Partitioning of photosynthetic electron flow between CO2and O2reduction in a C3leaf (Phaseolus vulgarisL.) at different CO2concentrations and during drought stress

摘要

Photosystem II chlorophyll fluorescence and leaf net gas exchanges (CO2and H2O) were measured simultaneously on bean leaves (Phaseolus vulgarisL.) submitted either to different ambient CO2concentrations or to a drought stress. When leaves are under photorespiratory conditions, a simple fluorescence parameter ΔF/ Fm(B. Genty et al. 1989, Biochem. Biophys. Acta990, 87–92; ΔF = difference between maximum, Fm, and steady-state fluorescence emissions) allows the calculation of the total rate of photosynthetic electron-transport and the rate of electron transport to O2. These rates are in agreement with the measurements of leaf O2absorption using18O2and the kinetic properties of ribulose-1,5bisphosphate carboxylase/oxygenase. The fluorescence parameter, ΔF/Fm, showed that the allocation of photosynthetic electrons to O2was increased during the desiccation of a leaf. Decreasing leaf net CO2uptake, either by decreasing the ambient CO2concentration or by dehydrating a leaf, had the same effect on the partitioning of photosynthetic electrons between CO2and O2reduction. It is concluded that the decline of net CO2uptake of a leaf under drought stress is only due, at least for a mild reversible stress (causing at most a leaf water deficit of 35), to stomatal closure which leads to a decrease in leaf internal CO2concentration. Since, during the dehydration of a leaf, the calculated internal CO2concentration remained constant or even increased we conclude that this calculation is misleading under such condit