Inhibition of photosystem II by UV-B radiation and the conditions for recovery in the liverwort Conocephalum conicum Dum

Inhibition of photosynthesis by UV-B was investigated in the thalloid liverwort Conocephalum conicum Dum. UV-B irradiance was adjusted to a strength producing 50% inhibition of the rate of photosynthesis during 10 min of irradiation. A linear relationship of the fluorescence terms F-v/F-m of photosystem (PS) II and J(p) was observed following a UV-B irradiation. This suggested that PS II was a major site of UV-B-induced damage of photosynthesis. The apparent inhibition of F-v/F-m was much smaller when electron flow to the secondary PS II acceptor Q(B) was inhibited by DCMU or when F-v/F-m was measured at 77 K. Apparently, the major target of UV-B effects was electron donation to the PS II reaction center, rather than electron transfer reactions at the PS II acceptor side. The time required for repair of PS II from UV-B-induced damage was light-dependent and minimal at a flux density of 5 mu E m(-2) s(-1). Low temperatures and the presence of streptomycin inhibited the repair processes of PS II, indicating that protein synthesis may be involved in the recovery of PS II. The data indicate that UV-B irradiation on bright and cool winter days may be most harmful for photosynthesis of C. conicum. A repeated irradiation of the thalli with UV-B induced tolerance of photosynthesis which was related to an accumulation of pigments with a maximum of absorption around 315 nm.