Parameterization of a coupled CO ₂ and H ₂O gas exchange model at the leaf scale of Populus euphratica

The following two models were combined to simultaneously predict CO ₂ and H ₂O gas exchange at the leaf scale of Populus euphratica: a Farquhar et al. type biochemical sub-model of photosynthesis (Farquhar et al., 1980) and a Ball et al. type stomatal conductance sub-model (Ball et al., 1987). The photosynthesis parameters [including maximum carboxylation rate allowed by ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) carboxylation rate ( V_cmax), potential light-saturated electron transport rate ( J_max), triose phosphate utilization (TPU) and day respiration ( R_d)] were determined by using the genetic algorithm (GA) method based on A/C_i data. Values of V_cmax and J_max standardized at 25 °C were 75.09±1.36 (mean ± standard error), 117.27±2.47, respectively. The stomatal conductance sub-model was calibrated independently. Prediction of net photosynthesis by the coupled model agreed well with the validation data, but the model tended to underestimate transpiration rates. Overall, the combined model generally captured the diurnal patterns of CO ₂ and H ₂O exchange resulting from variation in temperature and irradiation.