Estimating the Photorespiratory CO2 Compensation Point and CO2 Release in the Light Using the Laisk Method Combined With Photosynthetic Theory.

The photorespiratory CO2 compensation point (Γ*) and the rate of CO2 release in the light (DL) are critical parameters for understanding the carbon dynamics of C3 plants. These two parameters can be derived from the widely-used Laisk method as the intercept of linear regression lines fitted to net assimilation rates (Anet) vs. chloroplast CO2 partial pressures (Cc) obtained at different low-irradiance levels. However, photosynthetic theory indicates curvature in the Anet-Cc relationship which conflicts with the use of linear relationships for analysis. We, therefore, systematically evaluated the limitations of the use of linear relationships across temperatures from 5°C to 40°C and quantified the sensitivity of errors in Γ* and DL estimates to the selected Cc range. We found that wide CO2 ranges, especially when they exclude the expected Γ*, can introduce substantial biases in parameter estimation with linear regressions, particularly at lower temperatures. This can lead to marked underestimates of Γ*, and biologically unrealistic DL. We propose refining the Laisk method by using a photosynthesis model to analyse data. The model better represents the nonlinear Anet-Cc relationship and yields consistent Γ* and DL estimates, regardless of the CO2 range used.