Increasing temperature and vapour pressure deficit lead to hydraulic damages in the absence of soil drought

Stomata-Photosynthesis Synergy Mediates Combined Heat and Salt Stress Tolerance in Sugarcane Mutant M4209

Sugarcane (Saccharum officinarum L.) is an economically important long-duration crop which is currently facing concurrent heat waves and soil salinity. The present study evaluates an inducible salt-tolerant sugarcane mutant M4209, developed via radiation-induced mutagenesis of elite check variety Co 86032, under heat (42/30°C; day/night), NaCl (200 mM) or heat + NaCl (HS)-stress conditions. Though heat application significantly improved plant growth and biomass in both genotypes, this beneficial impact was partially diminished in Co 86032 under HS-stress conditions, coinciding with higher Na+ accumulation and lower triacylglycerol levels. Besides, heat broadly equalised the negative impact on NaCl stress in terms of various physiological and biochemical attributes in both the genotypes, indicating its spaciotemporal advantage. The simultaneous up- and downregulation of antagonistic regulators, epidermal patterning factor (EPF) 9 (SoEPF9) and SoEPF2, respectively attributed to the OSD (Open Small Dense) stomatal phenotype in M4209, which resulted into enhanced conductance, transpirational cooling and gaseous influx. This led to improved photoassimilation, which was supported by higher plastidic:nonplastidic lipid ratio, upregulation of SoRCA (Rubisco activase) and better source strength, resulting in overall plant growth enhancement across all the tested stress scenarios. Taken together, the present study emphasised the knowledge-driven harnessing of stomatal-photosynthetic synergy for ensuring global sugarcane productivity, especially under "salt-heat" coupled stress scenarios.

Elevated Growth Temperature Modifies Drought and Shade Responses of Fagus sylvatica Seedlings by Altering Growth, Gas Exchange, Water Relations, and Xylem Function

Climate change is increasing global temperatures and imposing new constraints on tree regeneration, especially in late-successional species exposed to simultaneous drought and low-light conditions. To disentangle the effects of warming from those of atmospheric drought, we conducted a multifactorial growth chamber experiment on Fagus sylvatica seedlings, manipulating temperature (25 °C and +7.5 °C above optimum), soil moisture (well-watered vs. water-stressed), and light intensity (high vs. low), while maintaining constant vapor pressure deficit (VPD). We assessed growth, biomass allocation, leaf gas exchange, water relations, and xylem hydraulic traits. Warming significantly reduced total biomass, leaf area, and water-use efficiency, while increasing transpiration and residual conductance, especially under high light. Under combined warming and drought, seedlings exhibited impaired osmotic adjustment, reduced leaf safety margins, and diminished hydraulic performance. Unexpectedly, warming under shade promoted a resource-acquisitive growth strategy through the production of low-cost leaves. These results demonstrate that elevated temperature, even in the absence of increased VPD, can compromise drought tolerance in beech seedlings and shift their ecological strategies depending on light availability. The findings underscore the need to consider multiple, interacting stressors when evaluating tree regeneration under future climate conditions.

A loss of stomata exposes a critical vulnerability to variable atmospheric humidity in ferns

Stomata confer both benefits and costs to plants, but assessing the magnitude of these effects is challenging. Some ferns have entirely lost stomata on their leaves, providing an opportunity to understand functional limitations associated with the inability to regulate transpiration. Here, we show that the loss of stomata and a massive reduction in xylem tissue investment in a filmy fern (Hymenophyllum flabellatum Labill.) leaves its vascular system exposed to catastrophic failure during relatively small reductions in atmospheric humidity. Hydraulic limitation, together with a sensitivity to fast desiccation, sets a clear lethal vapor pressure deficit threshold. This threshold enables a quantitative prediction of range contraction in H. flabellatum using a simple physical model. According to this threshold and climate projections, H. flabellatum may disappear from most of its native habitat in mainland Australia by 2050.

