Impacts of soil water stress on the acclimated stomatal limitation of photosynthesis: Insights from stable carbon isotope data

Abstract

Atmospheric aridity and drought both influence physiological function in plant leaves, but their relative contributions to changes in the ratio of leaf internal to ambient partial pressure of CO2 (χ) – an index of adjustments in both stomatal conductance and photosynthetic rate to environmental conditions – are difficult to disentangle. Many stomatal models predicting χ include the influence of only one of these drivers. In particular, the least‐cost optimality hypothesis considers the effect of atmospheric demand for water on χ but does not predict how soils with reduced water further influence χ, potentially leading to an overestimation of χ under dry conditions. Here, we use a large network of stable carbon isotope measurements in C3 woody plants to examine the acclimated response of χ to soil water stress. We estimate the ratio of cost factors for carboxylation and transpiration (β) expected from the theory to explain the variance in the data, and investigate the responses of β (and thus χ) to soil water content and suction across seed plant groups, leaf phenological types and regions. Overall, β decreases linearly with soil drying, implying that the cost of water transport along the soil–plant–atmosphere continuum increases as water available in the soil decreases. However, despite contrasting hydraulic strategies, the stomatal responses of angiosperms and gymnosperms to soil water tend to converge, consistent with the optimality theory. The prediction of β as a simple, empirical function of soil water significantly improves χ predictions by up to 6.3 ± 2.3% (mean ± SD of adjusted‐R2) over 1980–2018 and results in a reduction of around 2% of mean χ values across the globe. Our results highlight the importance of soil water status on stomatal functions and plant water‐use efficiency, and suggest the implementation of trait‐based hydraulic functions into the model to account for soil water stress.

Global decadal variability of plant carbon isotope discrimination and its link to gross primary production

“…The Prentice et al (2014) model is based on the least‐cost optimality hypothesis which assumes that leaves minimize the summed unit costs of transpiration and carboxylation so that:where β (unitless) is the ratio of cost factors for carboxylation and transpiration at 25°C, which may vary with changes in plant‐available soil water (Lavergne et al, 2020), but is set constant here because no mechanistic formulation for the soil water stress has been proposed yet. K is the effective Michaelis constant for Rubisco‐limited photosynthesis at a given partial pressure of O 2 (Pa).…”

Eco‐evolutionary optimality as a means to improve vegetation and land‐surface models

“…The hypothesis of Prentice et al . (2014) provided an equation with good predictive power for the responses of leaf and plant δ 13 C to the growth environment, but the one ‘universal’ parameter it includes has been shown to be influenced by soil moisture (Lavergne et al ., 2020a) and soil pH (Wang et al ., 2017; Paillassa et al ., 2020). Moreover, the variation of χ on long climatic moisture gradients appears to be significantly steeper than predicted by that equation (Dong et al ., 2020).…”

Plant hydraulics coordinated with photosynthetic traits and climate

“…Optimality theory allows testable predictions about trait-trait coordination and can also provide strong explanations for observed responses of traits to environment 22 . Among photosynthetic traits, analyses of δ 13 C data have shown quantitative agreement between observed and theoretically predicted environmental responses of χ 17,18,23 . Smith, et al 19 , similarly, used optimality theory to predict the observed environmental responses of V cmax in a global data set.…”