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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. Summary statement: Universal, near saturation of leaf air spaces is a long‐standing assumption in plant science. Here, we summarise historical and recent evidence which provides a compelling case for refuting this assumption and recognising significant unsaturation. Such recognition will enhance opportunities to promote plant productivity in the face of increasing evaporative demand. [ABSTRACT FROM AUTHOR]

SUMMARY: Stomatal pores in plant leaves mediate CO2 uptake for photosynthesis and water loss via transpiration. Altered stomatal density can affect plant photosynthetic capacity, water use efficiency, and growth, potentially providing either benefits or drawbacks depending on the environment. Here we explore, at different air humidity regimes, gas exchange, stomatal anatomy, and growth of Arabidopsis lines designed to combine increased stomatal density (epf1, epf2) with high stomatal sensitivity (ht1‐2, cyp707a1/a3). We show that the stomatal density and sensitivity traits combine as expected: higher stomatal density increases stomatal conductance, whereas the effect is smaller in the high stomatal sensitivity mutant backgrounds than in the epf1epf2 double mutant. Growth under low air humidity increases plant stomatal ratio with relatively more stomata allocated to the adaxial epidermis. Low relative air humidity and high stomatal density both independently impair plant growth. Higher evaporative demand did not punish increased stomatal density, nor did inherently low stomatal conductance provide any protection against low relative humidity. We propose that the detrimental effects of high stomatal density on plant growth at a young age are related to the cost of producing stomata; future experiments need to test if high stomatal densities might pay off in later life stages. [ABSTRACT FROM AUTHOR]

Summary: Current estimates of temperature effects on plants mostly rely on air temperature, although it can significantly deviate from leaf temperature (Tleaf). To address this, some studies have used canopy temperature (Tcan). However, Tcan fails to capture the fine‐scale variation in Tleaf among leaves and species in diverse canopies.We used infrared radiometers to study Tleaf and Tcan and how they deviate from air temperature (ΔTleaf and ΔTcan) in multispecies tropical tree plantations at three sites along an elevation and temperature gradient in Rwanda.Our results showed high Tleaf (up to c. 50°C) and ΔTleaf (on average 8–10°C and up to c. 20°C) of sun‐exposed leaves during 10:00 h–15:00 h, being close to or exceeding photosynthetic heat tolerance thresholds. These values greatly exceeded simultaneously measured values of Tcan and ΔTcan, respectively, leading to strongly overestimated leaf thermal safety margins if basing those on Tcan data. Stomatal conductance and leaf size affected Tleaf and Tcan in line with their expected influences on leaf energy balance.Our findings highlight the importance of leaf traits for leaf thermoregulation and show that monitoring Tcan is not enough to capture the peak temperatures and heat stress experienced by individual leaves of different species in tropical forest canopies. [ABSTRACT FROM AUTHOR]

Tropical forest photosynthesis can decline at high temperatures due to (1) biochemical responses to increasing temperature and (2) stomatal responses to increasing vapor pressure deficit (VPD), which is associated with increasing temperature. It is challenging to disentangle the influence of these two mechanisms on photosynthesis in observations, because temperature and VPD are tightly correlated in tropical forests. Nonetheless, quantifying the relative strength of these two mechanisms is essential for understanding how tropical gross primary production (GPP) will respond to climate change, because increasing atmospheric CO2 concentration may partially offset VPD‐driven stomatal responses, but is not expected to mitigate the effects of temperature‐driven biochemical responses. We used two terrestrial biosphere models to quantify how physiological process assumptions (photosynthetic temperature acclimation and plant hydraulic stress) and functional traits (e.g., maximum xylem conductivity) influence the relative strength of modeled temperature versus VPD effects on light‐saturated GPP at an Amazonian forest site, a seasonally dry tropical forest site, and an experimental tropical forest mesocosm. By simulating idealized climate change scenarios, we quantified the divergence in GPP predictions under model configurations with stronger VPD effects compared with stronger direct temperature effects. Assumptions consistent with stronger direct temperature effects resulted in larger GPP declines under warming, while assumptions consistent with stronger VPD effects resulted in more resilient GPP under warming. Our findings underscore the importance of quantifying the role of direct temperature and indirect VPD effects for projecting the resilience of tropical forests in the future, and demonstrate that the relative strength of temperature versus VPD effects in models is highly sensitive to plant functional parameters and structural assumptions about photosynthetic temperature acclimation and plant hydraulics. [ABSTRACT FROM AUTHOR]

Tree species diversity in forest ecosystems could reduce their vulnerability to extreme droughts through improved microclimate and below‐ground water source partitioning driven by contrasting species‐specific water use patterns. However, little is known about the seasonal dynamics of belowground water uptake that determine whether diversity positively or negatively impacts tree carbon assimilation and water exchange.Using a network of 30 permanent plots in Mediterranean forests with increasing tree species diversity (from monospecific to four‐species mixtures), we examined the seasonal patterns of in‐situ aboveground carbon and water relations and belowground water sources on 265 trees from four pine and oak species over 2 years using hydraulic and stable isotope approaches.We found that increasing species diversity in broadleaf and conifer mixtures induced strong soil water source partitioning between oak and pine species. As conditions became drier during the summer in mixed stands, oak species took up water from deeper soil sources, while pines were systematically limited to shallow ones. Despite significant belowground moisture partitioning, stronger drought‐induced reductions in photosynthesis, stomatal conductance, and leaf water potential were still observed in diverse compared with monospecific stands for pines but with some benefits for oaks.Synthesis: Our findings reveal that tree species diversity promoted belowground water source partitioning in mixed oak and pine stands, potentially reducing competition for water in more diverse ecosystems. Yet, our results show that it is insufficient to buffer the adverse impacts of severe droughts on aboveground tree carbon and water use, leading to higher water stress, especially for pines. [ABSTRACT FROM AUTHOR]

Stomatal opening in plant leaves is regulated through a balance of carbon and water exchange under different environmental conditions. Accurate estimation of stomatal regulation is crucial for understanding how plants respond to changing environmental conditions, particularly under climate change. A new generation of optimality‐based modelling schemes determines instantaneous stomatal responses from a balance of trade‐offs between carbon gains and hydraulic costs, but most such schemes do not account for biochemical acclimation in response to drought. Here, we compare the performance of six instantaneous stomatal optimisation models with and without accounting for photosynthetic acclimation. Using experimental data from 37 plant species, we found that accounting for photosynthetic acclimation improves the prediction of carbon assimilation in a majority of the tested models. Photosynthetic acclimation contributed significantly to the reduction of photosynthesis under drought conditions in all tested models. Drought effects on photosynthesis could not accurately be explained by the hydraulic impairment functions embedded in the stomatal models alone, indicating that photosynthetic acclimation must be considered to improve estimates of carbon assimilation during drought. Summary statement: Accounting for photosynthetic acclimation improves the predictions of carbon assimilation in all the stomatal optimisation models evaluated. The influence of drought on photosynthesis cannot be fully explained by the hydraulic impairment function of the stomatal models alone. [ABSTRACT FROM AUTHOR]

Summary: A long‐standing debate looks at whether air or soil dryness is more limiting to vegetation water use and productivity. The answer has large implications for future ecosystem functioning, as atmospheric dryness is predicted to increase globally while changes in soil moisture are predicted to be far more variable. Here, I review the complexities that contribute to this debate, including the strong coupling between atmospheric and soil dryness, and the widespread heterogeneity in vegetation hydraulic traits, acclimations, and adaptations to water stress. I discuss solutions to improve understanding and modeling of vegetation sensitivity to dryness, including how different types of observational data can be used together to gain insight into vegetation response to water stress across spatial and temporal scales. [ABSTRACT FROM AUTHOR]

Summary: Carbon isotope discrimination (∆) in leaf biomass (∆BL) and tree rings (∆TR) provides important proxies for plant responses to climate change, specifically in terms of intrinsic water‐use efficiency (iWUE). However, the nonphotosynthetic 12C/13C fractionation in plant tissues has rarely been quantified and its influence on iWUE estimation remains uncertain.We derived a comprehensive, ∆ based iWUE model (iWUEcom) which includes nonphotosynthetic fractionations (d) and characterized tissue‐specific d‐values based on global compilations of data of ∆BL, ∆TR and real‐time ∆ in leaf photosynthesis (∆online). iWUEcom was further validated with independent datasets.∆BL was larger than ∆online by 2.53‰, while ∆BL and ∆TR showed a mean offset of 2.76‰, indicating that ∆TR is quantitatively very similar to ∆online. Applying the tissue‐specific d‐values (dBL = 2.5‰, dTR = 0‰), iWUE estimated from ∆BL aligned well with those estimated from ∆TR or gas exchange. ∆BL and ∆TR showed a consistent iWUE trend with an average CO2 sensitivity of 0.15 ppm ppm−1 during 1975–2015.Accounting for nonphotosynthetic fractionations improves the estimation of iWUE based on isotope records in leaf biomass and tree rings, which is ultimate for inferring changes in carbon and water cycles under historical and future climate. [ABSTRACT FROM AUTHOR]

