Inhibition of photosynthesis in olive trees (Olea Europaea L.) during water stress and rewatering

The effect of high levels of natural light on leaf photosynthesis in olive trees (Olea europaea L. var. Coratina), grown in pots outdoors in the summer and subjected to water, stress, was studied. Net photosynthetic rates reached maximum values early in the morning in both control and stressed plants and subsequently declined gradually. This inactivation of photosynthetic activity was accompanied by changes in the fluorescence characteristics of the upper intact leaf surface. The maximum fluorescence yield (Fp) and the ratio Fv/Fp decreased at midday especially in water-stressed plants, but the initial fluorescence (Fo) rose to a maximum value at midday and declined again in the afternoon. In control plants the values of maximum fluorescence Fp and the ratio Fv/Fp increased again in the afternoon and had recovered almost completely by 8 p.m. as the leaf water potential recovered. In stressed plants this diurnal recovery was not complete, so that the photosynthetic rates and the ratio Fv/Fp declined gradually during the development of water stress. These results indicate that in olive trees subjected to severe water stress the non-stomatal component of photosynthesis was affected and perhaps a light-dependent inactivation of the primary photochemistry associated with photosystem II (PSII) occurred. Four to five days after rewatering severely stressed plants, the predawn leaf water potential, net photosynthetic rates and chlorophyll fluorescence indices recovered only partially.