Inoculation with plant growth-promoting bacteria mitigates the negative impacts of 2 °C warming on the photosynthesis, growth, and nutritional value of a tropical C4 grassland under field conditions

Human-induced climate change is causing Earth's temperature to rise, and models indicate a persistent increase in the next years. Temperature is one of the most important factors regulating the carbon flux of natural and managed ecosystems. In the last decades, the use of plant growth-promoting bacteria in C4 grasses has emerged as an important alternative to alleviate the negative impacts of abiotic factors on plant metabolism, growth, and forage nutritional quality. In this study, we investigated the effects of warming (+2 °C) on the photosynthesis, plant water status, growth, and nutritional quality of a managed pasture of Brachiaria (syn. Urochloa) Mavuno inoculated or not with Azospirillum brasilense and Pseudomonas fluorescens. We evaluated two levels of temperature (ambient and elevated) under two levels of inoculation (inoculated and non-inoculated) in a multifactorial design. Our results showed that inoculation stimulated root growth and increased photosynthetic rates through higher stomatal conductance and improved photosystem II performance, presumably resulting in higher productivity, crude protein content, and forage digestibility with reduced lignin and fiber fraction. Warming increased non-photochemical quenching and electron transport rate in the wet season, but decreased midday maximum quantum efficiency of PSII photochemistry during dry season, relative water content, productivity, and forage quality and digestibility. When inoculated plants developed under a warmer atmosphere, the positive effects of inoculation completely counteract the negative impacts of warming on photosynthesis, growth, nutritional quality, and digestibility, resulting in a pasture with reduced lignin content and improved heating dissipating capacity and digestibility. Our results demonstrated that A. brasilense and P. fluorescens co-inoculation is a sustainable option to fully mitigate the negative impacts of elevated temperature on Mavuno grass pastures. These findings highlight the potential of microbial inoculants in enhancing forage resilience and productivity under climate stress.

A high resolution, gridded product for vapor pressure deficit using Daymet

Vapor pressure deficit (VPD) is a critical variable in assessing drought conditions and evaluating plant water stress. Gridded products of global and regional VPD are not freely available from satellite remote sensing, model reanalysis, or ground observation datasets. We present two versions of the first gridded VPD product for the Continental US and parts of Northern Mexico and Southern Canada (CONUS+) at a 1 km spatial resolution and daily time step. We derived VPD from Daymet maximum daily temperature and average daily vapor pressure and scale the estimates based on (1) climate determined by the Köppen-Geiger classifications and (2) land cover determined by the International Geosphere-Biosphere Programme. Ground-based VPD data from 253 AmeriFlux sites representing different climate and land cover classifications were used to improve the Daymet-derived VPD estimates for every pixel in the CONUS+ grid to produce the final datasets. We evaluated the Daymet-derived VPD against independent observations and reanalysis data. The CONUS+ VPD datasets will aid in investigating disturbances including drought and wildfire, and informing land management strategies.

Unlocking the potential of cacao yield with full sun cultivation

Cacao and chocolate production is a global industry worth around $133 billion. Full sun cultivation is a modern approach aimed at increasing yields. We evaluated six cacao clones (PS 1319, CCN 10, CCN 51, PH 16, SJ 02, and CP 49) grown under full sun conditions to assess their leaf physiology, leaf structure, yield, and yield components. Leaf physiology was measured through seven gas exchange parameters, while leaf structure was analyzed using eight measurements. For fruit and seed, we evaluated seven yield components. The clones showed differences in gas exchange. Clones PH 16 and PS 1319 had higher net photosynthetic rates per unit of leaf area (A), transpiration rates, and lower leaf internal CO2 concentrations. These A high values suggest the clones are well-acclimatized to full sun cultivation. Water availability, nutrient supply, and appropriate plant architecture also contributed to this acclimatization. Under high light intensity, the potential quantum yield of photosystem II indicated no photoinhibition, and adaptations in the photosynthetic apparatus were observed, such as lower pigment concentration in clone PH 16. Clones differed in specific leaf area (SLA) and stomatal density (SD). CCN 51 had a higher SLA, while SJ 02 had a higher SD. A significant negative correlation (-0.89) was found between dry bean yield and leaf-to-air water vapor pressure deficit (VpdL), suggesting that VpdL is a crucial parameter for selecting high-performance clones for fertigated full sun cultivation. Yields ranged from 1,220 kg/ha (CCN 10) to 2,900 kg/ha (CCN 51). Full sun cacao farms have high yield potential due to a combination of cloning, management practices, and adequate water and nutrient availability.

