First, the predictive value of the model was checked against measurements obtained in assimilation chambers with the C 1 to C3 (cv. “Heidi’ ’ ) plants. The results (Fig. 6) showed that the agreement is satisfactory, although we note a slight overestimation for high levels of CO* and Z, (a = 1.94 + 0.30 pg CO% s- ’ per plant; b = 1.24 + 0.01 and R* = 0.95).