INACTIVATION OF PHOTOSYNTHETIC OXYGEN EVOLUTION BY UV-B IRRADIATION - A THERMOLUMINESCENCE STUDY

The influence of W-B irradiation on photosynthetic oxygen evolution by isolated spinach thylakoids has been investigated using thermoluminescence measurements. The thermoluminescence bands arising from the S(2)Q(B)(-) (B band) and S(2)Q(A)(-) (Q band) charge recombination disappeared with increasing UV-B irradiation time. In contrast, the C band at 50 degrees C, arising from the recombination of Q(A)(-) with an accessory donor of Photosystem II, was transiently enhanced by the UV-B irradiation. The efficiency of DCMU to block Q(A) to Q(B) electron transfer decreased after irradiation as detected by the incomplete suppression of the B band by DCMU. The flash-induced oscillatory pattern of the B band was modified in the UV-B irradiated samples, indicating a decrease in the number of centers with reduced Q(B). Based on the results of this study, W-B irradiation is suggested to damage both the donor and acceptor sides of Photosystem II, The damage of the water-oxidizing complex does not affect a specific S-state transition. Instead, charge stabilization is enhanced on an accessory donor. The acceptor-side modifications decrease the affinity of DCMU binding. This effect is assumed to reflect a structural change in the Q(B)/DCMU binding site. The preferential loss of dark stable Q(B)(-) may be related to the same structural change or could be caused by the specific destruction of reduced quinones by the W-B light.