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 o live trees (Olea europaea L. var. Coratina), grown in pots outdoors in the summer and subjected to water stress, was studied. Net photosynth etic rates reached maximum values early in the morning in both control and stressed plants and subsequently declined gradually, This inactiv ation of photosynthetic activity was accompanied by changes in the flu orescence characteristics of the upper intact leaf surface, The maximu m fluorescence yield (Fp) and the ratio Fv/Fp decreased at midday espe cially in water-stressed plants, but the initial fluorescence (Fo) ros e 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 complete ly by 8 p.m. as the leaf water potential recovered, In stressed plants this diurnal recovery was not complete, so that the photosynthetic ra tes and the ratio Fv/Fp declined gradually during the development of w ater stress, These results indicate that in olive trees subjected to s evere water stress the non-stomatal component of photosynthesis was af fected and perhaps a light-dependent inactivation of the primary photo chemistry associated with photosystem II (PSII) occurred, Four to five days after rewatering severely stressed plants, the predawn leaf wate r potential, net photosynthetic rates and chlorophyll fluorescence ind ices recovered only partially.