Effects of drought on photosynthesis, chlorophyll fluorescence and photoinhibition susceptibility in intact willow leaves

Abstract

Plants from clonal cuttings of Salix sp. were subjected to a drying cycle of 10 d in a controlled environment. Gas exchange and fluorescence emission were measured on attached leaves. The light-saturated photosynthetic CO₂ uptake became progressively inhibited with decreased leaf water potential both at high, and especially, at low intercellular CO₂ pressure. The maximal quantum yield of CO₂ uptake was more resistant. The inhibition of light-saturated CO₂ uptake at leaf water potentials around-10 bar, measured at a natural ambient CO₂ concentration, was equally attributable to stomatal and non-stomatal factors, but the further inhibition below this water-stress level was caused solely by non-stomatal factors. The kinetics of fluorescence emission was changed at severe water stress; the slow secondary oscillations of the induction curve were attenuated, and this probably indicates perturbations in the carbon reduction cycle. The influence of light level during the drought period was also studied. Provided the leaves were properly light-acclimated, drought at high and low light levels produced essentially the same effects on photosynthesis. However, low-light-acclimated leaves became more susceptible to photoinhibitory treatment under severe water stress, as compared with well-watered conditions.