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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]

Summary: Plant leaf temperatures can differ from ambient air temperatures. A temperature gradient in a gas mixture gives rise to a phenomenon known as thermodiffusion, which operates in addition to ordinary diffusion. Whilst transpiration is generally understood to be driven solely by the ordinary diffusion of water vapour along a concentration gradient, we consider the implications of thermodiffusion for transpiration.We develop a new modelling framework that introduces the effects of thermodiffusion on the transpiration rate, E.By applying this framework, we quantify the proportion of E attributable to thermodiffusion for a set of physiological and environmental conditions, varied over a wide range.Thermodiffusion is found to be most significant (in some cases > 30% of E) when a leaf‐to‐air temperature difference coincides with a relatively small water vapour concentration difference across the boundary layer; a boundary layer conductance that is large as compared to the stomatal conductance; or a relatively low transpiration rate. Thermodiffusion also alters the conditions required for the onset of reverse transpiration, and the rate at which this water vapour uptake occurs. See also the Commentary on this article by Vesala, 243: 1281–1283. [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]

Summary: Multivariate leaf trait correlations are hypothesized to originate from natural selection on carbon economics traits that control lifetime leaf carbon gain, and energy balance traits governing leaf temperatures, physiological rates, and heat injury. However, it is unclear whether macroevolution of leaf traits primarily reflects selection for lifetime carbon gain or energy balance, and whether photosynthetic heat tolerance is coordinated along these axes.To evaluate these hypotheses, we measured carbon economics, energy balance, and photosynthetic heat tolerance traits for 177 species (157 families) in a common garden that minimizes co‐variation of taxa and climate.We observed wide variation in carbon economics, energy balance, and heat tolerance traits. Carbon economics and energy balance (but not heat tolerance) traits were phylogenetically structured, suggesting macroevolution of leaf mass per area and leaf dry matter content reflects selection on carbon gain rather than energy balance. Carbon economics and energy balance traits varied along a common axis orthogonal to heat tolerance traits.Our results highlight a fundamental mismatch in the timescales over which morphological and heat tolerance traits respond to environmental variation. Whereas carbon economics and energy balance traits are constrained by species' evolutionary histories, photosynthetic heat tolerance traits are not and can acclimate readily to leaf microclimates. [ABSTRACT FROM AUTHOR]

Rising temperatures are influencing forests on many scales, with potentially strong variation vertically across forest strata. Using published research and new analyses, we evaluate how microclimate and leaf temperatures, traits, and gas exchange vary vertically in forests, shaping tree, and ecosystem ecology. In closed-canopy forests, upper canopy leaves are exposed to the highest solar radiation and evaporative demand, which can elevate leaf temperature (Tleaf ), particularly when transpirational cooling is curtailed by limited stomatal conductance. However, foliar traits also vary across height or light gradients, partially mitigating and protecting against the elevation of upper canopy Tleaf . Leaf metabolism generally increases with height across the vertical gradient, yet differences in thermal sensitivity across the gradient appear modest. Scaling from leaves to trees, canopy trees have higher absolute metabolic capacity and growth, yet are more vulnerable to drought and damaging Tleaf than their smaller counterparts, particularly under climate change. By contrast, understory trees experience fewer extreme high Tleaf 's but have fewer cooling mechanisms and thus may be strongly impacted by warming under some conditions, particularly when exposed to a harsher microenvironment through canopy disturbance. As the climate changes, integrating the patterns and mechanisms reviewed here into models will be critical to forecasting forest-climate feedback.

Summary: 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 H2O 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. [ABSTRACT FROM AUTHOR]

Summary: Radiation use efficiency (RUE) is a key crop adaptation trait that quantifies the potential amount of aboveground biomass produced by the crop per unit of solar energy intercepted. But it is unclear why elite maize and grain sorghum hybrids differ in their RUE at the crop level. Here, we used a non‐traditional top‐down approach via canopy photosynthesis modelling to identify leaf‐level photosynthetic traits that are key to differences in crop‐level RUE.A novel photosynthetic response measurement was developed and coupled with use of a Bayesian model fitting procedure, incorporating a C4 leaf photosynthesis model, to infer cohesive sets of photosynthetic parameters by simultaneously fitting responses to CO2, light, and temperature.Statistically significant differences between leaf photosynthetic parameters of elite maize and grain sorghum hybrids were found across a range of leaf temperatures, in particular for effects on the quantum yield of photosynthesis, but also for the maximum enzymatic activity of Rubisco and PEPc.Simulation of diurnal canopy photosynthesis predicted that the leaf‐level photosynthetic low‐light response and its temperature dependency are key drivers of the performance of crop‐level RUE, generating testable hypotheses for further physiological analysis and bioengineering applications. [ABSTRACT FROM AUTHOR]