Forest plant indicator values for moisture reflect atmospheric vapour pressure deficit rather than soil water content

Soil moisture shapes ecological patterns and processes, but it is difficult to continuously measure soil moisture variability across the landscape. To overcome these limitations, soil moisture is often bioindicated using community-weighted means of the Ellenberg indicator values of vascular plant species. However, the ecology and distribution of plant species reflect soil water supply as well as atmospheric water demand. Therefore, we hypothesized that Ellenberg moisture values can also reflect atmospheric water demand expressed as a vapour pressure deficit (VPD). To test this hypothesis, we disentangled the relationships among soil water content, atmospheric vapour pressure deficit, and Ellenberg moisture values in the understory plant communities of temperate broadleaved forests in central Europe. Ellenberg moisture values reflected atmospheric VPD rather than soil water content consistently across local, landscape, and regional spatial scales, regardless of vegetation plot size, depth as well as method of soil moisture measurement. Using in situ microclimate measurements, we discovered that forest plant indicator values for moisture reflect an atmospheric VPD rather than soil water content. Many ecological patterns and processes correlated with Ellenberg moisture values and previously attributed to soil water supply are thus more likely driven by atmospheric water demand.

Dry inside: progressive unsaturation within leaves with increasing vapour pressure deficit affects estimation of key leaf gas exchange parameters

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 (ei), we inferred ei from oxygen isotope discrimination of CO2 and water vapour. ei 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 ei overestimated the extent of reductions in stomatal conductance and CO2 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 ei was considered. We suggest that the possibility of unsaturated ei should not be overlooked in measurements related to leaf gas exchange and in stomatal models, especially at high VPD.

The stomatal response to vapor pressure deficit drives the apparent temperature response of photosynthesis in tropical forests

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.

Vapor-pressure-deficit-controlled temperature response of photosynthesis in tropical trees

Rising temperatures can affect stomatal and nonstomatal control over photosynthesis, through stomatal closure in response to increasing vapor pressure deficit (VPD), and biochemical limitations, respectively. To explore the independent effects of temperature and VPD, we conducted leaf-level temperature-response measurements while controlling VPD on three tropical tree species. Photosynthesis and stomatal conductance consistently decreased with increasing VPD, whereas photosynthesis typically responded weakly to changes in temperature when a stable VPD was maintained during measurements, resulting in wide parabolic temperature-response curves. We have shown that the negative effect of temperature on photosynthesis in tropical forests across ecologically important temperature ranges does not stem from direct warming effects on biochemical processes but from the indirect effect of warming, through changes in VPD. Understanding the acclimation potential of tropical trees to elevated VPD will be critical to anticipate the consequences of global warming for tropical forests.

In vivo X-ray microtomography locally affects stem radial growth with no immediate physiological impact

Microcomputed tomography (µCT) is a nondestructive X-ray imaging method used in plant physiology to visualize in situ plant tissues that enables assessments of embolized xylem vessels. Whereas evidence for X-ray-induced cellular damage has been reported, the impact on plant physiological processes such as carbon (C) uptake, transport, and use is unknown. Yet, these damages could be particularly relevant for studies that track embolism and C fluxes over time. We examined the physiological consequences of µCT scanning for xylem embolism over 3 mo by monitoring net photosynthesis (Anet), diameter growth, chlorophyll (Chl) concentration, and foliar nonstructural carbohydrate (NSC) content in 4 deciduous tree species: hedge maple (Acer campestre), ash (Fraxinus excelsior), European hornbeam (Carpinus betulus), and sessile oak (Quercus petraea). C transport from the canopy to the roots was also assessed through 13C labeling. Our results show that monthly X-ray application did not impact foliar Anet, Chl, NSC content, and C transport. Although X-ray effects did not vary between species, the most pronounced impact was observed in sessile oak, marked by stopped growth and stem deformations around the irradiated area. The absence of adverse impacts on plant physiology for all the tested treatments indicates that laboratory-based µCT systems can be used with different beam energy levels and doses without threatening the integrity of plant physiology within the range of tested parameters. However, the impacts of repetitive µCT on the stem radial growth at the irradiated zone leading to deformations in sessile oak might have lasting implications for studies tracking plant embolism in the longer-term.

