Partitioning of photosynthetic electron flow between CO2 and O2 reduction in a C3 leaf (Phaseolus vulgaris L.) at different CO2 concentrations and during drought stress
1991
Cornic, G. | Briantais, J.M.
Photosystem II chlorophyll fluorescence and leaf net gas exchanges (CO2 and H2O) were measured simultaneously on bean leaves (Phaseolus vulgaris L.) submitted either to different ambient CO2 concentrations or to a drought stress. When leaves are under photorespiratory conditions, a simple fluorescence parameter delta F/Fm (B. Genty et al. 1989. Biochem. Biophys. Acta 990, 87-92; delta F = 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 O2 absorption using (18)O2 and the kinetic properties of ribulose- 1,5-bisphosphate carboxylase:oxygenase. The fluorescence parameter, delta F/Fm, showed that the allocation of photosynthetic electrons to O2 was increased during the desiccation of a leaf. Decreasing leaf net CO2 uptake, either by decreasing the ambient CO2 concentration or by dehydrating a leaf, had the same effect on the partitioning of photosynthetic electrons between CO2 and O2 reduction. It is concluded that the decline of net CO2 uptake 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 CO2 concentration. Since, during the dehydration of a leaf, the calculated internal CO2 concentration remained constant or even increased we conclude that this calculation is misleading under such conditions.
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