The article titled "Tree Physiology" discusses the impact of climate change on gas exchange in tropical forests. It emphasizes the importance of vapor pressure deficit (VPD) in determining the photosynthetic temperature optimum in tropical trees. The study conducted on 42 species of trees in Panama concludes that VPD is the main driver of photosynthesis reduction in tropical trees, surpassing the influence of temperature alone. The authors highlight the need to consider VPD in accurately predicting carbon stock and forest growth in tropical forests under future climates. This research provides valuable insights into the potential effects of climate change on tropical forests and calls for further investigation in this area. [Extracted from the article]

Summary: Image‐based high‐throughput phenotyping promises the rapid determination of functional traits in large plant populations. However, interpretation of some traits – such as those related to photosynthesis or transpiration rates – is only meaningful if the irradiance absorbed by the measured leaves is known, which can differ greatly between different parts of the same plant and within canopies. No feasible method currently exists to rapidly measure absorbed irradiance in three‐dimensional plants and canopies.We developed a method and protocols to derive absorbed irradiance at any visible part of a canopy with a thermal camera, by fitting a leaf energy balance model to transient changes in leaf temperature. Leaves were exposed to short light pulses (30 s) that were not long enough to trigger stomatal opening but strong enough to induce transient changes in leaf temperature that was proportional to the absorbed irradiance.The method was successfully validated against point measurements of absorbed irradiance in plant species with relatively simple architecture (sweet pepper, cucumber, tomato, and lettuce). Once calibrated, the model was used to produce absorbed irradiance maps from thermograms.Our method opens new avenues for the interpretation of plant responses derived from imaging techniques and can be adapted to existing high‐throughput phenotyping platforms. [ABSTRACT FROM AUTHOR]

Summary: The threat of rising global temperatures may be especially pronounced for low‐latitude, lowland plant species that have evolved under stable climatic conditions. However, little is known about how these species may acclimate to elevated temperatures. Here, we leveraged a strong, steep thermal gradient along a natural geothermal river to assess the ability of woody plants in the Amazon to acclimate to elevated air temperatures.We measured leaf traits in six common tropical woody species along the thermal gradient to investigate whether individuals of these species: acclimate their thermoregulatory traits to maintain stable leaf temperatures despite higher ambient temperatures; acclimate their photosynthetic thermal tolerances to withstand hotter leaf temperatures; and whether acclimation is sufficient to maintain stable leaf thermal safety margins (TSMs) across different growth temperatures.Individuals of three species acclimated their thermoregulatory traits, and three species increased their thermal tolerances with growth temperature. However, acclimation was generally insufficient to maintain constant TSMs. Notwithstanding, leaf health was generally consistent across growth temperatures.Acclimation in woody Amazonian plants is generally too weak to maintain TSMs at high growth temperatures, supporting previous findings that Amazonian plants will be increasingly vulnerable to thermal stress as temperatures rise. See also the Commentary on this article by Posch, 241: 1381–1383. [ABSTRACT FROM AUTHOR]

Summary: Our ability to predict temperature responses of leaf respiration in light and darkness (RL and RDk) is essential to models of global carbon dynamics. While many models rely on constant thermal sensitivity (characterized by Q10), uncertainty remains as to whether Q10 of RL and RDk are actually similar.We measured short‐term temperature responses of RL and RDk in immature and mature leaves of two evergreen tree species, Castanopsis carlesii and Ormosia henry in an open field. RL was estimated by the Kok method, the Yin method and a newly developed Kok‐iterCc method.When estimated by the Yin and Kok‐iterCc methods, RL and RDk had similar Q10 (c. 2.5). The Kok method overestimated both Q10 and the light inhibition of respiration. RL/RDk was not affected by leaf temperature. Acclimation of respiration in summer was associated with a decline in basal respiration but not in Q10 in both species, which was related to changes in leaf nitrogen content between seasons. Q10 of RL and RDk in mature leaves were 40% higher than in immature leaves.Our results suggest similar Q10 values can be used to model RL and RDk while leaf development‐associated changes in Q10 require special consideration in future respiration models. [ABSTRACT FROM AUTHOR]