Summary: 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 CO2 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. [ABSTRACT FROM AUTHOR]

Increases in shrub height, biomass and canopy cover are key whole‐plant features of warming‐induced vegetation change in tundra. We investigated leaf functional traits underlying photosynthetic capacity of Arctic shrub species, particularly its main limiting processes such as mesophyll conductance. In this nutrient‐limited ecosystem, we expect leaf nitrogen concentration to be the main limiting factor for photosynthesis. We measured the net photosynthetic rate at saturated light (Asat) in three Salix species throughout a glacial valley in High‐Arctic tundra and used a causal approach to test relationships between leaf stomatal and mesophyll conductances (gsc, gm), carboxylation capacity (Vcmax), nitrogen and phosphorus concentration (Narea, Parea) and leaf mass ratio (LMA). Arctic Salix species showed no difference in Asat compared to a global data set, while being characterized by higher Narea, Parea and LMA. Vcmax, gsc and gm independently increased Asat, with Vcmax as its main limitation. We highlighted a nitrogen‐influenced pathway for increasing photosynthesis in the two prostrate mesic habitat species. In contrast, the erect wetland habitat Salix richardsonii mainly increased Asat with increasing gsc. Overall, our study revealed high photosynthetic capacities of Arctic Salix species but contrasting regulatory pathways that may influence shrub ability to respond to environmental changes in High Arctic tundra. Summary statement: The photosynthetic capacity and limitations in Arctic shrubs revealing contrasting adaptations among Salix species that may influence their ability to provide negative carbon feedback to the atmosphere in high‐Arctic tundra, despite their similar photosynthetic rate compared to a global data set. [ABSTRACT FROM AUTHOR]

SUMMARY: Tropospheric ozone [O3] is a secondary air pollutant formed from the photochemical oxidation of volatile organic compounds in the presence of nitrogen oxides, and it is one of the most damaging air pollutants to crops. O3 entry into the plant generates reactive oxygen species leading to cellular damage and oxidative stress, leading to decreased primary production and yield. Increased O3 exposure has also been shown to have secondary impacts on plants by altering the incidence and response to plant pathogens. We used the Capsicum annum (pepper)‐Xanthomonas perforans pathosystem to investigate the impact of elevated O3 (eO3) on plants with and without exposure to Xanthomonas, using a disease‐susceptible and disease‐resistant pepper cultivar. Gas exchange measurements revealed decreases in diurnal photosynthetic rate (A′) and stomatal conductance (gs′), and maximum rate of electron transport (Jmax) in the disease‐resistant cultivar, but no decrease in the disease‐susceptible cultivar in eO3, regardless of Xanthomonas presence. Maximum rates of carboxylation (Vc,max), midday A and gs rates at the middle canopy, and decreases in aboveground biomass were negatively affected by eO3 in both cultivars. We also observed a decrease in stomatal sluggishness as measured through the Ball–Berry–Woodrow model in all treatments in the disease‐resistant cultivar. We hypothesize that the mechanism conferring disease resistance to Xanthomonas in pepper also renders the plant less tolerant to eO3 stress through changes in stomatal responsiveness. Findings from this study help expand our understanding of the trade‐off of disease resistance with abiotic stresses imposed by future climate change. Significance Statement: This paper examines the impact of combined biotic and abiotic stress on plant physiology and finds that cultivars of pepper resistant to Xanthomonas infection have decreased leaf‐level carbon assimilation and stomatal conductance in response to elevated O3, regardless of inoculation status. Our data suggest that the mechanism conferring pathogen disease resistance in pepper also renders the plant less tolerant to elevated O3 stress through changes in stomatal responsiveness. [ABSTRACT FROM AUTHOR]

Embolism resistance of xylem tissue varies among species and is an important trait related to drought resistance, with anatomical attributes like pit membrane thickness playing an important role in avoiding embolism spread. Grafted Citrus trees are commonly grown in orchards, with the rootstock being able to affect the drought resistance of the whole plant. Here, we evaluated how rootstocks affect the vulnerability to embolism resistance of the scion using several rootstock/scion combinations. Scions of 'Tahiti' acid lime, 'Hamlin', 'Pera' and 'Valencia' oranges grafted on a 'Rangpur' lime rootstock exhibit similar vulnerability to embolism. In field‐grown trees, measurements of leaf water potential did not suggest significant embolism formation during the dry season, while stomata of Citrus trees presented an isohydric response to declining water availability. When 'Valencia' orange scions were grafted on 'Rangpur' lime, 'IAC 1710' citrandarin, 'Sunki Tropical' mandarin or 'Swingle' citrumelo rootstocks, variation in intervessel pit membrane thickness of the scion was found. The 'Rangpur' lime rootstock, which is known for its drought resistance, induced thicker pit membranes in the scion, resulting in higher embolism resistance than the other rootstocks. Similarly, the rootstock 'IAC 1710' citrandarin generated increased embolism resistance of the scion, which is highly relevant for citriculture. Summary statement: Different Citrus rootstocks influence scion vulnerability to embolism, with 'Rangpur' lime inducing less vulnerability and thicker pit membranes. Besides, in field‐grown trees, measurements of leaf water potential did not suggest significant embolism formation even during the dry season. [ABSTRACT FROM AUTHOR]

Hemiparasitic Cassytha filiformis commonly infects native host (Dillenia suffruticosa and Melastoma malabathricum) and introduced host (Acacia auriculiformis and Acacia mangium) species in threatened heath forests in Brunei. This study aims to investigate the impact of parasitism on the ecophysiology of these host species. This study addresses the research gap in understanding the ecophysiology of C. filiformis–host associations, particularly when native and introduced hosts were infected. We generated CO2 and light response curves to examine the effects of increasing CO2 and light levels of infected and uninfected hosts and examined gaseous exchange, mineral nutrients, and secondary bioactive compounds of host–parasite associations. Infected hosts were negatively impacted by C. filiformis as exhibited in the CO2 and light response curves, with C. filiformis–native host association performing better than introduced species. There were no significant differences in photosynthetic parameters between infected and uninfected hosts, except in D. suffruticosa. Certain nutrient contents showed significant differences, but total N, Ca, and K in uninfected hosts were similar to infected hosts. Total phenols and tannins were significantly higher in introduced hosts than native hosts. Our findings asserted that this hemiparasitic vine relies on both its photosynthetic efficiency and nutrient acquisition from its hosts. The parasitism did not significantly hinder the ecophysiological performance of infected hosts, suggesting a plausible co‐existence between the hosts and C. filiformis. This study provides essential ecophysiological information for future research on how C. filiformis can establish itself without negatively impacting the co‐habitating native hosts. [ABSTRACT FROM AUTHOR]

Summary: Elevated air temperature (Tair) and vapour pressure deficit (VPDair) significantly influence plant functioning, yet their relative impacts are difficult to disentangle.We examined the effects of elevated Tair (+6°C) and VPDair (+0.7 kPa) on the growth and physiology of six tropical tree species. Saplings were grown under well‐watered conditions in climate‐controlled glasshouses for 6 months under three treatments: (1) low Tair and low VPDair, (2) high Tair and low VPDair, and (3) high Tair and high VPDair. To assess acclimation, physiological parameters were measured at a set temperature.Warm‐grown plants grown under elevated VPDair had significantly reduced stomatal conductance and increased instantaneous water use efficiency compared to plants grown under low VPDair. Photosynthetic biochemistry and thermal tolerance (Tcrit) were unaffected by VPDair, but elevated Tair caused Jmax25 to decrease and Tcrit to increase. Sapling biomass accumulation for all species responded positively to an increase in Tair, but elevated VPDair limited growth.This study shows that stomatal limitation caused by even moderate increases in VPDair can decrease productivity and growth rates in tropical species independently from Tair and has important implications for modelling the impacts of climate change on tropical forests. [ABSTRACT FROM AUTHOR]

