A Model Predicting Stomatal Conductance and its Contribution to the Control of Photosynthesis under Different Environmental Conditions

Abstract

In the past, stomatal responses have generally been considered in relation to single environmental variables in part because the interactions between factors have appeared difficult to quantify in a simple way. A linear correlation between stomatal conductance (g) and CO₂ assimilation rate (A) has been reported when photon fluence was varied and when the photosynthetic capacity of leaves was altered by growth conditions, provided CO₂, air humidity and leaf temperature were constant (1). Temperature and humidity are, however, not consistent in nature. Lack of a concise description of stomatal responses to combinations of environmental factors has limited attempts to integrate these responses into quantitative models of leaf energy balance, photosynthesis, and transpiration. Moreover, this lack has hindered progress toward understanding the stomatal mechanism. We have taken a multi-variant approach to the study of stomatal conductance and we show that under many conditions the responses of stornata can be described by a set of linear relationships. This model can be linked to models of leaf carbon metabolism and the environment to predict fluxes of CO₂, H₂O and energy. In this paper, we show how the model of conductance can be linked to a description of CO₂ assimilation as a function of intercellular CO₂ (whether empirical or the output of a model) to predict the distribution of flux control between the stornata and leaf “biochemistry” under conditions in a gas-exchange cuvette.