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

Shifts in resource allocation and aggravation of foliage development restrict the growth rate of Picea abies under increasing atmospheric humidity at high latitudes

Global warming is accompanied by rising precipitation, atmospheric water vapour content, and specific humidity at high latitudes. The rising amount and frequency of rainfall increase the air relative humidity (RH) on a local scale, especially within forest canopies. We studied the effects of artificially elevated environmental humidity (RH and soil moisture) on leaf gas exchange, stomatal responses and growth of young Picea abies trees at the Free Air Humidity Manipulation site in eastern Estonia. Manipulation did not affect the net assimilation rate (An) but affected the stomatal responses, net photosynthetic efficiency (An/ci), and photosynthetic water-use efficiency (WUE). At an elevated air humidity (H), trees exhibited the highest stomatal conductance (gS) and lowest WUE, An/ci, and stomatal sensitivity to air vapour pressure deficit compared to trees growing under ambient conditions (C) and elevated soil moisture (I). Compared to C trees, H trees demonstrated reduced height growth, foliage biomass, and enhanced investments in fine roots referring to worsening soil nutrient availability. Tree growth decline can be explained by (1) foliage development retardation, (2) resource allocation changes, causing a shift in the photosynthetic to non-photosynthetic tissue ratio in favour of the latter, and (3) impaired nutrient uptake from the soil. Changes in stomatal responses make trees grown in a higher RH more vulnerable to weather extremes, also limiting tree growth and forest productivity. Increasing precipitation with concomitant increase in atmospheric humidity at high latitudes counteracts the expected enhancement of tree growth due to climate warming in mesic northern forests.

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.

Acclimation to Warming Shapes Gas Exchange and Metabolic Responses to Heat Shock in Pinus massoniana Seedlings

The sensitivity of physiological and metabolic processes in subtropical trees to temperature remains uncertain, limiting our ability to predict how subtropical forests will acclimate to future climates. In particular, our understanding of gas exchange and metabolic activity responses to warming and heat shocks is quite limited. Here, we exposed Pinus massoniana seedlings to three daytime growth temperatures (25°C, 3°C, and 35°C) for 65 days, followed by a heat shock up to 40°C, then immediately reduced to 25°C, to investigate physiological and metabolic responses. The optimal temperature of photosynthesis (ToptA) did not exhibit a significant shift with warming. Metabolism acclimated to rising growth temperature, resulting in enriched levels of key metabolites (tryptophan, indole, indoleacetate, and o-Phospho-L-serine) and key pathways (tryptophan metabolism). At 25°C, leaf dark respiration (Rd) decreased in warm-grown seedlings. At 40°C (heat shock period), warming reduced Rd, accumulated flavonoid metabolites, and upregulated tryptophan metabolism. After recovery to 25°C, higher growth temperatures decreased the net photosynthetic rate (Asat), accumulated prenol lipid metabolites, and led to enrichment in tryptophan metabolism, flavone, and flavonol biosynthesis pathways. Our findings suggest that photosynthesis in P. massoniana seedlings exhibits limited thermal acclimation, while respiration and metabolism can acclimate under short-term warming. However, acclimation to warming altered both physiological and metabolic responses to heat shock and during the subsequent recovery phase in seedlings.

Stomatal and Hydraulic Redundancy Allows Woody Species to Adapt to Arid Environments

Functional redundancy is considered a pivotal mechanism for maintaining the adaptability of species by preventing the loss of key functions in response to dehydration. However, we still lack a comprehensive understanding of the redundancy of leaf hydraulic systems along aridity gradients. Here, photosynthesis (An), stomatal conductance (gs) and leaf hydraulic conductance (Kleaf) during dehydration were measured in 20 woody species from a range of aridity index (AI) conditions and growing in a common garden to quantify stomatal redundancy (SR), the extent of stomatal opening beyond the optimum required for maximum photosynthesis (Amax), leaf hydraulic redundancy (HR), and the extent of leaf hydraulic conductance (Kleaf) beyond the optimum required for maximum gs (gs-max). The findings revealed that species from arid habitats tended to have higher SRs but lower HRs than did species from humid habitats. The relatively high SR in arid species arose from relatively high gs-max values. The relatively low HR arose from the relatively high Kleaf value at a 5% reduction in gs-max (Kleaf-gs). Our results suggest that greater stomatal redundancy and lower hydraulic redundancy prevent the loss of photosynthesis and water transportation, respectively, and thus might be the key adaptive mechanisms for plants to adapt to drought conditions.