The hysteresis response of tree sap flux (SF) to its main driving factor of incoming short‐wave radiation (Rsi) has been widely reported, affecting the accuracy of sap flux and transpiration estimates in forest ecosystems. The diurnal cycle of SF usually lags the Rsi cycle by certain hours, thereby generating a closed counterclockwise hysteresis pattern. However, a few studies have reported that diurnal SF cycle may advance Rsi cycle, and such a response pattern has not been fully explored. In this study, we reported a rarely seen crossed hysteresis response pattern of SF to Rsi in 1/3 trees of a young temperate pine forest. We found that the diurnal SF cycle advances Rsi cycle especially in the morning induced by the early stomatal closure, thereby generating the crossed hysteresis response of SF to Rsi. We also proposed a method to quantify the magnitude of hysteresis (Ahys) for both the crossed and closed hystereses. Our analysis suggests that a lower Ahys of two time series results in (a) a larger crossing degree of hysteresis, and (b) a stronger linear correlation between the two time series. The seasonal variation of soil water content can explain the variation in Ahys for the hysteresis response of SF to Rsi, and the crossed hysteresis of SF is more likely to occur under water stress conditions. This study contributes to advancing our understanding of forest transpiration and how forests may respond to drought stress, which are expected to become more frequent and longer under future climate change. Plain Language Summary: Sap flux (SF) is widely used as a surrogate of transpiration in field studies. It is broadly reported that the diurnal cycle of SF lags its driving force—incoming short‐wave radiation (Rsi). Correspondingly, a closed counterclockwise loop appears when plotting SF against Rsi on a diurnal basis. Such phenomenon is widely identified as a hysteresis response of SF to Rsi. In our study, we reported that the diurnal SF cycle can advance the Rsi cycle especially in the morning, with a crossed SF‐Rsi hysteresis loop emerging. The crossed hysteresis is more complex in terms of the direction and pattern than the closed hysteresis loop; we further found the crossed hysteresis pattern contributes to reducing the magnitude of hysteresis and improving the correlation between SF and Rsi. We developed a method to quantify the magnitude for both the crossed and closed hystereses. We found that the closure of stomatal conductance drives the early decline of the diurnal SF cycle, resulting in the crossed hysteresis. We also found that soil water content plays an important role in controlling the seasonal variation of hysteresis and the crossed hysteresis is more likely to emerge in water stress conditions. Key Points: A novel crossed hysteresis response of sap flux to solar radiation is reported and investigatedThe early stomatal closure drives the variation of the hysteresis pattern of forest transpirationSoil water stress induces the seasonal variation of the sap flux‐radiation hysteresis [ABSTRACT FROM AUTHOR]

Summary: The oxygen isotope composition of cellulose (δ18O values) has been suggested to contain information on stomatal conductance (gs) responses to rising pCO2. The extent by which pCO2 affects leaf water and cellulose δ18O values (δ18OLW and δ18OC) and the isotope processes that determine pCO2 effects on δ18OLW and δ18OC are, however, unknown.We tested the effects of pCO2 on gs, δ18OLW and δ18OC in a glasshouse experiment, where six plant species were grown under pCO2 ranging from 200 to 500 ppm.Increasing pCO2 caused a decline in gs and an increase in δ18OLW, as expected. Importantly, the effects of pCO2 on gs and δ18OLW were small and pCO2 effects on δ18OLW were not directly transferred to δ18OC but were attenuated in grasses and amplified in dicotyledonous herbs and legumes. This is likely because of functional group‐specific pCO2 effects on the model parameter pxpex.Our study highlights important uncertainties when using δ18OC as a proxy for gs. Specifically, pCO2‐triggered gs effects on δ18OLW and δ18OC are possibly too small to be detected in natural settings and a pCO2 effect on pxpex may render the commonly assumed negative linkage between δ18OC and gs to be incorrect, potentially confounding δ18OC based gs reconstructions. [ABSTRACT FROM AUTHOR]

Diagnosing and predicting evaporation through satellite‐based surface energy balance (SEB) and land surface models (LSMs) faces challenges due to the non‐linear responses of aerodynamic (ga) and stomatal conductance (gcs) to concurrent soil and atmospheric drought. Despite a soaring popularity to refine gcs formulation in LSMs by integrating soil‐plant hydraulics, SEB models often overlook the utility of gcs. This oversight is attributed to the overriding emphasis on reducing ga uncertainties and the lack of coordination between these two modeling communities. This disengagement between modeling communities poses a persistent challenge in understanding divergent evaporation estimates during intense soil‐atmospheric drought. Here we conducted a theoretical experiment over two contrasting European forest sites to examine the sensitivity of conductances and evaporative fluxes to a water‐stress factor (β‐factor), coupled with land surface temperature (LST) and vapor pressure deficit (representing soil and atmospheric drought proxy). Utilizing a non‐parametric diagnostic model (Surface Temperature Initiated Closure, STIC) and a prognostic model (Community Land Model, CLM5.0), the analysis revealed that the β‐factor, alongside different functional forms of conductances and the loose coupling of CLM5.0 conductances to LST, significantly influenced the response of the two models to soil and atmospheric drought. These discrepancies propagated in the estimates of evaporative fluxes between STIC and CLM5.0. The analysis reaffirms the need for a consensus on theory and models capturing the sensitivity of biophysical conductances to soil‐atmospheric drought interplay. It emphasizes the need for fostering collaboration between modeling communities to enhance the prediction of evaporation in complex environmental conditions. Plain Language Summary: The regulation of plant water loss through evaporation is governed by two key physical and biological attributes: aerodynamic and stomatal conductance. The magnitude and variability of these conductances and their degree of regulation on evaporation is heavily dependent on how the conductances respond to the conjugate dryness from the soil and the atmosphere. Because these conductances cannot be typically measured at a large scale, the majority of the global evaporation models use different mechanistic functions to estimate them, which involves many empirical parameters. Such methods do not fully capture the evaporation variability of ecosystems during water stress, leading to large errors in water cycle monitoring. Our model‐based synthetic experiment shows how two structurally different models with different functional forms of the conductances respond very differently to emerging soil‐atmospheric water stress and produce divergent estimates of evaporation in a variety of dry and wet conditions. This study not only provides insights into the role of conjugate effects of soil and atmospheric drought in explaining the conductances and evaporation variability but also suggests a novel perspective for reconciling predictive and remote sensing evaporation models, benefiting water management, testing plant water use theories, and understanding land‐atmosphere interactions. Key Points: Diagnostic (STIC) and prognostic (CLM5.0) evaporation models show distinct levels of sensitivity to water stressSTIC and CLM5.0 evaporation models better agree in simulating the energy fluxes as compared to underlying biophysical conductanceMajor differences in the simulated stomatal conductance are due to divergences in the physiological assumptions of the two evaporation modeling approaches [ABSTRACT FROM AUTHOR]

Environmental variation commonly drives functional trait diversity within species, among species, and across communities. Climate and shared evolutionary history can both influence trait–environment relationships. We studied variation in plant functional traits among closely related Costus species occurring across environmental gradients, the extent to which this variation occurs within single species, and how that variation may be influenced by shared evolutionary history. We measured leaf, aboveground stem, rhizome, and fine root traits of 17 species of Costus in eight sites in Costa Rica and Panama, which varied in elevation, temperature, and precipitation. We then assessed the relationships among traits and environmental variables and estimated the phylogenetic signal of the traits. We observed significant relationships between functional traits and climate. Stomatal conductance decreased, but stem density and rhizome dry matter content increased with decreasing mean annual temperature and precipitation seasonality in both cross‐species and single‐species analyses. This suggests that herbaceous species have a similar trade‐off between plant hydraulic efficiency and safety as found in woody plants. Mean annual temperature was a stronger driver of trait variation than mean annual precipitation. We also found phylogenetic signal in leaf and stem structural traits (i.e., closely related species are more similar than distantly related species), but not in physiological or belowground traits. Our results demonstrate significant trait variation within and among species of Costus, a widespread understory and herbaceous genus in the tropics, which is driven by both climate and shared evolutionary history. [ABSTRACT FROM AUTHOR]

Summary As temperature rises, net carbon uptake in tropical forests decreases, but the underlying mechanisms are not well understood. High temperatures can limit photosynthesis directly, for example by reducing biochemical capacity, or indirectly through rising vapor pressure deficit (VPD) causing stomatal closure. To explore the independent effects of temperature and VPD on photosynthesis we analyzed photosynthesis data from the upper canopies of two tropical forests in Panama with Generalized Additive Models. Stomatal conductance and photosynthesis consistently decreased with increasing VPD, and statistically accounting for VPD increased the optimum temperature of photosynthesis (Topt) of trees from a VPD‐confounded apparent Topt of c. 30–31°C to a VPD‐independent Topt of c. 33–36°C, while for lianas no VPD‐independent Topt was reached within the measured temperature range. Trees and lianas exhibited similar temperature and VPD responses in both forests, despite 1500 mm difference in mean annual rainfall. Over ecologically relevant temperature ranges, photosynthesis in tropical forests is largely limited by indirect effects of warming, through changes in VPD, not by direct warming effects of photosynthetic biochemistry. Failing to account for VPD when determining Topt misattributes the underlying causal mechanism and thereby hinders the advancement of mechanistic understanding of global warming effects on tropical forest carbon dynamics. [ABSTRACT FROM AUTHOR]

