Photoinhibition of photosynthesis: effect on chlorophyll fluorescence at 77K in intact leaves and in chloroplast membranes of Nerium oleander.

The effect of exposing intact leaves and isolated chloroplast membranes of Nerium oleander L. to excessive light levels under otherwise favorable conditions was followed by measuring photosynthetic CO2 uptake, electron transport and low-temperature (77K=-196°C) fluorescence kinetics. Photoinhibition, as manifested by a reduced rate and photon (quantum) yield of photosynthesis and a reduced electron transport rate, was accompanied by marked changes in fluorescence characteristics of the exposed upper leaf surface while there was little effect on the shaded lower surface. The most prominent effect of photoinhibitory treatment of leaves and chloroplasts was a strong quenching of the variable fluorescence emission at 692 nm (Fv,692) while the instantaneous fluorescence (Fo,692) was slightly increased. The maximum and the variable fluorescence at 734 nm were also reduced but not as much as FM,692 and Fv,692. The results support the view that photoinhibition involves an inactivation of the primary photochemistry of photosystem II by damaging the reaction-center complex. In intact leaves photoinhibition increased with increased light level, increased exposure time, and with decreased temperature. Increased CO2 pressure or decreased O2 pressure provided no protection against photoinhibition. With isolated chloroplasts, inhibition of photosystem II occurred even under essentially anaerobic conditions. Measurements of fluorescence characteristics at 77K provides a simple, rapid, sensitive and reproducible method for assessing photoinhibitory injury to leaves. The method should prove especially useful in studies of the occurrence of photoinhibition in nature and of interactive effects between high light levels and major environmental stress factors.