The impacts of rising vapour pressure deficit in natural and managed ecosystems

An exponential rise in the atmospheric vapour pressure deficit (VPD) is among the most consequential impacts of climate change in terrestrial ecosystems. Rising VPD has negative and cascading effects on nearly all aspects of plant function including photosynthesis, water status, growth and survival. These responses are exacerbated by land-atmosphere interactions that couple VPD to soil water and govern the evolution of drought, affecting a range of ecosystem services including carbon uptake, biodiversity, the provisioning of water resources and crop yields. However, despite the global nature of this phenomenon, research on how to incorporate these impacts into resilient management regimes is largely in its infancy, due in part to the entanglement of VPD trends with those of other co-evolving climate drivers. Here, we review the mechanistic bases of VPD impacts at a range of spatial scales, paying particular attention to the independent and interactive influence of VPD in the context of other environmental changes. We then evaluate the consequences of these impacts within key management contexts, including water resources, croplands, wildfire risk mitigation and management of natural grasslands and forests. We conclude with recommendations describing how management regimes could be altered to mitigate the otherwise highly deleterious consequences of rising VPD.

High heat tolerance, evaporative cooling, and stomatal decoupling regulate canopy temperature and their safety margins in three European oak species

Heatwaves and soil droughts are increasing in frequency and intensity, leading many tree species to exceed their thermal thresholds, and driving wide-scale forest mortality. Therefore, investigating heat tolerance and canopy temperature regulation mechanisms is essential to understanding and predicting tree vulnerability to hot droughts. We measured the diurnal and seasonal variation in leaf water potential (Ψ), gas exchange (photosynthesis Anet and stomatal conductance gs), canopy temperature (Tcan), and heat tolerance (leaf critical temperature Tcrit and thermal safety margins TSM, i.e., the difference between maximum Tcan and Tcrit) in three oak species in forests along a latitudinal gradient (Quercus petraea in Switzerland, Quercus ilex in France, and Quercus coccifera in Spain) throughout the growing season. Gas exchange and Ψ of all species were strongly reduced by increased air temperature (Tair) and soil drying, resulting in stomatal closure and inhibition of photosynthesis in Q. ilex and Q. coccifera when Tair surpassed 30°C and soil moisture dropped below 14%. Across all seasons, Tcan was mainly above Tair but increased strongly (up to 10°C > Tair) when Anet was null or negative. Although trees endured extreme Tair (up to 42°C), positive TSM were maintained during the growing season due to high Tcrit in all species (average Tcrit of 54.7°C) and possibly stomatal decoupling (i.e., Anet ≤0 while gs >0). Indeed, Q. ilex and Q. coccifera trees maintained low but positive gs (despite null Anet), decreasing Ψ passed embolism thresholds. This may have prevented Tcan from rising above Tcrit during extreme heat. Overall, our work highlighted that the mechanisms behind heat tolerance and leaf temperature regulation in oak trees include a combination of high evaporative cooling, large heat tolerance limits, and stomatal decoupling. These processes must be considered to accurately predict plant damages, survival, and mortality during extreme heatwaves.

Simulating atmospheric drought: Silica gel packets dehumidify mesocosm microclimates

As global temperatures rise, droughts are becoming more frequent and severe. To predict how drought might affect plant communities, ecologists have traditionally designed drought experiments with controlled watering regimes and rainout shelters. Both treatments have proven effective for simulating soil drought. However, neither are designed to directly modify atmospheric drought. Here, we detail the efficacy of a silica gel atmospheric drought treatment in outdoor mesocosms with and without a co-occurring soil drought treatment. At California State University, Los Angeles, we monitored relative humidity, temperature, and vapor pressure deficit every 10 min for 5 months in bare-ground, open-top mesocosms treated with soil drought (reduced watering) and/or atmospheric drought (silica dehumidification packets suspended 12 cm above soil). We found that silica packets dehumidified these mesocosm microclimates most effectively (-5% RH) when combined with reduced soil water, regardless of the ambient humidity levels of the surrounding air. Further, packets increased microclimate vapor pressure deficit most effectively (+0.4 kPa) when combined with reduced soil water and ambient air temperatures above 20°C. Finally, packets simulated atmospheric drought most consistently when replaced within 3 days of deployment. Our results demonstrate the use of silica packets as effective dehumidification agents in outdoor drought experiments. We emphasize that incorporating atmospheric drought in existing soil drought experiments can improve our understandings of the ecological impacts of drought.

Impacts of elevated temperature and vapour pressure deficit on leaf gas exchange and plant growth across six tropical rainforest tree species

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.