Determining the factors that set species' range limits is of critical importance in an era of rapid climate and biotic change. Mangroves dominate global tropical coastlines, but their subtropical range limits are thought to be mainly constrained by cold air. However, several tropical mangrove range limits are conspicuously warm, arid, and dominated by cold water countercurrents. Despite speculation that the combination of cold water and aridity controls these range limits, investigations of this phenomenon are rare.Here, we conducted an experimental study examining the separate and interactive effects of cold water and aridity on survival, growth, and ecophysiological traits of three mangrove species from the arid, countercurrent range limit in Baja California, Mexico. It is one of the few range edges worldwide where red and white mangroves (Rhizophora mangle and Laguncularia racemosa, respectively), not the putatively stress‐tolerant black mangroves (Avicennia germinans), are the most poleward distributed mangroves, providing further opportunity to examine how mangrove ecophysiological traits affect species‐specific range limits.Black mangroves were surprisingly intolerant of cold water regardless of aridity, with no seedlings surviving in the lowest water temperature (13°C) after 8 months. In contrast, both white and red mangrove seedlings were able to survive at 13°C when humidity was 65% (but not 40%).In cold water and arid conditions, leaf stomatal conductance was consistently lowest for white mangroves, intermediate for red mangroves, and highest for black mangroves, suggesting that white mangroves were the most resistant to transpiration water loss in the arid conditions typical of Baja.Similarly, when exposed to drought after the eight‐month experiment, white mangroves grown in cold water survived longer than red and black mangroves.Synthesis: Ultimately, our results suggest that the species‐specific range limits of mangroves in Baja California reflects their species‐specific adaptations (or lack thereof) to cold water and aridity, with white and red mangroves surprisingly outperforming black mangroves. Further, the difference in our results compared to the same species in other ranges underscores the need to incorporate species and range‐specific traits when conducting distribution modelling to predict the effects of climate change on organism range limits. [ABSTRACT FROM AUTHOR]

As the leading global grain crop, maize significantly impacts agricultural water usage. Presently, photosynthesis (Anet ${A}_{\text{net}}$) in leaves of modern maize crops is saturated with CO2 ${\text{CO}}_{2}$, implying that reducing stomatal conductance (gs ${g}_{{\rm{s}}}$) would not affect Anet ${A}_{\text{net}}$ but reduce transpiration (τ $\tau $), thereby increasing water use efficiency (WUE). While gs ${g}_{{\rm{s}}}$ reduction benefits upper canopy leaves under optimal conditions, the tradeoffs in low light and nitrogen‐deficient leaves under nonoptimal microenvironments remain unexplored. Moreover, gs ${g}_{{\rm{s}}}$ reduction increases leaf temperature (Tleaf ${T}_{\text{leaf}}$) and water vapor pressure deficit, partially counteracting transpiratory water savings. Therefore, the overall impact of gs ${g}_{{\rm{s}}}$ reduction on water savings remains unclear. Here, we use a process‐based leaf model to investigate the benefits of reduced gs ${g}_{{\rm{s}}}$ in maize leaves under different microenvironments. Our findings show that increases in Tleaf ${T}_{\text{leaf}}$ due to gs ${g}_{{\rm{s}}}$ reduction can diminish WUE gains by up to 20%. However, gs ${g}_{{\rm{s}}}$ reduction still results in beneficial WUE tradeoffs, where a 29% decrease in gs ${g}_{{\rm{s}}}$ in upper canopy leaves results in a 28% WUE gain without loss in Anet ${A}_{\text{net}}$. Lower canopy leaves exhibit superior tradeoffs in gs ${g}_{{\rm{s}}}$ reduction with 178% gains in WUE without loss in Anet ${A}_{\text{net}}$. Our simulations show that these WUE benefits are resilient to climate change. Summary statement: Reducing maize stomatal conductance boosts water use efficiency (WUE) by up to 26% in upper leaves and 170% in lower leaves, with minimal photosynthesis impact. However, higher leaf temperature could offset WUE gains by up to 16%. Stomatal conductance reduction tradeoffs are beneficial under diverse conditions and canopy leaf positions and are unaffected by climate change. [ABSTRACT FROM AUTHOR]

Wildfires are increasing across the USA. While smoke events affect human exposure and air quality, wildfire smoke effects on ecosystem‐atmosphere interactions are poorly understood. We investigate smoke effects on biogenic volatile organic compound (VOC) emissions and photosynthesis for ponderosa pines. During several wildfire smoke events, we observed photosynthetic reduction with evidence for stomatal plugging and changes in leaf‐level uptake and emission of both biogenic and wildfire VOCs. During intense smoke events, photosynthesis and VOC emissions were almost entirely suppressed, but increased dramatically upon stomatal opening. We propose four types of VOC responses to this burst in stomatal opening: post‐burst emissions, pulsed emissions, surge emissions, and post‐burst uptake. Our observations suggest that wildfire smoke can affect plant physiology and leaf‐atmosphere gas exchange. Plain Language Summary: Wildfires are becoming more prevalent in the United States. Wildfire smoke affects human health and air quality, but we know little about how smoke interacts with plants, or the subsequent feedback with the atmosphere. We present results from a field experiment during which smoke repeatedly spread across a pine forest. We measured smoke‐induced changes in plant physiology (namely photosynthesis) and a suite of chemical emissions. During prolonged periods of intense smoke, we found that both photosynthesis and leaf emissions of chemical compounds decreased. When we induced photosynthesis, we observed a burst of different chemicals and identified several different pathways for these chemical releases. Further analyses suggest that smoke intensity and exposure may change the known relationship between emissions and temperature. Key Points: Wildfire smoke interacts with stomates and leaf surfaces, suppressing photosynthesis and stomatal conductanceIntense smoke can alter emissions and uptake of biogenic and other volatile organic compounds through three mechanismsThe effects of wildfires on photosynthetic capacity and leaf emissions appear proportional to the intensity and duration of smoke events [ABSTRACT FROM AUTHOR]

Plant transpiration is a key component of the hydrological cycle that moves water from the soil through plants to the atmosphere and is largely determined by environmental conditions. The challenges of highly variable climatic conditions and water scarcity in the semi‐arid savannas of southern Africa drive trees to exhibit different water use strategies.An additional challenge for savanna trees arises from the browsing of large mammals, and its potential impact on transpiration has largely been overlooked.To assess water use strategies and the impact of browsing on transpiration in three common savanna woody species (Colophospermum mopane, Catophractes alexandri, Senegalia mellifera), we conducted an extensive field experiment in a wildlife reserve in Namibia. We established a browsing gradient ranging from 0% to 100% leaf removal to disentangle complex relationships, and we measured sap flow representing whole‐tree transpiration, stomatal conductance as a proxy for leaf level transpiration, and environmental factors including soil moisture, solar radiation, air temperature and air humidity.We discovered a unimodal relationship between browsing intensity and sap flow, consistent across all species, and peaking at low to moderate browsing levels equivalent to approximately 30% leaf removal. This universal pattern is remarkable since we found water use strategies to differ among species: Colophospermum mopane and C. alexandri exhibit a water‐saving behaviour to reduce water loss under unfavourable conditions while S. mellifera demonstrates water‐spending characteristics.The deviation from a negative linear effect of browsing does not meet the functional expectation when browsing is translated into a simple leaf area reduction. Instead, it can be attributed to stomatal adjustments that partially compensate for transpiratory surface loss. We propose that this conceptual mechanism can be generalised to other woody species and various ecosystems. In addition, common transpiration models relying solely on leaf area indices and abiotic factors can be improved by including a non‐linear relationship to reproduce the effect of browsing accurately.Our findings highlight the role of herbivores in shaping the connection between the Earth's spheres and improve our understanding of ecohydrological and vegetation dynamics in Southern African savannas. Read the free Plain Language Summary for this article on the Journal blog. [ABSTRACT FROM AUTHOR]

Vegetation and precipitation are known to fundamentally influence each other. However, this interdependence is not fully represented in climate models because the characteristics of land surface (canopy) conductance to water vapor and CO2 are determined independently of precipitation. Working within a coupled atmosphere and land modelling framework (CAM6/CLM5; coupled Community Atmosphere Model v6/Community Land Model v5), we have developed a new theoretical approach to characterizing land surface conductance by explicitly linking its dynamic properties to local precipitation, a robust proxy for moisture available to vegetation. This will enable regional surface conductance characteristics to shift fluidly with climate change in simulations, consistent with general principles of co‐evolution of vegetation and climate. Testing within the CAM6/CLM5 framework shows that climate simulations incorporating the new theory outperform current default configurations across several error metrics for core output variables when measured against observational data. In climate simulations for the end of this century the new, adaptive stomatal conductance scheme provides a revised prognosis for average and extreme temperatures over several large regions, with increased primary productivity through central and east Asia, and higher rainfall through North Africa and the Middle East. The new projections also reveal more frequent heatwaves than originally estimated for the south‐eastern US and sub‐Saharan Africa but less frequent heatwaves across east Europe and northeast Asia. These developments have implications for evaluating food security and risks from extreme temperatures in areas that are vulnerable to climate change. [ABSTRACT FROM AUTHOR]

This article is a Commentary on Griffani et al. (2024), 243: 1301–1311. [ABSTRACT FROM AUTHOR]

