Comparative study of the action of ultraviolet‐C and ultraviolet‐ B radiation on photosynthetic electron transport

The effect of ultraviolet‐C (UV‐C, mainly 254 nm radiation) and ultraviolet‐B (UV‐B, 290‐320 nm) radiation on the photosynthetic electron transport reactions has been investigated. The rates of Hill activity mediated by ferricyanide and dichlorodimethoxy‐p‐benzoquinone (DCDMQ) were differently sensitive to UV‐C but equally inhibited by UV‐B. Replacement of water with diphenylcarbazide was ineffective in restoring the activity of dichlorophenol indophenol (DCPIP) Hill reaction in UV‐B treated chloroplasts, but had significant effect in UV‐C treated chloroplasts.Photobleaching of carotenoids in the presence of carbonyl cyanide‐m‐chlorophenyl‐hydrazone, an indicator of the photochemical reaction associated with the reaction centre of photosystem II, was suppressed and is paralleled by the changes in Hill activity only in UV‐B‐treated chloroplasts. Carotenoid photobleaching occurred even in UV‐C treated chloroplasts showing no measurable Hill activity. UV‐C and UV‐B irradiation diminished variable fluorescence. With UV‐B treated, but not with UV‐C treated chloroplasts, an increase in the fluorescence yield was observed upon the addition of 3‐(3,4‐dichIorophenyl)‐l,l‐dimethylurea (DCMU) and/or Na dithionite.Photosystem I activity was found to be unaffected by both UV‐C and UV‐B radiation at the fluences tested. Kinetics of P700 photooxidation and dark reversal in UV treated chloroplasts indicate that only the electron flow from photosystem II to photosystem I is impaired. It is concluded that while UV‐B radiation inactivates specifically the photosystem II reaction centre, UV‐C radiation acts at plastoquinone, the quencher Q, and the water oxidizing enzyme system.