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

Quantification 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. A 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). RL 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 RL 13C. Using A/Cc curves instead of A/Ci curves, a higher photocompensation point Γ* (by 5 μmol mol-1) was found but the correction had no influence on RL Laisk estimates. The 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.