Summary: Mixing species with contrasting resource use strategies could reduce forest vulnerability to extreme events. Yet, how species diversity affects seedling hydraulic responses to heat and drought, including mortality risk, is largely unknown.Using open‐top chambers, we assessed how, over several years, species interactions (monocultures vs mixtures) modulate heat and drought impacts on the hydraulic traits of juvenile European beech and pubescent oak. Using modeling, we estimated species interaction effects on timing to drought‐induced mortality and the underlying mechanisms driving these impacts.We show that mixtures mitigate adverse heat and drought impacts for oak (less negative leaf water potential, higher stomatal conductance, and delayed stomatal closure) but enhance them for beech (lower water potential and stomatal conductance, narrower leaf safety margins, faster tree mortality). Potential underlying mechanisms include oak's larger canopy and higher transpiration, allowing for quicker exhaustion of soil water in mixtures.Our findings highlight that diversity has the potential to alter the effects of extreme events, which would ensure that some species persist even if others remain sensitive. Among the many processes driving diversity effects, differences in canopy size and transpiration associated with the stomatal regulation strategy seem the primary mechanisms driving mortality vulnerability in mixed seedling plantations. See also the Commentary on this article by Wright, 241: 955–957. [ABSTRACT FROM AUTHOR]

As heatwave frequency increases, they are more likely to coincide with other disturbances like insect defoliation. But it is unclear if high temperatures after defoliation impact canopy recovery or leaf traits which may affect response to further stressors like drought. To examine these stressor interactions, we subjected defoliated (DEF) and undefoliated (UNDEF) oak saplings to a simulated spring heatwave of +10°C for 25 days. We measured gas exchange, leaf area recovery, carbohydrate storage, turgor loss point (ΨTLP), and minimum leaf conductance (gmin). During the heatwave, stem respiration exhibited stronger thermal acclimation in DEF than UNDEF saplings, while stomatal conductance and net photosynthesis increased. The heatwave did not affect leaf area recovery or carbohydrate storage of DEF saplings, but reflush leaves had higher gmin than UNDEF leaves, and this was amplified by the heatwave. Across all treatments, higher gmin was associated with higher daytime stomatal conductance and a lower ΨTLP. The results suggest defoliation stress may not be exacerbated by higher temperatures. However, reflush leaves are less conservative in their water use, limiting their ability to minimise water loss. While lower ΨTLP could help DEF trees maintain gas exchange under mild drought, they may be more vulnerable to dehydration under severe drought. Summary Statement: Leaf area recovery and carbohydrate storage after spring defoliation were not impacted by a coinciding heatwave. However, reflush leaves had higher minimum rates of water loss—and this effect was exacerbated by the heatwave, suggesting reduced drought tolerance. [ABSTRACT FROM AUTHOR]

Plants of 16 Italian cultivars of durum wheat at the tillering stage were exposed to a single pulse of ozone (O3, 200 ppb, 5 h) to reveal the mechanisms explaining their O3 tolerance/sensitivity by considering some leaf physiochemical traits [e.g., stomatal conductance, photosystem II (PSII) efficiency, antioxidant enzyme activity, and phytohormone synthesis]. At the end of the recovery period (48 h from the beginning of the exposure), all cultivars showed visible foliar injury in the form of widespread chlorosis which developed in bifacial ivory necrotic lesions scattered among the leaf veins of completely expanded leaves. Mongibello leaves showed not only the highest severity of visible injury (McKinney index = 62%), but also a reduction of PSII performance (Fv/Fm ratio: −22% compared with controls) together with an activation of the dissipation of the excess excitation energy as heat (qNP: +35%). The photosynthetic impairment observed throughout the whole experiment, was likely due to stomatal limitations (gs: −36% as average) and indicated that this cultivar was very sensitive to a pulse of O3. Similar results were obtained for Latino and Meridiano. Conversely, Iride, Orobel, and Portorico showed the lowest severity of visible injury (McKinney index around 13%) suggesting that they could be considered tolerant to a pulse of O3. These cultivars showed none or only transient changes due to O3 in terms of photosynthetic activity (e.g., A values), while the slight decrease of Fv/Fm ratio did not determine the photoinhibition of the PSII activity, since mechanisms for dissipating excess excitation energy were being activated. Ozone treatment induced a significant reduction of CO2 assimilation rate (−34%) in Orobel and this is not attributable to stomatal and non‐stomatal limitations. The remaining cultivars were considered moderately sensitive to O3, with McKinney index in the range of 20–43%. The measured biochemical traits are not decisive in determining the O3 sensitivity. In particular, the activity of phenylalanine ammonia lyase increased only in seven cultivars; among them, only in Latino, Vetrodur, and Simeto, this enhancement reflected in an accumulation of phenolic compounds suggesting potential higher protection against reactive oxygen species. This work highlights the relevance of identifying O3‐sensitive cultivars, in the optic to develop sustainable strategies to mitigate the impact of peak O3 episodes on wheat production. [ABSTRACT FROM AUTHOR]

Understanding interactions between environmental stress and genetic variation is crucial to predict the adaptive capacity of species to climate change. Leaf temperature is both a driver and a responsive indicator of plant physiological response to thermal stress, and methods to monitor it are needed. Foliar temperatures vary across leaf to canopy scales and are influenced by genetic factors, challenging efforts to map and model this critical variable. Thermal imagery collected using unoccupied aerial systems (UAS) offers an innovative way to measure thermal variation in plants across landscapes at leaf‐level resolutions. We used a UAS equipped with a thermal camera to assess temperature variation among genetically distinct populations of big sagebrush (Artemisia tridentata), a keystone plant species that is the focus of intensive restoration efforts throughout much of western North America. We completed flights across a growing season in a sagebrush common garden to map leaf temperature relative to subspecies and cytotype, physiological phenotypes of plants, and summer heat stress. Our objectives were to (1) determine whether leaf‐level stomatal conductance corresponds with changes in crown temperature; (2) quantify genetic (i.e., subspecies and cytotype) contributions to variation in leaf and crown temperatures; and (3) identify how crown structure, solar radiation, and subspecies‐cytotype relate to leaf‐level temperature. When considered across the whole season, stomatal conductance was negatively, non‐linearly correlated with crown‐level temperature derived from UAS. Subspecies identity best explained crown‐level temperature with no difference observed between cytotypes. However, structural phenotypes and microclimate best explained leaf‐level temperature. These results show how fine‐scale thermal mapping can decouple the contribution of genetic, phenotypic, and microclimate factors on leaf temperature dynamics. As climate‐change‐induced heat stress becomes prevalent, thermal UAS represents a promising way to track plant phenotypes that emerge from gene‐by‐environment interactions. [ABSTRACT FROM AUTHOR]

Summary: Populus fremontii is among the most dominant, and ecologically important riparian tree species in the western United States and can thrive in hyper‐arid riparian corridors. Yet, P. fremontii forests have rapidly declined over the last decade, particularly in places where temperatures sometimes exceed 50°C.We evaluated high temperature tolerance of leaf metabolism, leaf thermoregulation, and leaf hydraulic function in eight P. fremontii populations spanning a 5.3°C mean annual temperature gradient in a well‐watered common garden, and at source locations throughout the lower Colorado River Basin.Two major results emerged. First, despite having an exceptionally high Tcrit (the temperature at which Photosystem II is disrupted) relative to other tree taxa, recent heat waves exceeded Tcrit, requiring evaporative leaf cooling to maintain leaf‐to‐air thermal safety margins. Second, in midsummer, genotypes from the warmest locations maintained lower midday leaf temperatures, a higher midday stomatal conductance, and maintained turgor pressure at lower water potentials than genotypes from more temperate locations.Taken together, results suggest that under well‐watered conditions, P. fremontii can regulate leaf temperature below Tcrit along the warm edge of its distribution. Nevertheless, reduced Colorado River flows threaten to lower water tables below levels needed for evaporative cooling during episodic heat waves. [ABSTRACT FROM AUTHOR]

Summary: Limitations and utility of three measures of water use characteristics were evaluated: water use efficiency (WUE), intrinsic WUE and marginal water cost of carbon gain (∂E/∂A) estimated, respectively, as ratios of assimilation (A) to transpiration (E), of A to stomatal conductance (gs) and of sensitivities of E and A with variation in gs. Only the measure ∂E/∂A estimates water use strategy in a way that integrates carbon gain relative to water use under varying environmental conditions across scales from leaves to communities. This insight provides updated and simplified ways of estimating ∂E/∂A and adds depth to understanding ways that plants balance water expenditure against carbon gain, uniquely providing a mechanistic means of predicting water use characteristics under changing environmental scenarios. [ABSTRACT FROM AUTHOR]

