Inhibition of chloroplasts by UV-irradiation and heat-treatment.

The site of inhibition in UV-irradiated and heat-treated chloroplasts was examined by using artificial electron donor compounds such as p-phenylenediamine and hydroquinone which donated electrons specifically to photosystem II. In both cases the electron donors restored the photoreduction of nicotinamide adenine dinucleotide phosphate and the restored activity was inhibited by 3-(3,4-dichlorophenyl)-1,1-dimethyl urea. The fluorescence of variable yield was eliminated by both inhibitory treatments and was partially restored by the electron donors in the heat-treated but not the UV-irradiated chloroplasts. The results suggest that the sites of inhibition of UV-radiation and heat treatment are in the photosynthetic electron transport chain between water and photosystem II.