ALTERNATION IN PHOTOSYNTHETIC CHARACTERISTICS OF ANABAENA-DOLIOLUM FOLLOWING EXPOSURE TO UVB AND PB

Anabaena doliolum, when exposed to either ultraviolet-B (UVB) radiatio n or Pb, showed reduced growth rate, carbon fixation, O-2-evolution, p hotosynthetic electron transport activity and ATP pool size, The rate of respiration was found to increase in UVB-treated cells; this increa se was more pronounced in the cells exposed to UVB and Pb simultaneous ly. The UVB-induced inhibition of 2,6-dichlorophenol indophenol (DCPIP ) photoreduction and lowering of chlorophyll a fluorescence could not be reversed by artificial electron donors (diphenyl carbazide, NH2OH a nd MnCl2). These electron donors, however, substantially reversed the inhibition caused by Pb, thereby suggesting that UVB primarily inhibit s the photosystem II (PS II) reaction center and Pb arrests the electr on flow at the water splitting site, Nevertheless, the suppressed fluo rescence intensity and the reduced emission and excitation peaks of ph ycobilisomes indicate the involvement of Pb in inhibition of PS II, Al l combinations of UVB and Pb inhibited the different metabolic process es in a synergistic manner.