In search of a reversible stage of photoinhibition in a higher plant: No changes in the amount of functional Photosystem II accompany relaxation of variable fluorescence after exposure of lincomycin-treated Cucurbita pepo leaves to high light

Pumpkin (Cucurbita pepo L.) leaves in which chloroplast protein synthesis was inhibited with lincomycin were exposed to strong photoinhibitory light, and changes in F-O, FM, F-V/F-M and in the amount of functional Photosystem II (O-2 evolution induced by saturating single-turnover flashes) were monitored during the high-light exposure and subsequent dark or low-light incubation. In the course of the photoinhibitory illumination, F-M, F-V/F-M and the amount of functional PS II declined continuously whereas F-O dropped rapidly to some extent and then slowly increased. If the experiments were done at room temperature, termination of the photoinhibitory illumination resulted in partial relaxation of the F-V/F-M ratio and in an increase in F-O and F-M. The relaxation was completed in 10-15 min after short-term (15 min) photoinhibitory treatment but continued 30-40 min if the exposure to high light was longer than 1 h. No changes in the amount of functional PS II accompanied the relaxation of F-V/F-M in darkness or in low light, in the presence of lincomycin. Transferring the leaves to low temperature (+4 degrees C) after the room-temperature illumination (2 h) completely inhibited the relaxation of F-V/F-M. Low temperature did not suppress the relaxation if the photoinhibitory illumination had also been done at low temperature. The results indicate that illumination of lincomycin-poisoned pumpkin leaves at room temperature does not lead to accumulation of a reversibly photoinactivated intermediate.