PHOTOSYSTEM-II REACTION CENTERS STAY INTACT DURING LOW-TEMPERATURE PHOTOINHIBITION

Photoinhibition of photosynthesis was studied in intact barley leaves at 5 and 20-degrees-C, to reveal if Photosystem II becomes predisposed to photoinhibition at low temperature by 1) creation of excessive excitation of Photosystem II or, 2) inhibition of the repair process of Photosystem II. The light and temperature dependence of the reduction state of Q(A) was measured by modulated fluorescence. Photon flux densities giving 60% of Q(A) in a reduced state at steady-state photosynthesis (300 mu mol m-2 s-1 at 5-degrees-C and 1200 mumol m-2 s-1 at 20-degrees-C) resulted in a depression of the photochemical efficiency of Photosystem II (F(v)/F(m)) at both 5 and 20-degrees-C. Inhibition of F(v)/F(m) occurred with initially similar kinetics at the two temperatures. After 6 h, F(v)/F(m), was inhibited by 30% and had reached steady-state at 20-degrees-C. However, at 5-degrees-C, F(v)/F(m) continued to decrease and after 10 h, F(v)/F(m) was depressed to 55% of control. The light response of the reduction state of Q(A) did not change during photoinhibition at 20-degrees-C, whereas after photoinhibition at 5-degrees-C, the proportion of closed reaction centres at a given photon flux density was 10-20% lower than before photoinhibition. Changes in the D1-content were measured by immunoblotting and by the atrazine binding capacity during photoinhibition at high and low temperatures, with and without the addition of chloramphenicol to block chloroplast encoded protein synthesis. At 20-degrees-C, there was a close correlation between the amount of D1-protein and the photochemical efficiency of photosystem II, both in the presence or in the absence of an active repair cycle. At 5-degrees-C, an accumulation of inactive reaction centres occurred, since the photochemical efficiency of Photosystem II was much more depressed than the loss of D1-protein. Furthermore, at 5-degrees-C the repair cycle was largely inhibited as concluded from the finding that blockage of chloroplast encoded protein synthesis did not enhance the susceptibility to photoinihibition at 5-degrees-C. It is concluded that, the kinetics of the initial decrease of F(v)/F(m) was determined by the reduction state of the primary electron acceptor Q(A), at both temperatures. However, the further suppression of F(v)/F(m) at 5-degrees-C after several hours of photoinhibition implies that the inhibited repair cycle started to have an effect in determining the photochemical efficiency of Photosystem II.