Phytohormonal homeostasis, chloroplast stability, and heat shock transcription pathways related to the adaptability of creeping bentgrass species to heat stress

Creeping bentgrass (Agrostis stolonifera) is a cool-season perennial turfgrass and is frequently utilized in high-quality turf areas. However, a poor to moderate resistance to heat stress limits its promotion and utilization in transitional and worm climate zones. The objectives of the study were to assess the heat tolerance of 18 creeping bentgrass genotypes in the field and to further uncover differential mechanisms of heat tolerance between heat-tolerant and heat-sensitive genotypes. The results showed that 18 different genotypes had different heat tolerance during summer months of 2021 and 2022. Among them, 13 M was identified as the best heat-tolerant cultivar based on the subordinate function values analysis of five physiological indicators. Under controlled growth conditions, heat stress significantly inhibited photosynthetic capacity and also accelerated oxidative damage and chlorophyll (Chl) degradation in both heat-tolerant 13 M and heat-sensitive PA4. However, as compared with heat-sensitive PA4, 13 M maintained significantly higher net photosynthetic rate, water use efficiency, and total antioxidant capacity as well as less Chl degradation and damage to chloroplast ultrastructure. Significantly higher contents of abscisic acid, cytokinin, gibberellin, and polyamines (spermine, spermidine, and putrescine) were also detected in 13 M than that in PA4 in the later stage of heat stress, but 13 M exhibited significantly lower indoleacetic acid content than PA4 during heat stress. In addition, heat-upregulated genes involved in heat shock transcriptional pathways were more pronounced in 13 M than in PA4. These findings indicated that better heat tolerance of 13 M could be related to more stable Chl metabolism, better photosynthetic and antioxidant capacities, endogenous hormonal homeostasis, and more effective heat shock transcriptional pathway. 13 M is more appropriate for planting in transitional and subtropical zones instead of widely used PA4.

Nutrient limitations on photosynthesis: from individual to combinational stresses

Liebig's law of the minimum states that increasing photosynthetic productivity on nutrient-impoverished soils depends on addressing the most limiting nutrient. Research has identified the roles of different mineral nutrients in photosynthetic processes. However, diffusional and biochemical regulation of photosynthesis both feature patterns of cumulative effects that jointly determine photosynthetic capacity. More importantly, responses to multiple nutrient stresses are not simply additive and require a comprehensive understanding of how these stresses interact and impact photosynthetic performance. In this review we highlight key macroelements for photosynthesis - nitrogen, phosphorus, potassium, and magnesium - focusing on their unique functions and interactions in regulating carbon fixation under multiple nutrient deficiencies, with the goal of enhancing crop productivity through balanced nutrient applications.

Unsaturation of Leaf Air Spaces Sheds New Light on the Role of Aquaporins

Aquaporins have long been known to facilitate water transport across membranes inside leaves. However, their role in regulating the water potential (ψ) difference between the cytosol and cell wall has been questioned, as the ψ of the cytosol and cell wall would be in equilibrium under the assumption of water‐vapour‐saturated leaf intercellular air spaces. Recent advances suggest that intercellular air spaces are unsaturated at high vapour pressure deficit (VPD) and that aquaporins that up‐regulate water transport might simultaneously down‐regulate CO2 transport in a competitive manner. Therefore, the currently assumed mechanisms of CO2 and water transport in the mesophyll under varying VPD must be re‐examined. We incorporated the competitive aquaporin hypothesis into the leaf gas exchange pathways with unsaturated intercellular air spaces. We show that the putative competitive control of CO2‐ and water‐facilitating aquaporins is fully effective only when there is a large ψ gradient between the cytosol and cell wall at high VPD. In this context, the down‐regulation of water‐facilitating aquaporins and the up‐regulation of CO2‐facilitating aquaporins could protect the cytosol from drying and maintain the CO2 supply for photosynthesis, respectively. While it remains unclear whether unsaturation drives aquaporin activity or vice versa, we identify research challenges that need to be addressed.

Foliage development and resource allocation determine the growth responses of silver birch (Betula pendula) to elevated environmental humidity

Scenarios for future climate predict an increase in precipitation amounts and frequency of rain events, resulting in higher air humidity and soil moisture at high latitudes, including in northern Europe. We analysed the effects of artificially elevated environmental humidity (air relative humidity and soil moisture) on leaf gas exchange, water relations, growth and phenology of silver birch (Betula pendula) trees growing at the free air humidity manipulation experimental site situated in the hemiboreal vegetation zone, in eastern Estonia, with no occurring water deficit to the trees. The environmental humidity manipulation did not significantly affect the water relations traits but did affect some leaf gas exchange parameters, growth and phenology of the trees. Elevated air humidity (H) did not influence photosynthetic capacity and stomatal conductance, while the trees exhibited higher stomatal sensitivity to leaf-to-air vapour pressure difference compared with the trees at ambient conditions (C) or at elevated soil moisture (I). H trees demonstrated reduced height growth and foliage biomass, increased allocation to stem radial growth and prolonged leaf retention in autumn compared with the C trees. Increased air humidity supports longer leaf retention and growth period, but this does not translate into increased growth parameters at the tree level. The changes in tree growth in response to increasing atmospheric humidity could plausibly be explained by (i) retardation of foliage development and (ii) changes in resource allocation, causing a shift in the ratio of photosynthetic to non-photosynthetic tissues in favour of the latter. Under high atmospheric evaporative demand, higher stomatal sensitivity in H trees induces faster stomatal closure, which may result in carbon starvation. A future rise in atmospheric humidity at high latitudes may lead to reduced tree growth and forest productivity, in contrast to the predicted future of forests.