Physiological Responses of C4 Perennial Bioenergy Grasses to Climate Change: Causes, Consequences, and Constraints

C4 perennial bioenergy grasses are an economically and ecologically important group whose responses to climate change will be important to the future bioeconomy. These grasses are highly productive and frequently possess large geographic ranges and broad environmental tolerances, which may contribute to the evolution of ecotypes that differ in physiological acclimation capacity and the evolution of distinct functional strategies. C4 perennial bioenergy grasses are predicted to thrive under climate change-C4 photosynthesis likely evolved to enhance photosynthetic efficiency under stressful conditions of low [CO2], high temperature, and drought-although few studies have examined how these species will respond to combined stresses or to extremes of temperature and precipitation. Important targets for C4 perennial bioenergy production in a changing world, such as sustainability and resilience, can benefit from combining knowledge of C4 physiology with recent advances in crop improvement, especially genomic selection.

The sensitivity of root water uptake to cold root temperature follows species-specific upper elevational distribution limits of temperate tree species

Physiological water stress induced by low root temperatures might contribute to species-specific climatic limits of tree distribution. We investigated the low temperature sensitivity of root water uptake and transport in seedlings of 16 European tree species which reach their natural upper elevation distribution limits at different distances to the alpine treeline. We used 2H-H2O pulse-labelling to quantify the water uptake and transport velocity from roots to leaves in seedlings exposed to constant 15°C, 7°C or 2°C root temperature, but identical aboveground temperatures between 20°C and 25°C. In all species, low root temperatures reduced the water transport rate, accompanied by reduced stem water potentials and stomatal conductance. At 7°C root temperature, the relative water uptake rates among species correlated positively with the species-specific upper elevation limits, indicating an increasingly higher sensitivity to lower root zone temperatures, the lower a species' natural elevational distribution limit. Conversely, 2°C root temperature severely inhibited water uptake in all species, irrespective of the species' thermal elevational limits. We conclude that low temperature-induced hydraulic constraints contribute to the cold distribution limits of temperate tree species and are a potential physiological cause behind the low temperature limits of plant growth in general.

Twenty years of irrigation acclimation is driven by denser canopies and not by plasticity in twig- and needle-level hydraulics in a Pinus sylvestris forest

Climate change is predicted to increase atmospheric vapor pressure deficit, exacerbating soil drought, and thus enhancing tree evaporative demand and mortality. Yet, few studies have addressed the longer-term drought acclimation strategy of trees, particularly the importance of morphological versus hydraulic plasticity. Using a long-term (20 years) irrigation experiment in a natural forest, we investigated the acclimation of Scots pine (Pinus sylvestris) morpho-anatomical traits (stomatal anatomy and crown density) and hydraulic traits (leaf water potential, vulnerability to cavitation (Ψ50), specific hydraulic conductivity (Ks), and tree water deficit) to prolonged changes in soil moisture. We found that low water availability reduced twig water potential and increased tree water deficit during the growing season. Still, the trees showed limited adjustments in most branch-level hydraulic traits (Ψ50 and Ks) and needle anatomy. In contrast, trees acclimated to prolonged irrigation by increasing their crown density and hence the canopy water demand. This study demonstrates that despite substantial canopy adjustments, P. sylvestris may be vulnerable to extreme droughts because of limited adjustment potential in its hydraulic system. While sparser canopies reduce water demand, such shifts take decades to occur under chronic water deficits and might not mitigate short-term extreme drought events.

Restricted internal diffusion weakens transpiration-photosynthesis coupling during heatwaves: Evidence from leaf carbonyl sulphide exchange

Increasingly frequent and intense heatwaves threaten ecosystem health in a warming climate. However, plant responses to heatwaves are poorly understood. A key uncertainty concerns the intensification of transpiration when heatwaves suppress photosynthesis, known as transpiration-photosynthesis decoupling. Field observations of such decoupling are scarce, and the underlying physiological mechanisms remain elusive. Here, we use carbonyl sulphide (COS) as a leaf gas exchange tracer to examine potential mechanisms leading to transpiration-photosynthesis decoupling on a coast live oak in a southern California woodland in spring 2013. We found that heatwaves suppressed both photosynthesis and leaf COS uptake but increased transpiration or sustained it at non-heatwave levels throughout the day. Despite statistically significant decoupling between transpiration and photosynthesis, stomatal sensitivity to environmental factors did not change during heatwaves. Instead, midday photosynthesis during heatwaves was restricted by internal diffusion, as indicated by the lower internal conductance to COS. Thus, increased evaporative demand and nonstomatal limitation to photosynthesis act jointly to decouple transpiration from photosynthesis without altering stomatal sensitivity. Decoupling offered limited potential cooling benefits, questioning its effectiveness for leaf thermoregulation in xeric ecosystems. We suggest that adding COS to leaf and ecosystem flux measurements helps elucidate diverse physiological mechanisms underlying transpiration-photosynthesis decoupling.

