UV-B and cadmium induced changes in pigments, photosynthetic electron transport activity, antioxidant levels and antioxidative enzyme activities of Riccia sp.

UV-B and cadmium, alone and together, induced changes in photosynthetic pigment levels, photosynthetic electron transport activity, enzymatic and non-enzymatic (low molecular weight) antioxidants, level of hydrogen peroxide and lipid peroxidation in Riccia sp. were evaluated. Chlorophyll content was found to decrease with the rising concentration of cadmium and UV-B exposure alone and its level further declined when both the stresses were applied together. In contrast to this, carotenoids exhibited varied response, as it showed enhancement with UV-B (15, 30 and 45 min exposure) and low concentration of Cd (1 and 10 muM) treatment alone and in combination. Both the stresses caused strong inhibitory effect on PS II activity (H2O --> p-BQ), while PS I activity (DCPIP/ASC --> MV) appeared to be less sensitive. Total peroxide content increased with simultaneous increase in lipid peroxidation. The level of non-enzymatic antioxidant ascorbate and enzymatic antioxidants superoxide dismutase and peroxidase activity were found to increase with simultaneous decrease in catalase activity following UV-B and Cd treatments. These results indicate that 45 min of UV-B exposure and 10, 100 and 1000 muM cadmium alone and together, strongly arrested electron flow through PS II which caused accelerated generation of reactive oxygen species (H2O2) and excess accumulation of H2O2 due to significant inhibition of catalase activity, led to the oxidative damage in Riccia sp.