Summary: We present the Fast Assimilation‐Temperature Response (FAsTeR) method, a new method for measuring plant assimilation‐temperature (AT) response that reduces measurement time and increases data density compared with conventional methods.The FAsTeR method subjects plant leaves to a linearly increasing temperature ramp while taking rapid, nonequilibrium measurements of gas exchange variables. Two postprocessing steps are employed to correct measured assimilation rates for nonequilibrium effects and sensor calibration drift. Results obtained with the new method are compared with those from two conventional stepwise methods.Our new method accurately reproduces results obtained from conventional methods, reduces measurement time by a factor of c. 3.3 (from c. 90 to 27 min), and increases data density by a factor of c. 55 (from c. 10 to c. 550 observations). Simulation results demonstrate that increased data density substantially improves confidence in parameter estimates and drastically reduces the influence of noise.By improving measurement speed and data density, the FAsTeR method enables users to ask fundamentally new kinds of ecological and physiological questions, expediting data collection in short‐field campaigns, and improving the representativeness of data across species in the literature. [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: Leaves are the main photosynthetic organs in plants, and their anatomy is optimized for light interception and gas exchange. Although each species has a characteristic leaf anatomy, which depends on the genotype, leaves also show a large degree of developmental plasticity. Light and temperature regulate leaf development from primordia differentiation to late stages of blade expansion. While the molecular mechanisms of light and temperature signaling have been mostly studied in seedlings, in the latest years, research has focused on leaf development. Here, I will describe the latest work carried out in the environmental regulation of Arabidopsis leaf development, comparing signaling mechanisms between leaves and seedlings, highlighting the new discoveries, and pointing out the most exciting open questions. [ABSTRACT FROM AUTHOR]

Summary: Portable gas exchange analysers provide critical data for understanding plant‐atmosphere carbon and water fluxes, and for parameterising Earth system models that forecast climate change effects and feedbacks.We characterised temperature measurement errors in the Li‐Cor LI‐6400XT and LI‐6800, and estimated downstream errors in derived quantities, including stomatal conductance (gsw) and leaf intercellular CO2 concentration (Ci).The LI‐6400XT exhibited air temperature errors (differences between reported air temperature and air temperature measured near the leaf) up to 7.2°C, leaf temperature errors up to 5.3°C, and relative errors in gsw and Ci that increased as temperatures departed from ambient. This caused errors in leaf‐to‐air temperature relationships, assimilation–temperature curves and CO2 response curves. Temperature dependencies of maximum Rubisco carboxylation rate (Vcmax) and maximum RuBP regeneration rate (Jmax) showed errors of 12% and 35%, respectively. These errors are likely to be idiosyncratic and may differ among machines and environmental conditions. The LI‐6800 exhibited much smaller errors.Earth system model predictions may be erroneous, as much of their parametrisation data were measured on the LI‐6400XT system, depending on the methods used. We make recommendations for minimising errors and correcting data in the LI‐6400XT. We also recommend transitioning to the LI‐6800 for future data collection. [ABSTRACT FROM AUTHOR]

Summary: Although stomata are typically found in greater numbers on the abaxial surface, wheat flag leaves have greater densities on the adaxial surface. We determine the impact of this less common stomatal patterning on gaseous fluxes using a novel chamber that simultaneously measures both leaf surfaces.Using a combination of differential illuminations and CO2 concentrations at each leaf surface, we found that mesophyll cells associated with the adaxial leaf surface have a higher photosynthetic capacity than those associated with the abaxial leaf surface, which is supported by an increased stomatal conductance (driven by differences in stomatal density).When vertical gas flux at the abaxial leaf surface was blocked, no compensation by adaxial stomata was observed, suggesting each surface operates independently. Similar stomatal kinetics suggested some co‐ordination between the two surfaces, but factors other than light intensity played a role in these responses.Higher photosynthetic capacity on the adaxial surface facilitates greater carbon assimilation, along with higher adaxial stomatal conductance, which would also support greater evaporative leaf cooling to maintain optimal leaf temperatures for photosynthesis. Furthermore, abaxial gas exchange contributed c. 50% to leaf photosynthesis and therefore represents an important contributor to overall leaf gas exchange. [ABSTRACT FROM AUTHOR]

