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 synthe sis was inhibited with lincomycin were exposed to strong photoinhibito ry light, and changes in F-O, FM, F-V/F-M and in the amount of functio nal Photosystem II (O-2 evolution induced by saturating single-turnove r flashes) were monitored during the high-light exposure and subsequen t dark or low-light incubation. In the course of the photoinhibitory i llumination, 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 slowl y increased. If the experiments were done at room temperature, termina tion of the photoinhibitory illumination resulted in partial relaxatio n of the F-V/F-M ratio and in an increase in F-O and F-M. The relaxati on was completed in 10-15 min after short-term (15 min) photoinhibitor y 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 accompan ied the relaxation of F-V/F-M in darkness or in low light, in the pres ence of lincomycin. Transferring the leaves to low temperature (+4 deg rees C) after the room-temperature illumination (2 h) completely inhib ited the relaxation of F-V/F-M. Low temperature did not suppress the r elaxation if the photoinhibitory illumination had also been done at lo w temperature. The results indicate that illumination of lincomycin-po isoned pumpkin leaves at room temperature does not lead to accumulatio n of a reversibly photoinactivated intermediate.