Forest disturbances increase the proportion of fast‐growing tree species compared to slow‐growing ones. To understand their relative capacity for carbon uptake and their vulnerability to climate change, and to represent those differences in Earth system models, it is necessary to characterise the physiological differences in their leaf‐level control of water use efficiency and carbon assimilation. We used wood density as a proxy for the fast‐slow growth spectrum and tested the assumption that trees with a low wood density (LWD) have a lower water‐use efficiency than trees with a high wood density (HWD). We selected 5 LWD tree species and 5 HWD tree species growing in the same location in an Amazonian tropical forest and measured in situ steady‐state gas exchange on top‐of‐canopy leaves with parallel sampling and measurement of leaf mass area and leaf nitrogen content. We found that LWD species invested more nitrogen in photosynthetic capacity than HWD species, had higher photosynthetic rates and higher stomatal conductance. However, contrary to expectations, we showed that the stomatal control of the balance between transpiration and carbon assimilation was similar in LWD and HWD species and that they had the same dark respiration rates. Summary Statement: We measured leaf gas exchange in the central Amazon to inform representation of tropical forest physiology in Earth system models. Contrary to expectations, we found no difference in stomatal control of leaf water use efficiency between early and late successional species. [ABSTRACT FROM AUTHOR]

Vapour pressure deficit (VPD) plays a crucial role in regulating plant carbon and water fluxes due to its influence on stomatal behaviour and transpiration. Yet, characterising stomatal responses of the whole plant to VPD remains challenging due to methodological limitations. Here, we develop a novel method for in situ assessment of whole‐plant stomatal responses (gc) to VPD in the herbaceous plant Tanacetum cinerariifolium. To do this, we examine the relationship between daytime VPD and the corresponding soil–stem water potential gradient (ΔΨ) monitored using the optical dendrometry in well‐hydrated plants under nonlimiting light in both glasshouse and field conditions. In glasshouse plants, ΔΨ increased proportionally with the VPD up to a threshold of 1.53 kPa, beyond which the slope decreased, suggesting a two‐phase response in gc. This pattern aligned with corresponding gravimetrically measured gc behaviour, which also showed a decline when VPD exceeded a similar threshold. This response was then compared with that of field plants monitored using the optical dendrometry technique over a growing season under naturally variable VPD conditions and nonlimiting light and water supply. Field plants exhibited a similar threshold‐type response to VPD but were more sensitive than glasshouse individuals with a VPD threshold of 0.74 kPa. The results showed that whole‐plant gc responses to VPD can be characterised optically in T. cinerariifolium, introducing a new tool for the monitoring and characterisation of stomatal behaviour in situ. Summary statement: Vapour pressure deficit (VPD) plays a crucial role in regulating plant carbon and water fluxes. Yet, characterising whole‐plant stomatal responses (gc) to VPD remains challenging due to technical limitations. Here, we develop a novel method for in situ assessment of gc to VPD in the herbaceous plant Tanacetum cinerariifolium. [ABSTRACT FROM AUTHOR]

The efficiency‐safety tradeoff has been thoroughly investigated in plants, especially concerning their capacity to transport water and avoid embolism. Stomatal regulation is a vital plant behaviour to respond to soil and atmospheric water limitation. Recently, a stomatal efficiency‐safety tradeoff was reported where plants with higher maximum stomatal conductance (gmax) exhibited greater sensitivity to stomatal closure during soil drying, that is, less negative leaf water potential at 50% gmax (ψgs50). However, the underlying mechanism of this gmax‐ψgs50 tradeoff remains unknown. Here, we utilized a soil‐plant hydraulic model, in which stomatal closure is triggered by nonlinearity in soil‐plant hydraulics, to investigate such tradeoff. Our simulations show that increasing gmax is aligned with less negative ψgs50. Plants with higher gmax (also higher transpiration) require larger quantities of water to be moved across the rhizosphere, which results in a precipitous decrease in water potential at the soil‐root interface, and therefore in the leaves. We demonstrated that the gmax‐ψgs50 tradeoff can be predicted based on soil‐plant hydraulics, and is impacted by plant hydraulic properties, such as plant hydraulic conductance, active root length and embolism resistance. We conclude that plants may therefore adjust their growth and/or their hydraulic properties to adapt to contrasting habitats and climate conditions. Summary Statement: The gmax‐ψgs50 tradeoff can be predicted based on soil‐plant hydraulics and is impacted by plant hydraulic properties, such as plant hydraulic conductance, active root length and embolism resistance. [ABSTRACT FROM AUTHOR]

Summary: Global nocturnal temperatures are rising more rapidly than daytime temperatures and have a large effect on crop productivity. In particular, stomatal conductance at night (gsn) is surprisingly poorly understood and has not been investigated despite constituting a significant proportion of overall canopy water loss.Here, we present the results of 3 yr of field data using 12 spring Triticum aestivum genotypes which were grown in NW Mexico and subjected to an artificial increase in night‐time temperatures of 2°C.Under nocturnal heating, grain yields decreased (1.9% per 1°C) without significant changes in daytime leaf‐level physiological responses. Under warmer nights, there were significant differences in the magnitude and decrease in gsn, values of which were between 9 and 33% of daytime rates while respiration appeared to acclimate to higher temperatures. Decreases in grain yield were genotype‐specific; genotypes categorised as heat tolerant demonstrated some of the greatest declines in yield in response to warmer nights.We conclude the essential components of nocturnal heat tolerance in wheat are uncoupled from resilience to daytime temperatures, raising fundamental questions for physiological breeding. Furthermore, this study discusses key physiological traits such as pollen viability, root depth and irrigation type may also play a role in genotype‐specific nocturnal heat tolerance. [ABSTRACT FROM AUTHOR]

The 18O enrichment (Δ18O) of leaf water affects the Δ18O of photosynthetic products such as sucrose, generating an isotopic archive of plant function and past climate. However, uncertainty remains as to whether leaf water compartmentation between photosynthetic and nonphotosynthetic tissue affects the relationship between Δ18O of bulk leaf water (Δ18OLW) and leaf sucrose (Δ18OSucrose). We grew Lolium perenne (a C3 grass) in mesocosm‐scale, replicated experiments with daytime relative humidity (50% or 75%) and CO2 level (200, 400 or 800 μmol mol−1) as factors, and determined Δ18OLW, Δ18OSucrose and morphophysiological leaf parameters, including transpiration (Eleaf), stomatal conductance (gs) and mesophyll conductance to CO2 (gm). The Δ18O of photosynthetic medium water (Δ18OSSW) was estimated from Δ18OSucrose and the equilibrium fractionation between water and carbonyl groups (εbio). Δ18OSSW was well predicted by theoretical estimates of leaf water at the evaporative site (Δ18Oe) with adjustments that correlated with gas exchange parameters (gs or total conductance to CO2). Isotopic mass balance and published work indicated that nonphotosynthetic tissue water was a large fraction (~0.53) of bulk leaf water. Δ18OLW was a poor proxy for Δ18OSucrose, mainly due to opposite Δ18O responses of nonphotosynthetic tissue water (Δ18Onon‐SSW) relative to Δ18OSSW, driven by atmospheric conditions. Summary Statement: The 18O enrichment of water in the two functionally distinct water pools of grasses—photosynthetic and nonphotosynthetic tissue water—revealed opposite responses to atmospheric humidity in Lolium perenne. 18O enrichment of sucrose was close to equilibrium with evaporative site enrichment. [ABSTRACT FROM AUTHOR]

Summary: Polyploidy plays an important role in plant evolution, but knowledge of its eco‐physiological consequences, such as of the putatively enlarged stomata of polyploid plants, remains limited. Enlarged stomata should disadvantage polyploids at low CO2 concentrations (namely during the Quaternary glacial periods) because larger stomata are viewed as less effective at CO2 uptake.We observed the growth, physiology, and epidermal cell features of 15 diploids and their polyploid relatives cultivated under glacial, present‐day, and potential future atmospheric CO2 concentrations (200, 400, and 800 ppm respectively).We demonstrated some well‐known polyploidy effects, such as faster growth and larger leaves, seeds, stomata, and other epidermal cells. The stomata of polyploids, however, tended to be more elongated than those of diploids, and contrary to common belief, they had no negative effect on the CO2 uptake capacity of polyploids. Moreover, polyploids grew comparatively better than diploids even at low, glacial CO2 concentrations. Higher polyploids with large genomes also showed increased operational stomatal conductance and consequently, a lower water‐use efficiency.Our results point to a possible decrease in growth superiority of polyploids over diploids in a current and future high CO2 climatic scenarios, as well as the possible water and/or nutrient dependency of higher polyploids. [ABSTRACT FROM AUTHOR]

