Differential response of Senna occidentalis L. to arsenic and cadmium contaminated soil

Abstract

We investigated the phytoremediation potential of Senna occidentalis L., a pantropical plant that has been associated with tolerance to heavy metal-contaminated soils around mining sites. Seedlings of S. occidentalis were exposed to cadmium chloride (CdCl2) and sodium arsenate (Na3AsO4) at concentrations of 200, 300, and 400 mg L-1 under greenhouse conditions. Heavy metal tolerance was assessed by comparing biomass and stress indicators such as chlorophyll, proline, and hydrogen peroxide content. Arsenic treatment had more toxic effects than cadmium on Senna physiology. Regardless of concentration of arsenic applied, the biomass decreased by 50% as compared to control and cadmium-treated plants. Chlorophyll content decreased with exposure to both heavy metals. Higher concentration of Cd and As (400 mg L-1) resulted in 50% reduction in chlorophyll content. Proline and hydrogen peroxide levels were higher in arsenic-treated plants compared to controls and cadmium-treated plants, indicating an enhanced stress response when exposed to arsenic. When heavy metal content was measured, there was a significant accumulation of arsenic in the leaves, stems, and roots, indicating that arsenic in these tissues was responsible for the profound changes in biomass, proline, and hydrogen peroxide content. In contrast, although significant, there was less cadmium uptake by Senna and tolerance can be seen, which was reflected by normal biomass, proline, and hydrogen peroxide levels. High translocation of metals from soil into roots and low translocation from root to shoot tissues suggests the potential for S. occidentalis to be used for phytostabilization of arsenic- and cadmium-contaminated soils.