The model was fitted using non-linear regression techniques (Proc NLIN, SAS). The value of Hd was fixed to 200 kJmol−1 according to Medlyn et al. (2002) in order to reduce the number of parameters estimated by the model. However, the model underestimates the value of Topt. Consequently, we first estimated Topt using eqn (9) and then used the obtained value to solve for Ha and ΔS using eqns (11) and (12) simultaneously.