Laisk measurements in the nonsteady state: Tests in plants exposed to warming and variable CO(2) 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 CO(2) 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 CO(2) 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 CO(2) 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 CO(2); 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 CO(2) pre-treatments. We propose that changes in the export of glycine from photorespiration may explain these apparent differences in C(i)*.