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. x 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*. Nonsteady-state Laisk measurements provide similar estimates of light respiration to steady-state methods but can be completed 4 times faster.