Bioaccumulation and physiological effects of mercury in Sesbania drummondii

The accumulation of mercury and its effect on growth, photosynthesis and antioxidative responses were studied in Sesbania drummondii seedlings. Mercury concentration in shoots as well as in the roots increased with increasing Hg concentrations in the growth solution. The accumulation of Hg was more in roots than shoots. At 100 mg l^-1 Hg concentration, shoots accumulated 998 mg Hg kg^-1 dry weight (dw) while roots accumulated 41 403 mg Hg kg^-1 dw. Seedlings growth was not significantly affected at lower concentrations of Hg. A concentration of 100 mg l^-1 Hg inhibited growth by 36.8%, with respect to control. Photosynthetic activity was assessed by measuring chlorophyll a fluorescence by determination of F_v/F_m and F_v/F_o values. Photosynthetic integrity was not affected up to 50 mg l^-1 Hg concentration, however, concentrations higher than 50 mg l^-1 affected photosynthetic integrity. Sesbania responded to Hg induced oxidative stress by modulating non-enzymatic antioxidants [glutathione (GSH) and non-protein thiols (NPSH)] and enzymatic antioxidants: superoxide dismutase (SOD), ascorbate peroxidase (APX) and glutathione reductase (GR). Glutathione content and GSH/GSSG ratio increased up to a concentration of 50 mg l^-1 while slight down at 100 mg l^-1 Hg. The content of NPSH significantly increased with increasing Hg concentrations in the growth medium. The activities of antioxidative enzymes, SOD, APX and GR followed the same trends as antioxidants first increased up to a concentration of 50 mg l^-1 Hg and then slight decreased. The results of present study suggest that Sesbania plants were able to accumulate and tolerate Hg induced stress using an effective antioxidative defense mechanisms.