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

Abstract

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 .