Combining PSII photochemistry and hydraulics improves predictions of photosynthesis and water use from mild to lethal drought

Rising temperatures and increases in drought negatively impact the efficiency and sustainability of both agricultural and forest ecosystems. Although hydraulic limitations on photosynthesis have been extensively studied, a solid understanding of the links between whole plant hydraulics and photosynthetic processes at the cellular level under changing environmental conditions is still missing, hampering our predictive power for plant mortality. Here, we examined plant hydraulic traits and CO2 assimilation rate under progressive water limitation by implementing Photosystem II (PSII) dynamics with a whole plant process model (TREES). The photosynthetic responses to plant water status were parameterized based on measurements of chlorophyll a fluorescence, gas exchange and water potential for Brassica rapa (R500) grown in a greenhouse under fully watered to lethal drought conditions. The updated model significantly improved predictions of photosynthesis, stomatal conductance and leaf water potential. TREES with PSII knowledge predicted a larger hydraulic safety margin and a decrease in percent loss of conductivity. TREES predicted a slower decrease in leaf water potential, which agreed with measurements. Our results highlight the pressing need for incorporating PSII drought photochemistry into current process models to capture cross-scale plant water dynamics from cell to whole plant level.

Uncoupling of stomatal conductance and photosynthesis at high temperatures: mechanistic insights from online stable isotope techniques

The strong covariation of temperature and vapour pressure deficit (VPD) in nature limits our understanding of the direct effects of temperature on leaf gas exchange. Stable isotopes in CO2 and H2 O vapour provide mechanistic insight into physiological and biochemical processes during leaf gas exchange. We conducted combined leaf gas exchange and online isotope discrimination measurements on four common European tree species across a leaf temperature range of 5-40°C, while maintaining a constant leaf-to-air VPD (0.8 kPa) without soil water limitation. Above the optimum temperature for photosynthesis (30°C) under the controlled environmental conditions, stomatal conductance (gs ) and net photosynthesis rate (An ) decoupled across all tested species, with gs increasing but An decreasing. During this decoupling, mesophyll conductance (cell wall, plasma membrane and chloroplast membrane conductance) consistently and significantly decreased among species; however, this reduction did not lead to reductions in CO2 concentration at the chloroplast surface and stroma. We question the conventional understanding that diffusional limitations of CO2 contribute to the reduction in photosynthesis at high temperatures. We suggest that stomata and mesophyll membranes could work strategically to facilitate transpiration cooling and CO2 supply, thus alleviating heat stress on leaf photosynthetic function, albeit at the cost of reduced water-use efficiency.

Interactions between beech and oak seedlings can modify the effects of hotter droughts and the onset of hydraulic failure

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.

Plant-plant interactions can mitigate (or exacerbate) hot drought impacts

This article is a Commentary on Mas et al. (2024), 241: 1021–1034.

Atmospheric drying and soil drying: Differential effects on grass community composition

Global surface temperatures are projected to increase in the future; this will modify regional precipitation regimes and increase global atmospheric drying. Despite many drought studies examining the consequences of reduced precipitation, there are few experimental studies exploring plant responses to atmospheric drying via relative humidity and vapor pressure deficit (VPD). We examined eight native California perennial grass species grown in pots in a greenhouse in Los Angeles, California for 34 weeks. All pots were well-watered for 21 weeks, at which point we reduced watering to zero and recorded daily growth and dormancy for 3 weeks. We used this information to better understand the drought tolerance of our species in a larger soil drying × atmospheric drying experiment. In this larger experiment, we grew all eight species together in outdoor mesocosms and measured changes in community composition after 4 years of growth. Soil drying in our small pot experiment mirrored compositional shifts in the larger experiment. Namely, our most drought-tolerant species in our pot experiment was Poa secunda, due to a summer dormancy strategy. Similarly, the grass community shifted toward P. secunda in the driest soils as P. secunda was mostly unaffected by either soil drying or atmospheric drying. We found that some species responded strongly to soil drying (Elymus glaucus, Festuca idahoensis, and Hordeum b. californicum), while others responded strongly to atmospheric drying (Bromus carinatus and Stipa cernua). As result, community composition shifted in different and interacting ways in response to soil drying, atmospheric drying, and their combination. Further study of community responses to increasing atmospheric aridity is an essential next step to predicting the future consequences of climate change.

