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 C-3 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-i[K-m(CO2)(i)] used for all C-3 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.