Is arsenite more toxic than arsenate in plants?

In order to evaluate the differential absorption and toxicity of arsenate (As^V) and arsenite (As^III), Lemna valdiviana plants were grown in a nutrient solution and subjected to 0.0 (control); 0.5; 1.0; 1.5; 2.0; 3.0; 4.0; 5.0 and 7.5 mg L^-1 of As^III or As^V for three days. Exposure to both chemical forms resulted in As bioaccumulation, although As^III-grown plants showed higher As content in tissues. In As^V-grown plants, the relative growth rate (RGR) decreased to 50%, at a concentration of 4.0 mg L^-1, while for treatments with As^III, the same decrease was observed at 1.0 mg L^-1. The tolerance index decreased with increasing concentrations, with lower values for As^III. Plants treated with As^III showed increased superoxide anion levels, whilst higher levels of hydrogen peroxide were present in AsV-treated plants. Moreover, malondialdehyde (MDA) levels were higher for plants subjected to As^III when compared to As^V at lower concentrations. Concentrations of 1 mg L^-1 of As^III and 4 mg L^-1 of As^V showed equivalent MDA levels. Superoxide dismutase and catalase activities were increased at low concentrations and were inhibited at higher concentrations of As^III and As^V, whereas peroxidase activity was positively modulated by increased As^III or As^V concentrations. In conclusion, L. valdiviana plants took up and accumulated arsenic as As^III or As^V, demonstrating the potential for phytoremediation of this metalloid. Furthermore, As^III-exposed plants showed enhanced toxicity when compared to As^V, at the same applied concentration, although toxicity was more related to internal As concentrations, regardless of the chemical form applied.