Influence of temperature on measurements of the CO(2) compensation point: differences between the Laisk and O(2)-exchange methods

The CO(2) compensation point in the absence of day respiration (Γ*) is a key parameter for modelling leaf CO(2) 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 CO(2) (g (m)) and (ii) measurements of O(2) 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 O(2)-exchange methods in Nicotiana tabacum demonstrated that the Laisk-derived model of Γ* was more sensitive to temperature compared with the O(2)-exchange model. Measurements in Arabidopsis thaliana indicated that the Laisk and O(2)-exchange methods produced similar Γ* at 25 °C; however, Γ* values from O(2) 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 O(2) exchange values of Γ* at 35 °C could be explained by temperature sensitivity of α in WT and/or errors in the assumptions of O(2) exchange. The differences between Γ* measured using the Laisk and O(2)-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.