Influence of different arsenic species on uptake, speciation and efflux of arsenic in hydroponic rice plants

Uptake, translocation and speciation of As in rice plants have been investigated through hydroponic cultivations under stress from different As species. After germination, rice seedlings were treated with arsenite [As(III)], arsenate [As(V)], monomethylarsonic acid (MMA) or dimethylarsinic acid (DMA) at concentrations of 50, 100, 150, 200 and 250 mu g/L for 24 days. Only inorganic As species were detected in the rice plants treated with As(III) or As(V), indicating that rice seedlings could not methylate inorganic As in hydroponic culture. As(V) in the rice roots was readily reduced to As(III) after uptake; thus, As(III) was the dominant species in the rice roots ( > 60%) and shoots ( > 80%) regardless of As(III) or As(V) treatment. The increased As(III) proportion in the nutrient solutions was due to the efflux of As(III) from the rice roots. MMA with relatively low stability in the blank nutrient solution was demethylated to As(III). Moreover, demethylation and methylation of MMA might occur simultaneously in rice plants. Specific proportions of MMA and AsB were observed in the rice roots treated with DMA, implying that MMA and AsB were the DMA metabolites in rice roots after detoxification.