Tolerance mechanisms in mercury-exposed Chromolaena odorata (l.f.) R.M. King et H. Robinson, a potential phytoremediator

Abstract

:Chromolaena odorata (L.f.) R.M. King et H. Robinson plants were grown in Hoagland's solutions with 0.00 ppm and 1.00 ppm Hg(NO 3 ) 2 . The calcium, magnesium, iron, and sulfur levels in the leaves were found to be not significantly affected by presence of the uptaken Hg 2+. The chlorophyll a, chlorophyll b, and total chlorophyll contents of its leaves also remained within normal levels, which may indicate that the photosynthetic machinery of the Hg-exposed C. odorata was unaffected by the presence of Hg 2+. The results of the ICP-AES analyses of the Hg 2+ contents established the presence of Hg 2+ in all the subcellular components obtained from the leaves of the Hg-treated C. odorata plants, and that the ultimate localization of Hg 2+ is in the vacuoles. The findings revealed no significant differences in the degree of oxidative injury between the cells from the control and Hg-treated plants, as evidenced by the low lipid peroxidation levels obtained with the TBARS assay. The SH-containing biomolecules that were initially detected through DTNB assay manifested a predominant peak in the RP-HPLC chromatographs of both the control and Hg-treated plants, with their retention times falling within the ranges of GSH, MT, and cysteine standards. However, the concentrations of the GSH-and/or MT-like, Cys-containing biomolecules detected in the leaves of Hg-treated C. odorata plants were ten times higher than those of the control. The findings of this study suggest that the enhanced antioxidative capacity, the production of Hg-binding biomolecules, and the localization of Hg 2+ ions ultimately in the vacuoles of the leaves are the mechanisms which bring about Hg 2+ tolerance and homeostasis in C. odorata plant. These results indicate that C. odorata is a potentially effective phytoremediator for Hg 2+ .