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

摘要

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