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

The photorespiratory CO 2 compensation point (Γ*) and the rate of CO 2 release in the light (D L ) are critical parameters for understanding the carbon dynamics of C 3 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 CO 2 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 CO 2 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 CO 2 range used.