Transient Depression of Photosynthesis in Bean Leaves During Rapid Water Loss

Abstract

The response of the photosynthetic capacity of mesophyll to a rapid loss of water from the leaf was evaluated by measuring transpiration, net CO₂ uptake and temperature in dwarf bean leaves exposed to saturating CO₂ partial pressures in a leaf chamber before and after cutting the petiole in air. Some plants were pre-exposed to low or high photon flux density, or to water deficit before measurements. During 3-5 min after cutting, when the turgorpassive opening of the stomata accelerated the water loss from the leaf, the rate of CO₂ uptake was depressed; the depression was more pronounced in young leaves, under higher CO₂ partial pressure, and in leaves pre-exposed to low photon flux density or to slight water deficit. During subsequent closure of the stomata, the CO₂ uptake rate accelerated, although the leaf water content and the internal CO₂ partial pressure declined. The depression in CO₂ uptake was poorly correlated with the water loss rate after cutting, but it was positively correlated with the increase in water loss rate relative to the transpiration rate before the cutting. It is speculated that sharp perturbation in leaf water regime may induce a transient leakage of cellular membranes and redistribution of metabolites between cell compartments, resulting in a depression of photosynthesis. Subsequent restoration of the concentrations of metabolites in compartments tends to restore the rate of photosynthesis, which then declines due to substantial stomatal closure.