Characterization of a lead hyperaccumulator shrub, Sesbania drummondii

Lead phytoextraction can be economically feasible only when the developed systems employ high biomass plants that can accumulate greater than 1% Pb in their shoots. In this study Sesbania drummondii, a leguminous shrub occurring in the wild, was used to demonstrate its capability for greater than 1% Pb accumulation in shoots when grown in a Pb-contaminated nutrient solution. Shoot concentrations of >4% Pb were obtained from Sesbania plants grown on modified Hoagland's solution containing 1 g Pb(NO3)2/L. The accumulation of Pb in the tissue was found to be dependent on the concentration of Pb in the nutrient solution. Addition of EDTA (100 microM) in the medium containing 1 g Pb(NO3)2/L increased uptake by 21%. Lower pH also favored Pb translocation to shoot. Results also indicate the path of Pb transport through root tissues. Scanning electron microscopy revealed the distribution of Pb granules in the cells from epidermis to the central axis, indicating both apoplastic and symplastic modes of transport. Transmission electron microscopy and X-ray microanalysis of root sections demonstrated the localization of Pb granules in the plasma membrane and cell wall, and also in the vacuoles. This investigation shows that S. drummondii satisfies the prerequisites for a hyperaccumulator, and thus might be useful, particularly, in the restoration of disturbed vegetation.