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 FO, FM, FV/FM and in the amount of functional Photosystem II (O2 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, FM, FV/FM and the amount of functional PS II declined continuously whereas FO 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 FV/FM ratio and in an increase in FO and FM. 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 FV/FM in darkness or in low light, in the presence of lincomycin. Transferring the leaves to low temperature (+4°C) after the room-temperature illumination (2 h) completely inhibited the relaxation of FV/FM. 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.