Determination of leaf respiration in the light: comparison between an isotopic disequilibrium method and the Laisk method

Quantification of leaf respiration is important for understanding plant physiology and ecosystem biogeochemical processes. Leaf respiration continues in the light (R_L) but supposedly at a lower rate than in the dark (R_{D k}). However, there is no method for direct measurement of R_L and the available methods require nonphysiological measurement conditions. A method based on isotopic disequilibrium quantified R_L (R_{L 13C}) and mesophyll conductance of young and old fully expanded leaves of six species. R_{L 13C} was compared to R_L determined by the Laisk method (R_{L Laisk}) on the very same leaves with a minimum time lag. R_{L 13C} and R_{L Laisk} were generally lower than R_{D k}, and were not significantly affected by leaf ageing. R_{L Laisk} and R_{L 13C} were positively correlated (r² = 0.35), and both were positively correlated with R_{D k} (r² ≥ 0.6). R_{L Laisk} was systematically lower than R_{L 13C} by 0.4 μmol m⁻² s⁻¹. Using A/C_c instead of A/C_i curves, a higher photocompensation point Γ* (by 5 μmol mol⁻¹) was found but no influence on R_{L Laisk} estimates was observed. The results imply that the Laisk method underestimates actual R_L significantly, probably related to the measurement condition of low CO₂ and irradiance. The isotopic disequilibrium method is useful for assessing responses of R_L to irradiance and CO₂, improving our mechanistic understanding of R_L.