Laisk measurements in the nonsteady state: Tests in plants exposed to warming and variable CO₂ concentrations

Light respiration (R_L) is an important component of plant carbon balance and a key parameter in photosynthesis models. R_L is often measured using the Laisk method, a gas exchange technique that is traditionally employed under steady-state conditions. However, a nonsteady-state dynamic assimilation technique (DAT) may allow for more rapid Laisk measurements. In 2 studies, we examined the efficacy of DAT for estimating R_L and the parameter C_i* (the intercellular CO₂ concentration where Rubisco's oxygenation velocity is twice its carboxylation velocity), which is also derived from the Laisk technique. In the first study, we compared DAT and steady-state R_L and C_i* estimates in paper birch (Betula papyrifera) growing under control and elevated temperature and CO₂ concentrations. In the second, we compared DAT-estimated R_L and C_i* in hybrid poplar (Populus nigra L. × P. maximowiczii A. Henry “NM6”) exposed to high or low CO₂ concentration pre-treatments. The DAT and steady-state methods provided similar R_L estimates in B. papyrifera, and we found little acclimation of R_L to temperature or CO₂; however, C_i* was higher when measured with DAT compared to steady-state methods. These C_i* differences were amplified by the high or low CO₂ pre-treatments. We propose that changes in the export of glycine from photorespiration may explain these apparent differences in C_i*.