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

Virtually all current estimates of the maximum carboxylation rate (V_cmax) of ribulose‐1,5‐bisphosphate carboxylase/oxygenase (Rubisco) and the maximum electron transport rate (J_max) for C₃ species implicitly assume an infinite CO₂ transfer conductance (g_i). And yet, most measurements in perennial plant species or in ageing or stressed leaves show that g_i 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 CO₂ concentration (C_i) are incorrect for leaves where g_i limits photosynthesis. We show how conventional A–C_i curve (net CO₂ assimilation rate of a leaf –A_n– as a function of C_i) fitting methods which rely on a rectangular hyperbola model under the assumption of infinite g_i can significantly underestimate V_cmax for such leaves. Alternative parameterizations of the conventional method based on a single, apparent Michaelis–Menten constant for CO₂ evaluated at C_iK_m(CO₂)_i] used for all C₃ plants are also not acceptable since the relationship between V_cmax and g_i is not conserved among species. We present an alternative A–C_i curve fitting method that accounts for g_i 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–C_i curve fitting method and provides V_cmax estimates that are virtually insensitive to g_i. Finally, we show how the new A–C_i curve fitting method can be used to estimate the value of the kinetic constants of Rubisco in vivo is presented