Estimating the Photorespiratory CO ₂ Compensation Point and CO ₂ 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 (D_L) 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 (A_net) vs. chloroplast CO₂ partial pressures (C_c) obtained at different low-irradiance levels. However, photosynthetic theory indicates curvature in the A_net-C_c 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 D_L estimates to the selected C_c 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 D_L. We propose refining the Laisk method by using a photosynthesis model to analyse data. The model better represents the nonlinear A_net-C_c relationship and yields consistent Γ* and D_L estimates, regardless of the CO₂ range used.