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.