## Introduction

The model of C_{3} photosynthesis first described by Farquhar, von Caemmerer & Berry (1980; referred to as FCB model hereafter) has been adopted almost universally in models describing CO_{2} exchange at multiple scales ranging from individual leaves (e.g. Harley *et al*. 1992), plant canopies (e.g. Leuning *et al*. 1995; Harley & Baldocchi 1995; De Pury & Farquhar 1997; Wang & Leuning 1998), through to landscapes in Global Climate Models (Sellers *et al*. 1996).

Many authors have shown that the FCB accurately describes leaf-level photosynthesis, provided values of the eight parameters of the model are known. There is considerable variation between and within species in two key parameters, *V*_{cmax} and *J*_{max}, although some of this variation may be ascribed to differences in leaf nitrogen concentration (Field 1983; Leuning, Cromer & Rance 1991; Harley *et al*.; 1992; Kellomäki & Wang 1997). Another problem in utilizing the FCB model at the leaf scale is that *V*_{cmax} and *J*_{max} are temperature dependent, and the dependence varies significantly between and within species.

Variation in the temperature dependence of the parameters in the FCB model also affects our ability to model photosynthesis at the canopy scale. Wang & Leuning (1998) incorporated the FCB model into a model of canopy photosynthesis and energy partitioning, and Leuning, Dunin & Wang (1998) found that the model accurately simulated the exchanges of heat, water vapour and CO_{2} for wheat fields. In a subsequent sensitivity analysis, Wang *et al*. (2001) showed that a maximum of only four parameters in the canopy model (including *V*_{cmax}) could be estimated independently from micrometeorological measurements of heat, water vapour and CO_{2} fluxes over crops and pastures. Their analysis used previously published temperature functions and parameter values to describe the temperature dependence of *V*_{cmax} and the other parameters in the FCB model. Without these assumptions, the problem of estimating parameters for the non-linear canopy model would have been extremely difficult. Similar problems will confront others wishing to utilize the highly parameterized FCB model at all spatial scales.

We may thus ask whether it is possible to utilize the biochemical FCB model in models of canopy photosynthesis when there are significant differences in the temperature dependence of *V*_{cmax} and *J*_{max} between and within plant species? To answer this question, the present note examines published data for the temperature dependence of *V*_{cmax} and *J*_{max} to determine whether a single temperature response function, with fixed parameter values for each of *V*_{cmax} and *J*_{max}, may be used for modelling purposes, and if so, what degree of error does this introduce into these parameters?