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Under future climates, leaf temperature (T l ) will be higher and more variable. This will affect plant carbon (C) balance because photosynthesis and respiration both respond to short-term (subdaily) fluctuations in T l and acclimate in the longer term (days to months). This study asks the question: To what extent can the potential and speed of photosynthetic acclimation buffer leaf C gain from rising and increasing variable T l ? We quantified how increases in the mean and variability of growth temperature affect leaf performance (mean net CO 2 assimilation rates, A net ; its variability; and time under near-optimal photosynthetic conditions), as mediated by thermal acclimation. To this aim, the probability distribution of A net was obtained by combining a probabilistic description of short- and long-term changes in T l with data on A net responses to these changes, encompassing 75 genera and 111 species, including both C3 and C4 species. Our results show that (a) expected increases in T l variability will decrease mean A net and increase its variability, whereas the effects of higher mean T l depend on species and initial T l , and (b) acclimation reduces the effects of leaf warming, maintaining A net at >80% of its maximum under most thermal regimes., (© 2019 John Wiley & Sons Ltd.)

C4 photosynthesis evolved independently numerous times, probably in response to declining atmospheric CO2 concentrations, but also to high temperatures and aridity, which enhance water losses through transpiration. Here, the environmental factors controlling stomatal behaviour of leaf-level carbon and water exchange were examined across the evolutionary continuum from C3 to C4 photosynthesis at current (400 μmol mol(-1)) and low (280 μmol mol(-1)) atmospheric CO2 conditions. To this aim, a stomatal optimization model was further developed to describe the evolutionary continuum from C3 to C4 species within a unified framework. Data on C3, three categories of C3-C4 intermediates, and C4 Flaveria species were used to parameterize the stomatal model, including parameters for the marginal water use efficiency and the efficiency of the CO2-concentrating mechanism (or C4 pump); these two parameters are interpreted as traits reflecting the stomatal and photosynthetic adjustments during the C3 to C4 transformation. Neither the marginal water use efficiency nor the C4 pump strength changed significantly from C3 to early C3-C4 intermediate stages, but both traits significantly increased between early C3-C4 intermediates and the C4-like intermediates with an operational C4 cycle. At low CO2, net photosynthetic rates showed continuous increases from a C3 state, across the intermediates and towards C4 photosynthesis, but only C4-like intermediates and C4 species (with an operational C4 cycle) had higher water use efficiencies than C3 Flaveria. The results demonstrate that both the marginal water use efficiency and the C4 pump strength increase in C4 Flaveria to improve their photosynthesis and water use efficiency compared with C3 species. These findings emphasize that the advantage of the early intermediate stages is predominantly carbon based, not water related., (© The Author 2014. Published by Oxford University Press on behalf of the Society for Experimental Biology.)

Summary: Stomatal optimization models can improve estimates of water and carbon fluxes with relatively low complexity, yet there is no consensus on which formulations are most appropriate for ecosystem‐scale applications. We implemented three existing analytical equations for stomatal conductance, based on different water penalty functions, in a big‐leaf comparison framework, and determined which optimization principles were most consistent with flux tower observations from different biomes.We used information theory to dissect controls of soil water supply and atmospheric demand on evapotranspiration in wet to dry conditions and to quantify missing or inadequate information in model variants. We ranked stomatal optimization principles based on parameter uncertainty, parsimony, predictive accuracy, and functional accuracy of the interactions between soil moisture, vapor pressure deficit, and evapotranspiration.Performance was high for all model variants. Water penalty functions with explicit representation of plant hydraulics did not substantially improve predictive or functional accuracy of ecosystem‐scale evapotranspiration estimates, and parameterizations were more uncertain, despite having physiological underpinnings at the plant level.Stomatal optimization based on water use efficiency thus provided more information about ecosystem‐scale evapotranspiration compared to those based on xylem vulnerability and proved more useful in improving ecosystem‐scale models with less complexity. [ABSTRACT FROM AUTHOR]

Plant hydraulic traits have been conjectured to be coordinated, thereby providing plants with a balanced hydraulic system that protects them from cavitation while allowing an efficient transport of water necessary for photosynthesis. In particular, observations suggest correlations between the water potentials at which xylem cavitation impairs water movement and the one at stomatal closure, and between maximum xylem and stomatal conductances, begging the question as to whether such coordination emerges as an optimal water-use strategy under unpredictable rainfall. Here mean transpiration < E> is used as a proxy for long-term plant fitness and its variations as a function of the water potentials at 50% loss of stem conductivity due to cavitation and at 90% stomatal closure are explored. It is shown that coordination between these hydraulic traits is necessary to maximize < E>, with rainfall patterns altering the optimal range of trait values. In contrast, coordination between ecosystem-level conductances appears not necessary to maximize < E>. The optimal trait ranges are wider under drier than under mesic conditions, suggesting that in semiarid systems different water use strategies may be equally successful. Comparison with observations across species from a range of ecosystems confirms model predictions, indicating that the coordinated functioning of plant organs might indeed emerge from an optimal response to rainfall variability. [ABSTRACT FROM AUTHOR]

Highlights: [•] Stomatal optimization theories have neglected the effect of dynamic soil moisture. [•] Here soil moisture is considered to provide a closure for the optimization problem. [•] The new solutions link marginal water use efficiency to air [CO2] and soil water. [•] Previously inconsistent theoretical results and data are generalized and explained. [Copyright &y& Elsevier]

Despite the observed impact of water stress on photosynthesis, some of the most used models of CO2 assimilation in C3 and C4 functional types do not directly account for it. We discuss an extension of these models, which explicitly includes the metabolic and diffusive limitations due to water stress on photosynthesis. Functional relationships describing the photosynthetic processes and CO2 diffusion inside leaves are modified to account for leaf water status on the basis of experimental results available in the literature. Extensive comparison with data shows that the model is suitable to describe the reduction in CO2 assimilation rate with decreasing leaf water potentials in various species. A simultaneous analysis of photosynthesis, transpiration and soil moisture dynamics is then carried out to explore the actual impact of drought on different photosynthesis processes and on the overall plant activity. The model well reproduces measured CO2 assimilation rate as a function of soil moisture and could be useful to formulate hypotheses for detailed experiments as well as to simulate in detail transpiration and photosynthesis dynamics under water stress. [ABSTRACT FROM AUTHOR]

In the future, boreal forests will face warmer and in some cases drier conditions, potentially resulting in extreme leaf temperatures and reduced photosynthesis. One potential and still partially unexplored avenue to prepare boreal forest for future climates is the identification of plant traits that may support photosynthetic rates under a changing climate. However, the interplay among plant traits, soil water depletion and the occurrence of heat stress has been seldom explored in boreal forests. Here, a mechanistic model describing energy and mass exchanges among the soil, plant and atmosphere is employed to identify which combinations of growing conditions and plant traits allow trees to simultaneously keep high photosynthetic rates and prevent thermal damage under current and future growing conditions. Our results show that the simultaneous lack of precipitation and warm temperatures is the main trigger of thermal damage and reduction of photosynthesis. Traits that facilitate the coupling of leaves to the atmosphere are key to avoid thermal damage and guarantee the maintenance of assimilation rates in the future. Nevertheless, the same set of traits may not maximize forest productivity over current growing conditions. As such, an effective trait selection needs to explicitly consider the expected changes in the growing conditions, both in terms of averages and extremes. [ABSTRACT FROM AUTHOR]