Arsenic speciation in the phloem exudates of rice and its role in arsenic accumulation in rice grains.

Arsenic (As) speciation in the phloem sap of rice plants and its role in As accumulation in rice grains remain largely uncharacterized. In the present study, we tested As chemical species in the phloem exudates of rice treated with arsenate [As(V)], arsenite [As(III)], monomethylarsonic acid [MMA(V)], or dimethylarsinic acid [DMA(V)]. As(V) was the main species (58%) in the phloem exudates of As(V)-exposed rice, whereas As(III) predominated (69%) in As(III)-exposed rice. A large proportion of As(V) (41-45%) was observed in the phloem exudates when rice was treated with methylated As species. High concentrations of phytochelatins were detected in the phloem exudates when the As(V) treatment level was increased. The role of phloem transport was analyzed by applying a ±stem-girdling treatment to the rice plants, limiting phloem transport to the grain in rice pulsed with As(III), As(V), MMA(V), or DMA(V). The findings of the present study indicate that organic As is more mobile than inorganic As during phloem transport. Phloem transport accounted for 54% of As(III), 56% of As(V), 100% of MMA(V), and 89% of DMA(V) transport to the grain. The total As concentration and As(III) percentage in rice phloem and grain were significantly affected by increasing the phosphate concentration in the medium.

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