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

Abstract

ABSTRACT 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 CO 2 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 CO 2 concentration ( C i ) are incorrect for leaves where g i limits photosynthesis. We show how conventional A–C i curve (net CO 2 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 CO 2 evaluated at C i [ K m (CO 2 ) 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