On the need to incorporate sensitivity to CO2 transfer conductance into the Farquhar–von Caemmerer–Berry leaf photosynthesis model

Abstract

Virtually all current estimates of the maximum carboxylation rate (Vcmax) of ribulose‐1,5‐bisphosphate carboxylase/oxygenase (Rubisco) and the maximum electron transport rate (Jmax) for C 3 species implicitly assume an infinite CO 2 transfer conductance (gi). And yet, most measurements in perennial plant species or in ageing or stressed leaves show that gi imposes a significant limitation on photosynthesis. Herein, we demonstrate that many current parameterizations of the photosynthesis model of Farquhar, von Caemmerer & Berry (Planta 149, 78–90, 1980) based on the leaf intercellular CO 2 concentration (Ci) are incorrect for leaves where gi limits photosynthesis. We show how conventional A–Ci curve (net CO 2 assimilation rate of a leaf –An– as a function of Ci) fitting methods which rely on a rectangular hyperbola model under the assumption of infinite gi can significantly underestimate Vcmax for such leaves. Alternative parameterizations of the conventional method based on a single, apparent Michaelis–Menten constant for CO 2 evaluated at Ci[Km(CO 2)i] used for all C 3 plants are also not acceptable since the relationship between Vcmax and gi is not conserved among species. We present an alternative A–Ci curve fitting method that accounts for gi through a non‐rectangular hyperbola version of the model of Farquhar et al. (1980). Simulated and real examples are used to demonstrate how this new approach eliminates the errors of the conventional A–Ci curve fitting method and provides Vcmax estimates that are virtually insensitive to gi. Finally, we show how the new A–Ci curve fitting method can be used to estimate the value of the kinetic constants of Rubisco in vivo is presented