[Effects of Boron Treatment on Arsenic Uptake and Efflux in Rice Seedlings]

The impacts of boron (B) root application and foliar spray on arsenic (As) uptake, translocation, and efflux by/in rice seedlings (Oryza sativa L.) were investigated in three hydroponic experiments. The addition of B to culture medium did not alter concentrations of arsenite (As[Ⅲ]), arsenate (As[Ⅴ]), and total As, nor did it alter transfer coefficients or uptake efficiency of As in rice seedlings under either As(Ⅲ) or As(Ⅴ) exposure. Foliar B supply increased shoot B concentrations 15.8-fold, and decreased root As concentrations and As uptake efficiency by 20.9% and 18.0% under As(Ⅴ) treatment, and by 12.6% and 13.8% under As(Ⅲ) treatment, respectively, yet did not significantly decrease shoot As concentrations (P>0.05). Interestingly, foliar B supply reduced root B concentrations by up to 47.1% under exposure to As(Ⅴ) but not As(Ⅲ), and corresponding root B concentrations were 85.3% higher in As(Ⅴ) treatment than in As(Ⅲ) treatment on average (P<0.05). Both total As and As(Ⅴ) concentrations were positively related to B concentration in rice roots under As(Ⅴ) treatment following foliar B supply (P<0.05). Rice seedlings extruded 105.2% more As after As(Ⅲ)-pretreatment than after As(Ⅴ)-pretreatment. Foliar B supply increased the amount of As excreted by As(Ⅲ)-pretreated rice root by 14.0%-16.9% (P>0.05), and had no effect on the As efflux of As(Ⅴ)-pretreatment seedlings. A range of 45.9%-70.7% of root As was excreted to solution during one week. These results indicate that the root application of B at four times the concentration of As can slightly decrease As accumulation by rice, whereas foliar B supply is conducive to a decline in As acquisition by rice roots. It is likely that the B channel is at least not the main pathway for As(Ⅲ) entering into rice roots, and the As(Ⅴ) distribution mechanism in rice plants may be shared with that of B.