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In this stochastic rainfall scenario, we can exchange the long-term average in Equation 3 with the ensemble average so that the long-term mean net carbon gain rate defined with Equation 3 becomes (Lu et al., 2016) 10 HT ht Implementation In the single dry-down scenario (but not in the stochastic rainfall scenario), the simulations were run in discrete time with all the state variables updated daily. Stomatal regulation is a cornerstone process that links the exchange of carbon and water between vegetation and atmosphere, and how it is affected by plant water stress. [Extracted from the article]

Highlights: [•] Optimal stomatal conductance under CO2-light co-limitation is analytically obtained. [•] It reconciles existing formulations based on single limitations to photosynthesis. [•] Model predictions are compared to known stomatal responses to micro-environment. [•] Marginal water use efficiency response to short- and long-term CO2 shifts are discussed. [ABSTRACT FROM AUTHOR]

Soil and plant hydraulics constrain ecosystem productivity by setting physical limits to water transport and hence carbon uptake by leaves. While more negative xylem water potentials provide a larger driving force for water transport, they also cause cavitation that limits hydraulic conductivity. An optimum balance between driving force and cavitation occurs at intermediate water potentials, thus defining the maximum transpiration rate the xylem can sustain (denoted as Emax). The presence of this maximum raises the question as to whether plants regulate transpiration through stomata to function near Emax., To address this question, we calculated Emax across plant functional types and climates using a hydraulic model and a global database of plant hydraulic traits., The predicted Emax compared well with measured peak transpiration across plant sizes and growth conditions ( R = 0.86, P < 0.001) and was relatively conserved among plant types (for a given plant size), while increasing across climates following the atmospheric evaporative demand. The fact that Emax was roughly conserved across plant types and scales with the product of xylem saturated conductivity and water potential at 50% cavitation was used here to explain the safety-efficiency trade-off in plant xylem., Stomatal conductance allows maximum transpiration rates despite partial cavitation in the xylem thereby suggesting coordination between stomatal regulation and xylem hydraulic characteristics. [ABSTRACT FROM AUTHOR]

Quantification of stomatal responses to environmental variables, in particular to soil water status, is needed to model carbon and water exchange rates between plants and the atmosphere. Models based on stomatal optimality theory successfully describe leaf gas exchange under different environmental conditions, but the effects of water availability on the key optimization parameter [the marginal water use efficiency (WUE), λ = ∂ A/∂ E] has resisted complete theoretical treatment. Building on previous optimal leaf gas exchange models, we developed an analytical equation to estimate λ from gas exchange observations along gradients of soil water availability. This expression was then used in a meta-analysis of about 50 species to investigate patterns of variation in λ. Assuming that cuticular water losses are negligible λ increases under mild water stress but decreases when severe water stress limits photosynthesis. When cuticular conductance is considered, however, λ increases monotonically with increasing water stress, in agreement with previous theoretical predictions. Moreover, the shape of these response curves to soil water availability changes with plant functional type and climatic conditions. In general, λ is lower in species grown in dry climates, indicating lower marginal WUE. The proposed parameterization provides a framework to assess the responses of leaf gas exchange to changes in water availability. Moreover, the model can be extended to scale leaf-level fluxes to the canopy and ecosystem level. [ABSTRACT FROM AUTHOR]

Using a stomatal optimization model, an abrupt change is foundnd in the relationship between carbon and water fluxes along the evolutionary gradient from C3 to C4 photosynthetic types.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 C4Flaveria 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 C3Flaveria. The results demonstrate that both the marginal water use efficiency and the C4 pump strength increase in C4Flaveria 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. [ABSTRACT FROM PUBLISHER]