THERMOLUMINESCENCE STUDIES ON THE FUNCTION OF PHOTOSYSTEM-II IN THE DESICCATION TOLERANT LICHEN CLADONIA-CONVOLUTA

The effect of desiccation and rehydration on the function of Photosyst em II has been studied in the desiccation tolerant lichen Cladonia con voluta by thermoluminescence. We have shown that in functional fully h ydrated thalli thermoluminescence signals can be observed from the rec ombination of the S-2(3)Q(B)(-) (B band), S(2)Q(A)(-) (Q band), Tyr-D Q(A)(-) (C band) and Tyr-Z(+) (His(+))Q(A)(-) (A band) charge stabili zation states. These thermoluminescence signals are completely absent in desiccated thalli, but rapidly reappear on rehydration. Flash-induc ed oscillation in the amplitude of the thermoluminescence band from th e S-2(3) Q(B)(-) recombination shows the usual pattern with maxima aft er 2 and 6 flashes when rehydration takes place in light. However, aft er rehydration in complete darkness, there is no thermoluminescence em ission after the Ist flash, and the maxima of the subsequent oscillati on are shifted to the 3rd and 7th flashes. It is concluded that desicc ation of Cladonia convoluta converts PS II into a nonfunctional state. This state is characterized by the lack of stable charge separation a nd recombination, as well as by a one-electron reduction of the water- oxidizing complex. Restoration of PS II function during rehydration ca n proceed both in the light and in darkness. After rehydration in the dark, the first charge separation act is utilized in restoring the usu al oxidation state of the water-oxidizing complex.