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 ) but supposedly at a lower rate than in the dark (R ). However, there is no method for direct measurement of R and the available methods require nonphysiological measurement conditions. A method based on isotopic disequilibrium quantified R (R ) and mesophyll conductance of young and old fully expanded leaves of six species. R was compared to R determined by the Laisk method (R ) on the very same leaves with a minimum time lag. R and R were generally lower than R , and were not significantly affected by leaf ageing. R and R were positively correlated (r  = 0.35), and both were positively correlated with R (r  ≥ 0.6). R was systematically lower than R by 0.4 μmol m  s . Using A/C instead of A/C curves, a higher photocompensation point Γ* (by 5 μmol mol ) was found but no influence on R estimates was observed. The results imply that the Laisk method underestimates actual R significantly, probably related to the measurement condition of low CO and irradiance. The isotopic disequilibrium method is useful for assessing responses of R to irradiance and CO , improving our mechanistic understanding of R .