This article is a Commentary on Guo et al. (2023), 238: 1004–1018. [ABSTRACT FROM AUTHOR]

Summary: Warming climate increases the risk for harmful leaf temperatures in terrestrial plants, causing heat stress and loss of productivity. The heat sensitivity may be particularly high in equatorial tropical tree species adapted to a thermally stable climate.Thermal thresholds of the photosynthetic system of sun‐exposed leaves were investigated in three tropical montane tree species native to Rwanda with different growth and water use strategies (Harungana montana, Syzygium guineense and Entandrophragma exselsum). Measurements of chlorophyll fluorescence, leaf gas exchange, morphology, chemistry and temperature were made at three common gardens along an elevation/temperature gradient.Heat tolerance acclimated to maximum leaf temperature (Tleaf) across the species. At the warmest sites, the thermal threshold for normal function of photosystem II was exceeded in the species with the highest Tleaf despite their higher heat tolerance. This was not the case in the species with the highest transpiration rates and lowest Tleaf. The results point to two differently effective strategies for managing thermal stress: tolerance through physiological adjustment of leaf osmolality and thylakoid membrane lipid composition, or avoidance through morphological adaptation and transpiratory cooling.More severe photosynthetic heat stress in low‐transpiring montane climax species may result in a competitive disadvantage compared to high‐transpiring pioneer species with more efficient leaf cooling. [ABSTRACT FROM AUTHOR]

Summary: Temperature is a key control over biological activities from the cellular to the ecosystem scales. However, direct, high‐precision measurements of surface temperature of small objects, such as leaves, under field conditions with large variations in ambient conditions remain rare. Contact methods, such as thermocouples, are prone to large errors. The use of noncontact remote‐sensing methods, such as thermal infrared measurements, provides an ideal solution, but their accuracy has been low (c. 2°C) owing to the necessity for corrections for material emissivity and fluctuations in background radiation Lbg.A novel 'dual‐reference' method was developed to increase the accuracy of infrared needle‐leaf surface temperature measurements in the field. It accounts for variations in Lbg and corrects for the systematic camera offset using two reference plates.We accurately captured surface temperature and leaf‐to‐air temperature differences of needle‐leaves in a forest ecosystem with large diurnal and seasonal temperature fluctuations with an uncertainty of ± 0.23°C and ± 0.28°C, respectively.Routine high‐precision leaf temperature measurements even under harsh field conditions, such as demonstrated here, opens the way for investigating a wide range of leaf‐scale processes and their dynamics. [ABSTRACT FROM AUTHOR]

Summary: The drier climates predicted for many regions will result in reduced evaporative cooling, leading to leaf heat stress and enhanced mortality. The extent to which nonevaporative cooling can contribute to plant resilience under these increasingly stressful conditions is not well known at present.Using a novel, high accuracy infrared system for the continuous measurement of leaf temperature in mature trees under field conditions, we assessed leaf‐to‐air temperature differences (ΔTleaf–air) of pine needles during drought.On mid‐summer days, ΔTleaf–air remained < 3°C, both in trees exposed to summer drought and in those provided with supplemental irrigation, which had a more than 10‐fold higher transpiration rate. The nonevaporative cooling in the drought‐exposed trees must be facilitated by low resistance to heat transfer, generating a large sensible heat flux, H. ΔTleaf–air was weakly related to variations in the radiation load and mean wind speed in the lower part of the canopy, but was dependent on canopy structure and within‐canopy turbulence that enhanced the H.Nonevaporative cooling is demonstrated as an effective cooling mechanism in needle‐leaf trees which can be a critical factor in forest resistance to drying climates. The generation of a large H at the leaf scale provides a basis for the development of the previously identified canopy‐scale 'convector effect'. [ABSTRACT FROM AUTHOR]