Chilling can decrease stomatal sensitivity to abscisic acid (ABA) in some legumes, although hormonal mechanisms involved are unclear. After evaluating leaf gas exchange of 16 European soybean genotypes at 14°C, 6 genotypes representing the range of response were selected. Further experiments combined low (L, 14°C) and high (H, 24°C) temperature exposure from sowing until the unifoliate leaf was visible and L or H temperature until full leaf expansion, to impose four temperature treatments: LL, LH, HL, and HH. Prolonged chilling (LL) substantially decreased leaf water content but increased leaf ethylene evolution and foliar concentrations of the ethylene precursor 1‐aminocyclopropane‐1‐carboxylic acid, indole‐3‐acetic acid, ABA and jasmonic acid. Across genotypes, photosynthesis linearly increased with stomatal conductance (Gs), with photosynthesis of HH plants threefold higher than LL plants at the same Gs. In all treatments except LL, Gs declined with foliar ABA accumulation. Foliar ABA sprays substantially decreased Gs of HH plants, but did not significantly affect LL plants. Thus low temperature compromised stomatal sensitivity to endogenous and exogenous ABA. Applying the ethylene antagonist 1 methyl‐cyclopropene partially reverted excessive stomatal opening of LL plants. Thus, chilling‐induced ethylene accumulation may mediate stomatal insensitivity to ABA, offering chemical opportunities for improving seedling survival in cold environments. Summary statement: Prolonged chilling of soybean increased foliar ethylene evolution, abscisic acid concentrations and stomatal conductance resulting in extremely low leaf water contents. Applying the ethylene antagonist 1‐MCP partially reversed this maladaptive stomatal response. [ABSTRACT FROM AUTHOR]

Plants can detect herbivore‐induced plant volatiles (HIPVs) from their damaged neighbours and respond by enhancing or priming their defenses against future herbivore attack. Plant communication and defense priming by volatile cues has been well documented, however, the extent to which plants are able to perceive and respond to these cues across different environmental contexts remains poorly understood. We investigated how abiotic changes that modulate stomatal conductance and/or defense signalling affect the ability of maize plants to perceive HIPVs and respond by priming their defenses. During light exposure, when stomata were open and conditions allowed for defense signal biosynthesis, the individual compounds indole and (Z)−3‐hexenyl acetate primed maize defenses. Neither compound primed defenses under environmental conditions that closed stomata and/or altered defense signalling. Moreover, plants were not primed when exposed to indole or (Z)−3‐hexenyl acetate in darkness (while stomata were closed) and then subjected to simulated herbivory in the light, to ensure defense induction. The full blend of HIPVs primed maize defenses in light conditions but suppressed defense induction during dark exposure and wounding. These findings indicate that environmental context is important for plant communication and defense priming and suggest that stomata play a role in plant perception of HIPVs. Summary Statement: This work indicates that environmental conditions impact the ability of Zea mays to detect volatile compounds and prime defense responses, lending support to the idea that plant stomata are involved in volatile uptake and perception. [ABSTRACT FROM AUTHOR]

Summary: Little is known about long‐distance mesophyll‐driven signals that regulate stomatal conductance. Soluble and/or vapor‐phase molecules have been proposed. In this study, the involvement of the gaseous signal ethylene in the modulation of stomatal conductance in Arabidopsis thaliana by CO2/abscisic acid (ABA) was examined.We present a diffusion model which indicates that gaseous signaling molecule/s with a shorter/direct diffusion pathway to guard cells are more probable for rapid mesophyll‐dependent stomatal conductance changes. We, therefore, analyzed different Arabidopsis ethylene‐signaling and biosynthesis mutants for their ethylene production and kinetics of stomatal responses to ABA/[CO2]‐shifts.According to our research, higher [CO2] causes Arabidopsis rosettes to produce more ethylene. An ACC‐synthase octuple mutant with reduced ethylene biosynthesis exhibits dysfunctional CO2‐induced stomatal movements. Ethylene‐insensitive receptor (gain‐of‐function), etr1‐1 and etr2‐1, and signaling, ein2‐5 and ein2‐1, mutants showed intact stomatal responses to [CO2]‐shifts, whereas loss‐of‐function ethylene receptor mutants, including etr2‐3;ein4‐4;ers2‐3, etr1‐6;etr2‐3 and etr1‐6, showed markedly accelerated stomatal responses to [CO2]‐shifts. Further investigation revealed a significantly impaired stomatal closure to ABA in the ACC‐synthase octuple mutant and accelerated stomatal responses in the etr1‐6;etr2‐3, and etr1‐6, but not in the etr2‐3;ein4‐4;ers2‐3 mutants.These findings suggest essential functions of ethylene biosynthesis and signaling components in tuning/accelerating stomatal conductance responses to CO2 and ABA. [ABSTRACT FROM AUTHOR]

Summary: Terrestrial biosphere models (TBMs) include the representation of vertical gradients in leaf traits associated with modeling photosynthesis, respiration, and stomatal conductance. However, model assumptions associated with these gradients have not been tested in complex tropical forest canopies.We compared TBM representation of the vertical gradients of key leaf traits with measurements made in a tropical forest in Panama and then quantified the impact of the observed gradients on simulated canopy‐scale CO2 and water fluxes.Comparison between observed and TBM trait gradients showed divergence that impacted canopy‐scale simulations of water vapor and CO2 exchange. Notably, the ratio between the dark respiration rate and the maximum carboxylation rate was lower near the ground than at the top‐of‐canopy, leaf‐level water‐use efficiency was markedly higher at the top‐of‐canopy, and the decrease in maximum carboxylation rate from the top‐of‐canopy to the ground was less than TBM assumptions.The representation of the gradients of leaf traits in TBMs is typically derived from measurements made within‐individual plants, or, for some traits, assumed constant due to a lack of experimental data. Our work shows that these assumptions are not representative of the trait gradients observed in species‐rich, complex tropical forests. [ABSTRACT FROM AUTHOR]

Summary: Plant water use theory has largely been developed within a plant‐performance paradigm that conceptualizes water use in terms of value for carbon gain and that sits within a neoclassical economic framework. This theory works very well in many contexts but does not consider other values of water to plants that could impact their fitness. Here, we survey a range of alternative hypotheses for drivers of water use and stomatal regulation. These hypotheses are organized around relevance to extreme environments, population ecology, and community ecology. Most of these hypotheses are not yet empirically tested and some are controversial (e.g. requiring more agency and behavior than is commonly believed possible for plants). Some hypotheses, especially those focused around using water to avoid thermal stress, using water to promote reproduction instead of growth, and using water to hoard it, may be useful to incorporate into theory or to implement in Earth System Models. [ABSTRACT FROM AUTHOR]

Monitoring and estimating drought impact on plant physiological processes over large regions remains a major challenge for remote sensing and land surface modeling, with important implications for understanding plant mortality mechanisms and predicting the climate change impact on terrestrial carbon and water cycles. The Orbiting Carbon Observatory 3 (OCO‐3), with its unique diurnal observing capability, offers a new opportunity to track drought stress on plant physiology. Using radiative transfer and machine learning modeling, we derive a metric of afternoon photosynthetic depression from OCO‐3 solar‐induced chlorophyll fluorescence (SIF) as an indicator of plant physiological drought stress. This unique diurnal signal enables a spatially explicit mapping of plants' physiological response to drought. Using OCO‐3 observations, we detect a widespread increasing drought stress during the 2020 southwest US drought. Although the physiological drought stress is largely related to the vapor pressure deficit (VPD), our results suggest that plants' sensitivity to VPD increases as the drought intensifies and VPD sensitivity develops differently for shrublands and grasslands. Our findings highlight the potential of using diurnal satellite SIF observations to advance the mechanistic understanding of drought impact on terrestrial ecosystems and to improve land surface modeling. [ABSTRACT FROM AUTHOR]

Plant strategies for coping with water limitation are likely to mediate restoration outcomes in degraded dryland ecosystems. Trade‐offs in traits related to water acquisition and use can intensify in more arid environments, making their effects on dryland restoration success even more salient. However, isolating the effects of drought responses from those of other environmental factors, as well as identifying the specific drought resistance traits that influence restoration success, can be difficult.In the present study, we couple a controlled dry‐down experiment with a cross‐site restoration field trial of out‐planted seedlings (RestoreNet) using a suite of dryland herbaceous plant species from the same seed sources. We quantified interspecific variation in physiological responses to drought, specifically reductions in stomatal conductance (gs) and stem water potential (SWP), by comparing well‐watered control plants to those experiencing decreasing soil moisture.Drought responses of SWP and gs varied independently among species, but both were related to survival in the cross‐site restoration field trial when effect sizes were aggregated across all sites. Responses were consistent with acquisitive water‐use strategies resulting in greater success, where species with greater declines in SWP or weaker declines in gs under drought had greater survival. The correlation between SWP drought response and survival also intensified in sites with lower accumulated precipitation following restoration.Differences among functional groups revealed two different paths to restoration success: forbs that maintain high gs and narrow safety margins to maximize exploitation of moisture pulses before going into drought dormancy, or C4 grasses that maintain efficient water uptake in drying soils while risking cavitation. C3 grass species varied between these two strategies.Synthesis and applications. Taken together, the results of this study and others conducted at RestoreNet sites indicate that while a diversity of physiological responses to drought may exist in dryland plant communities, successfully restoring herbaceous species through out‐planting in degraded conditions is likely to be achieved with species that maximize water uptake via one of two strategies, with tolerance of low SWP being particularly important in the most arid settings. [ABSTRACT FROM AUTHOR]