Conserved responses of water use to evaporative demand in mixed forest across seasons in low subtropical China

The positive correlation between diversity and production has been extensively documented. Given the intrinsic relationship between production and plant water consumption, it was anticipated that mixed forests would exhibit different water use compared to pure forests. In this study, the responses of water use to vapour pressure deficit were analyzed by monitoring the sap flow of Schima superba in both pure and mixed forests, as well as Castanopsis chinensis in mixed forest. Additionally, the relationships among leaf and stem traits were examined by measuring specific leaf area (SLA), N and P concentration per unit leaf mass, leaf δ18O and δ13C and wood density of sapwood (WD) during both wet and dry seasons. The results showed that S. superba demonstrated a comparable regulation of water use during both wet and dry seasons in mixed forest, whereas it exhibited less strict water use regulation during the wet season in comparison to the dry season in pure forest. Regardless of whether the forests were pure or mixed, both leaf δ13C and WD remained consistent across seasons, while there was an increase in SLA during the wet season compared to the dry season for S. superba. There was a different seasonal change in leaf δ18O for S. superba in pure and mixed forests. Water use and leaf economic spectrum may determine the adaptive strategies of coexisting species, and the coexisting tree species in mixed forest exhibited a resource-use differentiation, as indicated by seasonal variations in leaf and stem traits, likely explaining the conserved responses of sap flow to evaporative demand. Our research might provide insights into the impact of tree interaction on water use strategies and the water use-based forest management under current climate change.

Converging functional phenotyping with systems mapping to illuminate the genotype-phenotype associations

Illuminating the phenotype-genotype black box under complex traits is an ambitious goal for researchers. The generation of temporally or spatially phenotypic data today has far outpaced its interpretation, due to their highly dynamic nature depending on the environment and developmental stages. Here, we propose an integrated enviro-pheno-geno functional approach to pinpoint the major challenges of decomposing physiological traits. The strategy first features high-throughput functional physiological phenotyping (FPP) to efficiently acquire phenotypic and environmental data. It then features functional mapping (FM) and the extended systems mapping (SM) to tackle trait dynamics. FM, by modeling traits as continuous functions, can increase the power and efficiency in dissecting the spatiotemporal effects of QTLs. SM could enable reconstruction of a genotype-phenotype map from developmental pathways. We present a recent case study that combines FPP and SM to dissect complex physiological traits. This integrated approach will be an important engine to drive the translation of phenomic big data into genetic gain.

Warming triggers stomatal opening by enhancement of photosynthesis and ensuing guard cell CO2 sensing, whereas higher temperatures induce a photosynthesis-uncoupled response

Plants integrate environmental stimuli to optimize photosynthesis vs water loss by controlling stomatal apertures. However, stomatal responses to temperature elevation and the underlying molecular genetic mechanisms remain less studied. We developed an approach for clamping leaf-to-air vapor pressure difference (VPDleaf) to fixed values, and recorded robust reversible warming-induced stomatal opening in intact plants. We analyzed stomatal temperature responses of mutants impaired in guard cell signaling pathways for blue light, abscisic acid (ABA), CO2, and the temperature-sensitive proteins, Phytochrome B (phyB) and EARLY-FLOWERING-3 (ELF3). We confirmed that phot1-5/phot2-1 leaves lacking blue-light photoreceptors showed partially reduced warming-induced stomatal opening. Furthermore, ABA-biosynthesis, phyB, and ELF3 were not essential for the stomatal warming response. Strikingly, Arabidopsis (dicot) and Brachypodium distachyon (monocot) mutants lacking guard cell CO2 sensors and signaling mechanisms, including ht1, mpk12/mpk4-gc, and cbc1/cbc2 abolished the stomatal warming response, suggesting a conserved mechanism across diverse plant lineages. Moreover, warming rapidly stimulated photosynthesis, resulting in a reduction in intercellular (CO2). Interestingly, further enhancing heat stress caused stomatal opening uncoupled from photosynthesis. We provide genetic and physiological evidence that the stomatal warming response is triggered by increased CO2 assimilation and stomatal CO2 sensing. Additionally, increasing heat stress functions via a distinct photosynthesis-uncoupled stomatal opening pathway.