Summary: The frequency and severity of heatwave events are increasing, exposing species to conditions beyond their physiological limits. Species respond to heatwaves in different ways, however it remains unclear if plants have the adaptive capacity to successfully respond to hotter and more frequent heatwaves.We exposed eight tree populations from two climate regions grown under cool and warm temperatures to repeated heatwave events of moderate (40°C) and extreme (46°C) severity to assess adaptive capacity to heatwaves.Leaf damage and maximum quantum efficiency of photosystem II (Fv/Fm) were significantly impacted by heatwave severity and growth temperatures, respectively; populations from a warm‐origin avoided damage under moderate heatwaves compared to those from a cool‐origin, indicating a degree of local adaptation. We found that plasticity to heatwave severity and repeated heatwaves contributed to enhanced thermal tolerance and lower leaf temperatures, leading to greater thermal safety margins (thermal tolerance minus leaf temperature) in a second heatwave.Notably, while we show that adaptation and physiological plasticity are important factors affecting plant adaptive capacity to thermal stress, plasticity of thermal tolerances and thermal safety margins provides the opportunity for trees to persist among fluctuating heatwave exposures. [ABSTRACT FROM AUTHOR]

Summary: Leaf hydraulic and mesophyll CO2 conductance are both influenced by leaf anatomical traits, however it is poorly understood how the temperature response of these conductances differs between C4 and C3 species with distinct leaf anatomy.This study investigated the temperature response of leaf hydraulic conductance (Kleaf), stomatal (gs) and mesophyll (gm) conductance to CO2, and leaf anatomical traits in phylogenetically related Panicum antidotale (C4) and P. bisulcatum (C3) grasses.The C4 species had lower hydraulic conductance outside xylem (Kox) and Kleaf compared with the C3 species. However, the C4 species had higher gm compared with the C3 species. Traits associated with leaf water movement, Kleaf and Kox, increased with temperature more in the C3 than in the C4 species, whereas traits related to carbon uptake, Anet and gm, increased more with temperature in the C4 than the C3 species.Our findings demonstrate that, in addition to a CO2 concentrating mechanism, outside‐xylem leaf anatomy in the C4 species P. antidotale favours lower water movement through the leaf and stomata that provides an additional advantage for greater leaf carbon uptake relative to water loss with increasing leaf temperature than in the C3 species P. bisulcatum. [ABSTRACT FROM AUTHOR]

Summary: Nocturnal transpiration is widely observed across species and biomes, and may significantly impact global water, carbon, and energy budgets. However, it remains elusive why plants lose water at night and how to model it at large scales.We hypothesized that plants optimize nighttime leaf diffusive conductance (gwn) to balance potential daytime photosynthetic benefits and nocturnal transpiration benefits. We quantified nighttime benefits from respiratory reductions due to evaporative leaf cooling. We described nighttime costs in terms of a reduced carbon gain during the day because of water use at night. We measured nighttime stomatal responses and tested our model with water birch (Betula occidentalis) saplings grown in a glasshouse.The gwn of water birch decreased with drier soil, higher atmospheric CO2, wetter air, lower leaf temperature, and lower leaf respiration rate. Our model predicted all these responses correctly, except for the response of gwn to air humidity. Our results also suggested that the slow decrease in gwn after sunset could be associated with decreasing leaf respiration.The optimality‐based nocturnal transpiration model smoothly integrates with daytime stomatal optimization approaches, and thus has the potential to quantitatively predict nocturnal transpiration across space and time. [ABSTRACT FROM AUTHOR]

Summary: Short‐term temperature response curves of leaf dark respiration (R–T) provide insights into a critical process that influences plant net carbon exchange. This includes how respiratory traits acclimate to sustained changes in the environment.Our study analysed 860 high‐resolution R–T (10–70°C range) curves for: (a) 62 evergreen species measured in two contrasting seasons across several field sites/biomes; and (b) 21 species (subset of those sampled in the field) grown in glasshouses at 20°C : 15°C, 25°C : 20°C and 30°C : 25°C, day : night.In the field, across all sites/seasons, variations in R25 (measured at 25°C) and the leaf T where R reached its maximum (Tmax) were explained by growth T (mean air‐T of 30‐d before measurement), solar irradiance and vapour pressure deficit, with growth T having the strongest influence. R25 decreased and Tmax increased with rising growth T across all sites and seasons with the single exception of winter at the cool‐temperate rainforest site where irradiance was low. The glasshouse study confirmed that R25 and Tmax thermally acclimated.Collectively, the results suggest: (1) thermal acclimation of leaf R is common in most biomes; and (2) the high T threshold of respiration dynamically adjusts upward when plants are challenged with warmer and hotter climates. [ABSTRACT FROM AUTHOR]