The vast majority of the energy and substance required for maize growth and development and related metabolic processes come from the photosynthesis of leaves. Over the past 60 years, the amount of solar radiation reaching the earth's surface has decreased significantly, reducing the photosynthetic rate of crops and leading to a significant reduction in yields. However, (1) the effects of shading stress on stomatal characteristics, electron transport efficiency of summer maize leaves and (2) the adaptability of summer maize to low light are still unclear. In order to investigate this problems, maize hybrid Denghai605 (DH605) was used as the experimental materials, and two treatments of shading (shading degree is 60%) and CK (natural light as control) were set in the field. The results showed that shading stress resulted in chloroplast dysplasia, and the overall performance of the two photosystems was reduced by 27.85%. The number of stomata per unit area decreased by 15.6% and the proportion of stomatal opening decreased by 73.1% after shading, which resulted in a significant 27.7% reduction in intercellular CO2 concentration. Shading significantly reduced photosynthetic rate by affecting stomatal characteristics and electron transport, and the former was the main influencing factor. The decrease in carbon assimilation capacity resulted in insufficient assimilate supply and significantly reduced grain allocation ratio, resulting in a significant yield reduction of 57.5%. However, in the low‐light environment, the expression of chlorophyll‐binding protein in the leaf of summer maize was upregulated, the content of Lhcb1 protein of the light harvesting pigment protein complex II (LHCII) was increased, and the content of photosynthetic pigment, especially chlorophyll b, was increased, which increased the absorption and capture of blue light, improved the efficiency of light energy utilization, and was a stress manifestation of summer maize adapting to shading stress. [ABSTRACT FROM AUTHOR]

The páramos are Neotropical alpine tundra‐like ecosystems that play a crucial role as biodiversity hotspots and also act as water sources for the inter‐Andean regions and cities. Improving our understanding of hydrological processes, here evapotranspiration, is crucial, especially in the context of global changes. In páramos, most research have focused on estimating potential evapotranspiration (ETo) using the Penman–Monteith method. Only a few studies have quantified actual evapotranspiration using mostly the Eddy covariance method (EC) or volumetric lysimeters. Importantly, these studies focused only on tussock grass communities, and none have addressed the effect of other plant communities specific to páramos on the actual evapotranspiration of this ecosystem. In this research, portable closed chambers were installed for the first time in a páramo (in Los Nevados National Park, between 3900 and 4100 masl) to quantify actual evapotranspration (ETa) in March and May 2019 in three representative plant communities of the páramo (giant rosettes, shrubs and tussock grasses). The ETa rates measured were then compared with ETo estimated using the Penman–Monteith method. Also, environmental factors of solar radiation, temperature and relative humidity were recorded and their influence on ETa variation was analysed. Our results indicate that ETa daily rates were very low with a high daily variation (0.290 ± 0.266 mm day−1). The shrubs showed higher ETa rates even though differences among communities were not significant. ETo rates calculated via the Penman–Monteith method were significantly higher (1.017 ± 0.468 mm day−1) than those measured using the portable chambers. Portable closed chamber is a promising method to assess ETa at small spatial and time scales and under controlled environment; however, they should be improved to enable ETa measurements on longer time periods. This study confirms the highly variable and low evapotranspiration rate of the páramo vegetation, here confirmed across different plant communities and underlines the importance of solar radiation and temperature, which were positively correlated with evapotranspiration rates. [ABSTRACT FROM AUTHOR]

Summary: Guard cells control the opening of stomatal pores in the leaf surface, with the use of a network of protein kinases and phosphatases. Loss of function of the CBL‐interacting protein kinase 23 (CIPK23) was previously shown to decrease the stomatal conductance, but the molecular mechanisms underlying this response still need to be clarified.CIPK23 was specifically expressed in Arabidopsis guard cells, using an estrogen‐inducible system. Stomatal movements were linked to changes in ion channel activity, determined with double‐barreled intracellular electrodes in guard cells and with the two‐electrode voltage clamp technique in Xenopus oocytes.Expression of the phosphomimetic variant CIPK23T190D enhanced stomatal opening, while the natural CIPK23 and a kinase‐inactive CIPK23K60N variant did not affect stomatal movements. Overexpression of CIPK23T190D repressed the activity of S‐type anion channels, while their steady‐state activity was unchanged by CIPK23 and CIPK23K60N.We suggest that CIPK23 enhances the stomatal conductance at favorable growth conditions, via the regulation of several ion transport proteins in guard cells. The inhibition of SLAC1‐type anion channels is an important facet of this response. [ABSTRACT FROM AUTHOR]

Leaf hydraulic traits characterize plant drought tolerance and responses to climate change. Yet, plant hydraulics are biased towards northern hemisphere woody species. We collected rhizomes of several perennial grass species along a precipitation gradient in eastern Australia and grew them in an experimental pot study to investigate potential trade‐offs between drought tolerance and plant morphology.We measured the following leaf hydraulic traits: the leaf water potential (Ψleaf) at 50% and 88% loss of leaf hydraulic conductance (P50Kleaf and P88Kleaf), the Ψleaf at 50% loss of stomatal conductance (P50gs), leaf turgor loss point (TLP), leaf dry matter content (LDMC), leaf modulus of elasticity (ε), and the slope of the relationship between predawn and midday Ψleaf. We also measured basal area, tiller density, seed head density, root collar diameter, plant height, and aboveground biomass of each individual.As expected, grass species varied widely in leaf‐level drought tolerance, with loss of 88% hydraulic conductance occurring at a Ψleaf ranging from −1.52 to −4.01 MPa. However, all but one species lost leaf turgor, and most reached P50gs before this critical threshold. Taller more productive grass species tended to have drought vulnerable leaves characterized by low LDMC and less negative P88Kleaf. Species with greater tiller production experienced stomatal closure and lost turgor at more negative Ψleaf. Although our sample size was limited, we found no relationships between these species' traits and their climate of origin.Overall, we identified important hydraulic and morphological trade‐offs in Australian grasses that were surprisingly similar to those observed for woody plants: (1) xylem of taller species was less drought tolerant and (2) turgor loss occurs and stomatal closure begins before significant loss of Kleaf. These data build upon a small yet growing field of grass hydraulics and may be informative of species responses to further drought intensification in Australia. Read the free Plain Language Summary for this article on the Journal blog. [ABSTRACT FROM AUTHOR]

Summary: The representation of stomatal regulation of transpiration and CO2 assimilation is key to forecasting terrestrial ecosystem responses to global change. Given its importance in determining the relationship between forest productivity and climate, accurate and mechanistic model representation of the relationship between stomatal conductance (gs) and assimilation is crucial.We assess possible physiological and mechanistic controls on the estimation of the g1 (stomatal slope, inversely proportional to water use efficiency) and g0 (stomatal intercept) parameters, using diurnal gas exchange surveys and leaf‐level response curves of six tropical broadleaf evergreen tree species.g1 estimated from ex situ response curves averaged 50% less than g1 estimated from survey data. While g0 and g1 varied between leaves of different phenological stages, the trend was not consistent among species. We identified a diurnal trend associated with g1 and g0 that significantly improved model projections of diurnal trends in transpiration.The accuracy of modeled gs can be improved by accounting for variation in stomatal behavior across diurnal periods, and between measurement approaches, rather than focusing on phenological variation in stomatal behavior. Additional investigation into the primary mechanisms responsible for diurnal variation in g1 will be required to account for this phenomenon in land‐surface models. [ABSTRACT FROM AUTHOR]

Elevated CO2 (eCO2) reduces the impact of drought, but the mechanisms underlying this effect remain unclear. Therefore, we used a multidisciplinary approach to investigate the interaction of drought and eCO2 in Arabidopsis thaliana leaves. Transcriptome and subsequent metabolite analyses identified a strong induction of the aliphatic glucosinolate (GL) biosynthesis as a main effect of eCO2 in drought‐stressed leaves. Transcriptome results highlighted the upregulation of ABI5 and downregulation of WRKY63 transcription factors (TF), known to enhance and inhibit the expression of genes regulating aliphatic GL biosynthesis (e.g., MYB28 and 29 TFs), respectively. In addition, eCO2 positively regulated aliphatic GL biosynthesis by MYB28/29 and increasing the accumulation of GL precursors. To test the role of GLs in the stress‐mitigating effect of eCO2, we investigated the effect of genetic perturbations of the GL biosynthesis. Overexpression of MYB28, 29 and 76 improved drought tolerance by inducing stomatal closure and maintaining plant turgor, whereas loss of cyp79f genes reduced the stress‐mitigating effect of eCO2 and decreased drought tolerance. Overall, the crucial role of GL metabolism in drought stress mitigation by eCO2 could be a beneficial trait to overcome future climate challenges. Summary Statement: Elevated CO2 can mitigate drought stress, but the underlying mechanisms remain largely unclear. This study identifies the genetic regulation of aliphatic glucosinolate by eCO2 and its role in inducing drought tolerance by improving water relations through their role in stomatal regulation. [ABSTRACT FROM AUTHOR]