Temperature response of photosynthesis and internal conductance to CO2: results from two independent approaches

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anglais. The internal conductance to CO2 transfer from intercellular spaces to chloroplasts poses a major limitation to photosynthesis, but few studies have investigated its temperature response. The aim of this study was to determine the temperature response of photosynthesis and internal conductance between 10 deg C and 35 deg C in seedlings of a deciduous forest tree species, Quercus canariensis. Internal conductance was estimated via simultaneous measurements of gas exchange and chlorophyll fluorescence ('variable J method'). Two of the required parameters, the intercellular photocompensation point (Ci*) and rate of mitochondrial respiration in the light (Rd), were estimated by the Laisk method. These were used to calculate the chloroplastic photocompensation point ( GAMMA *) in a simultaneous equation with gi. An independent estimate of internal conductance was obtained by a novel curve-fitting method based on the curvature of the initial Rubisco-limited portion of an A/Ci curve. The temperature responses of the rate of Rubisco carboxylation (Vcmax) and the RuBP limited rate of electron transport (Jmax) were determined from chloroplastic CO2 concentrations. The rate of net photosynthesis peaked at 24 deg C. Ci* was similar to reports for other species with a Ci* of 39 micro mol mol-1 at 25 deg C and an activation energy of 34 kJ mol-1. GAMMA * was very similar to the published temperature response for Spinacia oleracea from 20 deg C to 35 deg C, but was slightly greater at 10 deg C and 15 deg C. Jmax peaked at 30 deg C, whereas Vcmax did not reach a maximum between 10 deg C and 35 deg C. Activation energies were 49 kJ mol-1 for Vcmax and 100 kJ mol-1 for Jmax. Both methods showed that internal conductance doubled from 10 deg C to 20 deg C, and then was nearly temperature-independent from 20 deg C to 35 deg C. Hence, the temperature response of internal conductance could not be fitted to an Arrhenius function. The best fit to estimated gi was obtained with a three-parameter log normal function (R2=0.98), with a maximum gi of 0.19 mol m-2 s-1 at 29 deg C.