Simulating atmospheric drought: Silica gel packets dehumidify mesocosm microclimates

1. As global temperatures rise, droughts are becoming more frequent and severe. To predict how drought might affect plant communities, ecologists have traditionally designed experiments with controlled watering regimes and rainout shelters. Both treatments have proven effective for simulating soil drought. However, neither are designed to directly modify atmospheric drought. 2. Here, we detail the efficacy of a silica gel atmospheric drought treatment in outdoor mesocosms with and without a cooccurring soil drought treatment. At California State University, Los Angeles, we monitored relative humidity (RH), temperature, and vapor pressure deficit (VPD) every 10 minutes for five months in a bare-ground experiment featuring mesocosms treated with soil drought (reduced watering) and/or atmospheric drought (silica packets suspended 12 cm above soil). 3. We found that silica packets dehumidified these microclimates most effectively (-5% RH) when combined with reduced soil water, regardless of the ambient humidity levels of the surrounding air. Further, packets increased microclimate VPD most effectively (+0.4 kPa) when combined with reduced soil water and ambient air temperatures above 20°C. Finally, packets simulated atmospheric drought most consistently when replaced within three days of deployment. 4. Our results demonstrate the use of silica packets as effective dehumidification agents in outdoor drought experiments. We emphasize that incorporating atmospheric drought in existing soil drought experiments can improve our understandings of the ecological impacts of drought.

Seasonal and Daily Xylem Radius Variations in Scots Pine Are Closely Linked to Environmental Factors Affecting Transpiration

Seasonal and daily radius variations in the xylem (XRV) and inner bark (IBV) of mature Scots pine trees (Pinus sylvestris) were determined during April 2019-October 2021 at a drought-prone inner alpine site (c. 750 m asl; Tyrol, Austria) by applying point dendrometers. XRVs were also related to environmental factors to evaluate the drivers of XRV during the growing season. XRV records revealed that the xylem width (i) started to shrink around the onset of radial stem growth in April, (ii) consistently decreased by c. 50 µm at the time when air temperature (T) and vapor pressure deficit (VPD) reached their maximum in late June through mid-July, and (iii) recovered until November/December. Although in daily cycles of radius variations XRV preceded IBV by about two hours and the daily amplitude of XRV was about 1/10 that of IBV, XRV and IBV (seasonal trends removed) were closely linked (ρ = 0.755; p < 0.001), indicating tight hydraulic coupling between these tissues. Furthermore, the daily amplitude of XRV was linearly and closely related to daily maximum T (ρ = 0.802; p < 0.001), mean daily solar radiation (ρ = 0.809; p < 0.001), and non-linearly related to daily maximum VPD (R2= 0.837; p < 0.001), indicating that the xylem of Pinus sylvestris reacts like a transpiration-driven passive hydraulic system.

The dynamic multi-functionality of leaf water transport outside the xylem

A surge of papers have reported low leaf vulnerability to xylem embolism during drought. Here, we focus on the less studied, and more sensitive, outside-xylem leaf hydraulic responses to multiple internal and external conditions. Studies of 34 species have resolved substantial vulnerability to dehydration of the outside-xylem pathways, and studies of leaf hydraulic responses to light also implicate dynamic outside-xylem responses. Detailed experiments suggest these dynamic responses arise at least in part from strong control of radial water movement across the vein bundle sheath. While leaf xylem vulnerability may influence leaf and plant survival during extreme drought, outside-xylem dynamic responses are important for the control and resilience of water transport and leaf water status for gas exchange and growth.

A herbaceous species provides insights into drought-driven plant adaptation

This article comments on: Thonglim A, Bortolami G, Delzon S, Larter M, Offringa R, Keurentjes JJB, Smets E, Balazadeh S, Lens F. 2023. Drought response in Arabidopsis displays synergistic coordination between stems and leaves. Journal of Experimental Botany 74, 1004–1021