-saving effect of stomatal regulation and water-related factors mostly determined the trend and variability in ET, respectively. This may also imply an increase in the limitation of water availability on ET in young forests, associated with the structural and compositional changes related to forest growth., (© 2024 John Wiley & Sons Ltd.)

Main Conclusion: The high intrinsic water-use efficiency of Erianthus may be due to the low abaxial stomatal density and the accumulation of leaf metabolites such as betaine and gamma-aminobutyric acid. Sugarcane is an important crop that is widely cultivated in tropical and subtropical regions of the world. Because drought is among the main impediments limiting sugarcane production in these regions, breeding of drought-tolerant sugarcane varieties is important for sustainable production. Erianthus arundinaceus, a species closely related to sugarcane, exhibits high intrinsic water-use efficiency (iWUE), the underlying mechanisms for which remain unknown. To improve the genetic base for conferring drought tolerance in sugarcane, in the present study, we performed a comprehensive comparative analysis of leaf gas exchange and metabolites in different organs of sugarcane and Erianthus under wet and dry soil-moisture conditions. Erianthus exhibited lower stomatal conductance under both conditions, which resulted in a higher iWUE than in sugarcane. Organ-specific metabolites showed gradations between continuous parts and organs, suggesting linkages between them. Cluster analysis of organ-specific metabolites revealed the effects of the species and treatments in the leaves. Principal component analysis of leaf metabolites confirmed a rough ordering of the factors affecting their accumulations. Compared to sugarcane leaf, Erianthus leaf accumulated more raffinose, betaine, glutamine, gamma-aminobutyric acid, and S-adenosylmethionine, which function as osmolytes and stress-response compounds, under both the conditions. Our extensive analyses reveal that the high iWUE of Erianthus may be due to the specific accumulation of such metabolites in the leaves, in addition to the low stomatal density on the abaxial side of leaves. The identification of drought-tolerance traits of Erianthus will benefit the generation of sugarcane varieties capable of withstanding drought stress., (© 2024. The Author(s).)

Water-use strategies play a crucial role in the adaptive capabilities of mangroves to the saline intertidal conditions, yet the intricacies of daily water-use patterns in mangrove species, which are pivotal for maintaining water balance, remain poorly understood. In this comprehensive study, we aimed to clarify the water use strategies of three co-occurring mangrove species, Avicennia marina, Aegiceras corniculatum and Kandelia obovata, through stem sap flow monitoring, leaf gas exchange and stem diameter change measurements. Our findings revealed that the daily sap flow density of Avicennia and Aegiceras reached the peak about 1 h earlier than that of Kandelia. When transpiration was strong, Kandelia and Aegiceras used stem storage to meet water demand, while Avicennia synchronized stem water storage. These three mangrove species adopted cross-peak water used and unique stem water storage to regulate their water balance. In Kandelia, the daily sap flow in per sapwood area was significantly lower, while water-use efficiency was significantly higher than those of Avicennia and Aegiceras, indicating that Kandelia adopted a more conservative and efficient water-use strategy. Sap flow in Avicennia was the most sensitive to environmental changes, while Kandelia limited water dissipation by tightly controlling stomata. Meteorological factors (photosynthetically active radiation, vapor pressure deficit and air temperature) were the main driving factors of sap flow. The increase of soil temperature can promote the water use of mangrove species, while the increase of salinity resulted in more conservative water use. Our results highlight the diversity of daily water-use strategies among the three co-occurring mangrove species, pinpointing Kandelia as the most adaptive at navigating the changing conditions of intertidal habitats in the future climate. In conclusion, our findings provide a mesoscale perspective on water-use characteristics of mangroves and also provides theoretical basis for mangroves afforestation and ecological restoration., (© The Author(s) 2024. Published by Oxford University Press. All rights reserved. For permissions, please e-mail: journals.permission@oup.com.)