Summary: Leaf optical properties impact leaf energy balance and thus leaf temperature. The effect of leaf development on mid‐infrared (MIR) reflectance, and hence thermal emissivity, has not been investigated in detail.We measured a suite of morphological characteristics, as well as directional‐hemispherical reflectance from ultraviolet to thermal infrared wavelengths (250 nm to 20 µm) of leaves from five temperate deciduous tree species over the 8 wk following spring leaf emergence.By contrast to reflectance at shorter wavelengths, the shape and magnitude of MIR reflectance spectra changed markedly with development. MIR spectral differences among species became more pronounced and unique as leaves matured. Comparison of reflectance spectra of intact vs dried and ground leaves points to cuticular development – and not internal structural or biochemical changes – as the main driving factor. Accompanying the observed spectral changes was a drop in thermal emissivity from about 0.99 to 0.95 over the 8 wk following leaf emergence.Emissivity changes were not large enough to substantially influence leaf temperature, but they could potentially lead to a bias in radiometrically measured temperatures of up to 3 K. Our results also pointed to the potential for using MIR spectroscopy to better understand species‐level differences in cuticular development and composition. [ABSTRACT FROM AUTHOR]

Summary: Thermoregulation of leaf temperature (Tleaf) may foster metabolic homeostasis in plants, but the degree to which Tleaf is moderated, and under what environmental contexts, is a topic of debate. Isotopic studies inferred the temperature of photosynthetic carbon assimilation to be a constant value of c. 20°C; by contrast, leaf biophysical theory suggests a strong dependence of Tleaf on environmental drivers. Can this apparent disparity be reconciled?We continuously measured Tleaf and whole‐crown net CO2 uptake for Eucalyptus parramattensis trees growing in field conditions in whole‐tree chambers under ambient and +3°C warming conditions, and calculated assimilation‐weighted leaf temperature (TL‐AW) across 265 d, varying in air temperature (Tair) from −1 to 45°C. We compared these data to TL‐AW derived from wood cellulose δ18O.Tleaf exhibited substantial variation driven by Tair, light intensity, and vapor pressure deficit, and Tleaf was strongly linearly correlated with Tair with a slope of c. 1.0. TL‐AW values calculated from cellulose δ18O vs crown fluxes were remarkably consistent; both varied seasonally and in response to the warming treatment, tracking variation in Tair.The leaves studied here were nearly poikilothermic, with no evidence of thermoregulation of Tleaf towards a homeostatic value. Importantly, this work supports the use of cellulose δ18O to infer TL‐AW, but does not support the concept of strong homeothermic regulation of Tleaf See also the Commentary on this article by Cavaleri, 228: 1455–1457. [ABSTRACT FROM AUTHOR]

Summary: The Kok and Laisk techniques can both be used to estimate light respiration Rlight. We investigated whether responses of Rlight to short‐ and long‐term changes in leaf temperature depend on the technique used to estimate Rlight.We grew Eucalyptus tereticornis in whole‐tree chambers under ambient temperature (AT) or AT + 3°C (elevated temperature, ET). We assessed dark respiration Rdark and light respiration with the Kok (RKok) and Laisk (RLaisk) methods at four temperatures to determine the degree of light suppression of respiration using both methods in AT and ET trees.The ET treatment had little impact on Rdark, RKok or RLaisk. Although the thermal sensitivities of RKok or RLaisk were similar, RKok was higher than RLaisk. We found negative values of RLaisk at the lowest measurement temperatures, indicating positive net CO2 uptake, which we propose may be related to phosphoenolpyruvate carboxylase activity. Light suppression of Rdark decreased with increasing leaf temperature, but the degree of suppression depended on the method used.The Kok and Laisk methods do not generate the same estimates of Rlight or light suppression of Rdark between 20 and 35°C. Negative rates of RLaisk imply that this method may become less reliable at low temperatures. [ABSTRACT FROM AUTHOR]

This article is a Commentary on Drake et al. (2020), 228: 1511–1523. [ABSTRACT FROM AUTHOR]

