Influence of temperature on measurements of the CO2 compensation point: Differences between the Laisk and O2-exchange methods

Abstract

The CO2 compensation point in the absence of day respiration (γ*) is a key parameter for modelling leaf CO2 exchange. γ* links the kinetics of ribulose-1,5- bisphosphate carboxylase-oxygenase (Rubisco) with the stoichiometry of CO 2 released per Rubisco oxygenation from photorespiration (α), two essential components of biochemical models of photosynthesis. There are two main gas-exchange methods for measuring γ*: (i) the Laisk method, which requires estimates of mesophyll conductance to CO2 (gm) and (ii) measurements of O2 isotope exchange, which assume constant values of α and a fixed stoichiometry between O 2 uptake and Rubisco oxygenation. In this study, the temperature response of γ* measured using the Laisk and O 2-exchange methods was compared under ambient (25 °C) and elevated (35 °C) temperatures to determine whether both methods yielded similar results. Previously published temperature responses of γ* estimated with the Laisk and O2-exchange methods in Nicotiana tabacum demonstrated that the Laisk-derived model of γ* was more sensitive to temperature compared with the O2-exchange model. Measurements in Arabidopsis thaliana indicated that the Laisk and O2-exchange methods produced similar γ* at 25 °C; however, γ* values from O2 exchange were lower at 35 °C compared with the Laisk method. Compared with a photorespiratory mutant (pmdh1pmdh2hpr) with increased α, wild-type (WT) plants had lower Laisk values of γ* at 25 °C but were not significantly different at 35 °C. These differences between Laisk and O2 exchange values of γ* at 35 °C could be explained by temperature sensitivity of α in WT and/or errors in the assumptions of O2 exchange. The differences between γ* measured using the Laisk and O2-exchange method with temperature demonstrate that assumptions used to measure γ*, and possibly the species-specific validity of these assumptions, need to be considered when modelling the temperature response of photosynthesis. © The Author [2013].