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 thermolumines cence measurements. The thermoluminescence bands arising from the S(2) Q(B)(-) (B band) and S(2)Q(A)(-) (Q band) charge recombination disappe ared with increasing UV-B irradiation time. In contrast, the C band at 50 degrees C, arising from the recombination of Q(A)(-) with an acces sory donor of Photosystem II, was transiently enhanced by the UV-B irr adiation. The efficiency of DCMU to block Q(A) to Q(B) electron transf er decreased after irradiation as detected by the incomplete suppressi on 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 decr ease 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 a nd acceptor sides of Photosystem II, The damage of the water-oxidizing complex does not affect a specific S-state transition. Instead, charg e stabilization is enhanced on an accessory donor. The acceptor-side m odifications decrease the affinity of DCMU binding. This effect is ass umed 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 s tructural change or could be caused by the specific destruction of red uced quinones by the W-B light.