The phenology of spring leaf unfolding plays a key role in the structure and functioning of ecosystems. The classical concept of heat requirement (growing degree days) for leaf unfolding was developed hundreds of years ago, but this model does not include the recently reported greater importance of daytime than night-time temperature., A manipulative experiment on daytime vs night-time warming with saplings of three species of temperate deciduous trees was conducted and a Bayesian method was applied to explore the different effects of daytime and night-time temperatures on spring phenology., We found that both daytime and night-time warming significantly advanced leaf unfolding, but the sensitivities to increased daytime and night-time temperatures differed significantly. Trees were most sensitive to daytime warming (7.4 ± 0.9, 4.8 ± 0.3 and 4.8 ± 0.2 d advancement per degree Celsius warming (d °C−1) for birch, oak and beech, respectively) and least sensitive to night-time warming (5.5 ± 0.9, 3.3 ± 0.3 and 2.1 ± 0.9 d °C−1). Interestingly, a Bayesian analysis found that the impact of daytime temperature on leaf unfolding was approximately three times higher than that of night-time temperatures., Night-time global temperature is increasing faster than daytime temperature, so model projections of future spring phenology should incorporate the effects of these different temperatures. [ABSTRACT FROM AUTHOR]

The thermal performance curve is an ecological concept relating the phenotype of organisms and temperature. It requires characterization of the leaf temperature for foliar fungal pathogens. Epidemiologists, however, use air temperature to assess the impacts of temperature on such pathogens. Leaf temperature can differ greatly from air temperature, either in controlled or field conditions. This leads to a misunderstanding of such impacts., Experiments were carried out in controlled conditions on adult wheat plants to characterize the response of Mycosphaerella graminicola to a wide range of leaf temperatures. Three fungal isolates were used. Lesion development was assessed twice a week, whereas the temperature of each leaf was monitored continuously., Leaf temperature had an impact on disease dynamics. The latent period of M. graminicola was related to leaf temperature by a quadratic relationship. The establishment of thermal performance curves demonstrated differences among isolates as well as among leaf layers., For the first time, the thermal performance curve of a foliar fungal pathogen has been established using leaf temperature. The experimental setup we propose is applicable, and efficient, for other foliar fungal pathogens. Results have shown the necessity of such an approach, when studying the acclimatization of foliar fungal pathogens. [ABSTRACT FROM AUTHOR]

Climate manipulation experiments are of key importance in identifying possible responses of plant communities and ecosystems to climate change. Experiments for warming the air under sunlit conditions are carried out in (partial) enclosures. These inevitably alter the energy balance inside, potentially altering tissue temperatures which affect metabolism and growth., Using an empirically validated energy balance model, we investigate effects of two widely used warming methods, climate-controlled glasshouses and passively warmed open-top chambers (OTCs), on leaf temperatures. The model applies standard energy balance formulas, supplemented with data on optical properties of glasshouse materials and wind conditions inside OTCs., Results show that the different radiation environment inside glasshouses did not produce large leaf temperature deviations compared with outside. Poor glasshouse design with significant radiation blockage by the structure or with insufficient ventilation did affect tissue temperatures more significantly. The drastic wind speed reduction inside OTCs approximately doubled the actual (canopy) warming compared with earlier reported increases in air temperature provided by this technique - an effect that was inflated if the plants' stomates closed., These results demonstrate that leaf temperatures were higher than previously considered in OTCs but not in climate-controlled glasshouses. [ABSTRACT FROM AUTHOR]

Summary [ABSTRACT FROM AUTHOR]

• Near-isogenic Brassica napus lines carrying/lacking resistance gene Rlm6 were used to investigate the effects of temperature and leaf wetness duration on phenotypic expression of Rlm6-mediated resistance. • Leaves were inoculated with ascospores or conidia of Leptosphaeria maculans carrying the effector gene AvrLm6. Incubation period to the onset of lesion development, number of lesions and lesion diameter were assessed. Symptomless growth of L. maculans from leaf lesions to stems was investigated using a green fluorescent protein (GFP) expressing isolate carrying AvrLm6. • L. maculans produced large grey lesions on Darmor (lacking Rlm6) at 5–25 °C and DarmorMX (carrying Rlm6) at 25°C, but small dark spots and ‘green islands’ on DarmorMX at 5–20°C. With increasing temperature/wetness duration, numbers of lesions/spots generally increased. GFP-expressing L. maculans grew from leaf lesions down leaf petioles to stems on DarmorMX at 25°C but not at 15°C. • We conclude that temperature and leaf wetness duration affect the phenotypic expression of Rlm6-mediated resistance in leaves and subsequent L. maculans spread down petioles to produce stem cankers. [ABSTRACT FROM AUTHOR]

