RELATIONSHIP BETWEEN PHOTOINHIBITION OF PHOTOSYNTHESIS, D1 PROTEIN-TURNOVER AND CHLOROPLAST STRUCTURE - EFFECTS OF PROTEIN-SYNTHESIS INHIBITORS

Irradiation of Spinacia oleracea intact leaf tissue and of mesophyll p rotoplasts of Valerianella locusta at 20 degrees C with strong light r esulted in severe (40-80%) inhibition of photosynthesis, measured as p hotosystem II electron transport activity in isolated thylakoids or as fluorescence parameter F-V/F-M on intact leaf disks. No net degradati on of the D1 protein of photosystem II was seen under these conditions . However, in the presence of streptomycin, an inhibitor of chloroplas t protein synthesis, net D1 degradation (up to about 80%) did occur wi th a half-time of 4-6 h, and photoinhibition was enhanced. Thylakoid u ltrastructure remained stable during photoinhibition, even when substa ntial degradation of D1 took place in the presence of streptomycin. Wh en leaf disks were irradiated at 2 degrees C, streptomycin did not inf luence the degree of photoinhibition, and net D1 degradation did not o ccur. These results suggest that in excess (photoinhibitory) light at 20 degrees C, turnover (coordinated degradation and synthesis) of D1 d iminished the degree of photoinhibition. The observed photoinhibition is thought to be due to the accumulation of inactive photosystem II re action centres still containing D1. In the presence of streptomycin, t he D1 protein was degraded (probably in the previously inactivated cen tres), but restoration of active centres via D1 synthesis was blocked, leading to more severe photoinhibition. Low temperature (2 degrees C) , by restricting both degradation and resynthesis of D1, favoured the accumulation of inactive centres. Streptomycin and chloramphenicol (an other inhibitor of chloroplast protein synthesis) were tested for side -effects on photosynthesis. Strong inhibitory effects of chloramphenic ol, but much less severe effects of streptomycin were observed.