Fig. 1.

Net CO₂ assimilation rate (A) as a function of intercellular CO₂ concentration (C_i). Numbers indicate the three incident irradiances (I_inc) under which measurements were carried out (in μmol m⁻² s⁻¹). Regression lines at these I_inc, fitted to data points that each represents the mean of measurements from four replicated plants, were forced to join at the common intersection point (C_i*, –R_d), where R_d is the estimated leaf respiration rate in the light (Laisk method) and C_i* is the C_i-based CO₂ compensation point in the absence of R_d. The dashed horizontal line is the line of A=0. The estimated R_d is negative for maize (b), indicating that the Laisk method does not work for C₄ species. Note that the scales in the two panels are different.