Summary Stomatal behaviour of plants in drying soil can be regulated by (long distance) chemical signals that provide the shoot with some measure of water availability. Although much emphasis has been placed on the plant hormone abscisic acid (ABA) as a central component of the signalling process, soil drying will modify the delivery to the shoot of a range of potential chemical signals. Here we consider the role that changes in the xylem sap pH might play in determining the access that ABA has to sites of action on the guard cells. We also show how redistribution of inorganic ions between different compartments in the leaf (localized chemical signalling) can provide sensitive control of stomata and water loss in response to potentially damaging changes in leaf temperature. Partial root zone drying is an irrigation technique that has been developed to allow exploitation of the plant’s long distance signalling system. When the system is optimized, stomatal behaviour, shoot water status and leaf growth can be regulated such that water use efficiency (fruit yield/water used) can be significantly increased. We show how an understanding of the drought stress physiology of the whole plant can lead to substantial saving of irrigation water in agriculture. [ABSTRACT FROM AUTHOR]

Summary • A common response of plants to elevated atmospheric CO2 concentration (CO2 ) is decreased leaf conductance. Consequently, leaf temperature is predicted to increase under elevated CO2 . • Diurnal patterns of leaf conductance and temperature were measured for three desert perennials, the C3 shrub Larrea tridentata , C3 tussock grass Achnatherum hymenoides and C4 tussock grass Pleuraphis rigida , at the Nevada Desert FACE facility. Measurements were made on ambient and c . 550 µmol mol-1 CO2 plots through both a wet and dry year. • Reductions in conductance were 35%, 20% and 13% for Pleuraphis , Achnatherum and Larrea , respectively. Decreased conductance occurred throughout the day only for Pleuraphis . Both C3 species had smaller CO2 effects during dry periods than wet. Leaf temperature did not differ significantly between elevated and ambient CO2 for any species. Comparisons of blower-control and nonring plots indicated that the FACE apparatus did not confound our results. • All three species exhibited decreased leaf conductance under elevated CO2 , although reductions were not uniform during the day or among years. Nonetheless, leaf energy balance was only minimally changed for these microphyllous desert perennials. [ABSTRACT FROM AUTHOR]

Populations of two continental-northern species, Carex ericetorum Poll. and Eriophorum latifolium Hoppe and two arctic-alpine species, Kobresia simpliciuscula (Wahlenb.) Mackenzie and Carex capillaris L., from the flora of Teesdale were grown under controlled conditions and their rates of photosynthesis, photorespiration and transpiration were measured in an open gas-analysis system over the leaf temperature range 5 to 30 °C over a range of irradiances. The maximum rates and the temperature optima for photosynthesis were similar for all the species. The rates of photosynthesis at low temperature and the light saturation characteristics of C. ericetorum and E. latifolium were significantly different from those of K. simpliciuscula and C. capillaris. The differences are discussed in relation to the habitats in which these species usually occur. [ABSTRACT FROM AUTHOR]

Widely different rates of carbon dioxide assimilation by apple leaves have been reported. Data from forty-nine papers are examined and likely reasons for their variation are deduced. Many studies were made on glasshouse-grown plants, the leaves of which can have a very different structure and photosynthetic capacity from those grown outdoors. Other biological aspects including the influence of fruits are discussed. Leaf temperatures have rarely been measured but there is evidence that excessive temperatures occurred in some studies and many other assimilation rates were probably decreased by less obviously extreme conditions. The rates of air flow employed have often been inadequate for true assimilation rates to be obtained. However, many rates higher than those normally quoted have been obtained by several workers including the author and it is concluded that the maximum photosynthetic rate of apple leaves in normal air is at least 35 mg CO[SUB2] dm[SUP-2] h[SUP-1]. [ABSTRACT FROM AUTHOR]

This article comments on an article by S. T. Michaletz and E. A. Johnson on the temperature of conifer buds. The author presents information on the difference of the temperatures of leaves, buds and stems from air temperature. He states that Michaletz and Johnson have not evaluate the entire energy balance of the structures. The purpose of the study is indicated.

Summary [ABSTRACT FROM AUTHOR]