Measurement of leaf day respiration using a new isotopic disequilibrium method compared with the Laisk method

O_LIQuantification of leaf respiration is of great importance for the understanding of plant physiology and ecosystem biogeochemical processes. Leaf respiration continues in light (RL) but supposedly at a lower rate compared to the dark (RD). Yet, there is no method for direct measurement of RL and most available methods require unphysiological measurement conditions.\nC_LIO_LIA method based on isotopic disequilibrium quantified RL (RL 13C) and mesophyll conductance of young and old fully-expanded leaves of six species compared RL 13C to RL values determined by the Laisk method (RL Laisk).\nC_LIO_LIRL 13C and RL Laisk were consistently lower than RD. Leaf ageing negatively affected photosynthetic performance, but had no significant effect on RL or RL/RD as determined by both methods. RL Laisk and RL 13C were measured successively on the same leaves and correlated positively (r2=0.38), but average RL Laisk was 28% lower than RL13C. Using A/Cc curves instead of A/Ci curves, a higher photocompensation point {Gamma}* (by 5 mol mol-1) was found but the correction had no influence on RL Laisk estimates.\nC_LIO_LIThe results suggest that the Laisk method underestimated RL. The isotopic disequilibrium method is useful for assessing responses of RL to irradiance and CO2, improving our mechanistic understanding of RL.\nC_LI