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.

On the importance of vapor pressure deficit for the determination of the photosynthetic temperature optimum in tropical trees

This article is a Commentary on Slot et al. (2024), 244: 1238–1249.

The response of mesophyll conductance to short-term CO2 variation is related to stomatal conductance

The response of mesophyll conductance (gm) to CO2 plays a key role in photosynthesis and ecosystem carbon cycles under climate change. Despite numerous studies, there is still debate about how gm responds to short-term CO2 variations. Here we used multiple methods and looked at the relationship between stomatal conductance to CO2 (gsc) and gm to address this aspect. We measured chlorophyll fluorescence parameters and online carbon isotope discrimination (Δ) at different CO2 mole fractions in sunflower (Helianthus annuus L.), cowpea (Vigna unguiculata L.), and wheat (Triticum aestivum L.) leaves. The variable J and Δ based methods showed that gm decreased with an increase in CO2 mole fraction, and so did stomatal conductance. There were linear relationships between gm and gsc across CO2 mole fractions. gm obtained from A-Ci curve fitting method was higher than that from the variable J method and was not representative of gm under the growth CO2 concentration. gm could be estimated by empirical models analogous to the Ball-Berry model and the USO model for stomatal conductance. Our results suggest that gm and gsc respond in a coordinated manner to short-term variations in CO2, providing new insight into the role of gm in photosynthesis modelling.

The interplay of short-term mesophyll and stomatal conductance responses under variable environmental conditions

Understanding the short-term responses of mesophyll conductance (gm) and stomatal conductance (gsc) to environmental changes remains a challenging yet central aspect of plant physiology. This review synthesises our current knowledge of these short-term responses, which underpin CO2 diffusion within leaves. Recent methodological advances in measuring gm using online isotopic discrimination and chlorophyll fluorescence have improved our confidence in detecting short-term gm responses, but results need to be carefully evaluated. Environmental factors like vapour pressure deficit and CO2 concentration indirectly impact gm through gsc changes, highlighting some of the complex interactions between the two parameters. Evidence suggests that short-term responses of gm are not, or at least not fully, mechanistically linked to changes in gsc, cautioning against using gsc as a reliable proxy for gm. The overarching challenge lies in unravelling the mechanistic basis of short-term gm responses, which will contribute to the development of accurate models bridging laboratory insights with broader ecological implications. Addressing these gaps in understanding is crucial for refining predictions of gm behaviour under changing environmental conditions.

The poorly-explored stomatal response to temperature at constant evaporative demand

Changes in leaf temperature are known to drive stomatal responses, because the leaf-to-air water vapour gradient (Δw) increases with temperature if ambient vapour pressure is held constant, and stomata respond to changes in Δw. However, the direct response of stomata to temperature (DRST; the response when Δw is held constant by adjusting ambient humidity) has been examined far less extensively. Though the meagre available data suggest the response is usually positive, results differ widely and defy broad generalisation. As a result, little is known about the DRST. This review discusses the current state of knowledge about the DRST, including numerous hypothesised biophysical mechanisms, potential implications of the response for plant adaptation, and possible impacts of the DRST on plant-atmosphere carbon and water exchange in a changing climate.

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.

Elevated aerosol enhances plant water-use efficiency by increasing carbon uptake while reducing water loss

Aerosols could significantly influence ecosystem carbon and water fluxes, potentially altering their interconnected dynamics, typically characterized by water-use efficiency (WUE). However, our understanding of the underlying ecophysiological mechanisms remains limited due to insufficient field observations. We conducted 4-yr measurements of leaf photosynthesis and transpiration, as well as 3-yr measurements of stem growth (SG) and sap flow of poplar trees exposed to natural aerosol fluctuation, to elucidate aerosol's impact on plant WUE. We found that aerosol improved sun leaf WUE mainly because a sharp decline in photosynthetically active radiation (PAR) inhibited its transpiration, while photosynthesis was less affected, as the negative effect induced by declined PAR was offset by the positive effect induced by low leaf vapor pressure deficit (VPDleaf). Conversely, diffuse radiation fertilization (DRF) effect stimulated shade leaf photosynthesis with minimal impact on transpiration, leading to an improved WUE. The responses were further verified by a strong DRF on SG and a decrease in sap flow due to the suppresses in total radiation and VPD. Our field observations indicate that, contrary to the commonly assumed coupling response, carbon uptake and water use exhibited dissimilar reactions to aerosol pollution, ultimately enhancing WUE at the leaf and canopy level.