Effects of drought stress on chlorophyll fluorescence and photosynthetic pigments in grapevine leaves (Vitis vinifera cv. 'White Riesling')

Drought stress is one of the most important abiotic stress factors which is generally accompanied by heat stress. Especially photosynthesis is inhibited under drought stress due to the perturbation of the biochemical processes involving the PSII reaction centres. With the aim to investigate the effects of drought stress on PSII functioning, irrigated pot grown grapevines (cv. 'White Riesling') were compared with not irrigated vines. During the 40 days of the trial leaf water potential, chlorophyll fluorescence, and leaf pigment contents were measured. Differences in water potential between stressed and irrigated plants were significant 20 days after the treatment head started, whereas the potential quantum efficiency of PSII (Fv/Fm) was unaffected. We found a significant correlation between Fv/Fm and water potential only in stressed plants, with a decrease in Fv/Fm when Psi(leaf) was below -0.9 MPa. Leaf pigment concentrations (Chl a, Chl h, carotenoids) were not influenced by water stress, but in stressed plants, the correlation between Fv/Fm and chlorophyll concentrations was significant. We conclude that photosynthetic efficiency is only affected at a relatively high intensity of drought. Fv/Fm could therefore be a good indicator to define the threshold between moderate and excessive drought stress in the field.