Aims The objective of this study was to compare the values of respiration under light derived by fitting a photosynthetic CO₂ response model and measurements, in order to provide information for model optimization.

Methods Using combined gas exchange measurements and a low O₂ (2% O₂) method, the responses of photosynthetic rate (P_n) to CO₂ at different light intensities (2000, 1500, 1000 and 500 μmol·m^-2·s^-1) in the flag leaves of wheat were measured. The measured data were fitted by a biochemical model, a rectangular hyperbola model and a modified rectangular hyperbola model of the photosynthetic response to intercellular CO₂ concentration (A/C_i) and air CO₂ concentration (A/C_a), aiming to approach the reasonability of the fitted results obtained from the models.

Important findings The sequence of fitting effect of the three CO₂ response models in descending order was as follows: modified rectangular hyperbola model > rectangular hyperbola model > biochemical model. Fitted values of A/C_a curve was more reasonable than A/C_i curve, because the photorespiration and mitochondrial respiration under light (R_d) estimated by the former better matched the measured values. However, there were significant differences in the whole between the fitted and measured values. The reason could be that the effect of CO₂ concentrations on R_d and apparent photorespiration (R_pa) is neglected in all the current CO₂ response models. Our results showed that CO₂ concentration had a marked effect on R_pa and R_d. With increasing CO₂ concentration, R_pa and R_d increased first, and then decreased sharply. Take 2000 μmol·m^-2·s^-1 for example, R_pa varied between 5.035 and 11.670 μmol CO₂·m^-2·s^-1, and R_d varied between 0.491 and 2.987 μmol CO₂·m^-2·s^-1. Regression analyses indicated that R_pa and R_d were well related to CO₂ concentrations at different light intensities.