The concentration of selenium matters: a field study on mercury accumulation in rice by selenite treatment in qingzhen, Guizhou, China

Aims The consumption of rice grain produced in mercury (Hg) contaminated soil was identified as a major route of dietary Hg exposure. The aims were 1) to determine the most suitable concentration of Se that can lead to least Hg accumulation in rice grain in real Hg contaminated paddy field in Qingzhen, and 2) to elucidate the possible mechanism of the protection against the phytotoxicity of Hg in rice by Se. Methods Rice plants were treated with different concentrations (0, 0.01, 0.1, 0.5, 1 and 5 μg/mL) of sodium selenite in a real paddy field in Qingzhen, Guizhou, China. The concentrations of Hg and Se in soil, stream water, rice tissues, and the seed setting rate (SSR) and thousand seed weight (TSW) were checked. The distribution and chemical forms of Hg and Se in rice root were studied by XRF and XAS. Results Treating the rice plants with 0.5 μg/mL of sodium selenite achieved the lowest Hg accumulation in rice grain while the highest SSR and TSW. In rice root, XRF found decreased Hg uptake, and XAS found Hg-Se complexes were formed. These findings, together with the formation of biological barriers like iron plaque, could explain the decreased accumulation of Hg in rice grain at Se levels below 0.5 μg/mL. Se concentrations over 0.5 μg/mL led to increased Hg accumulation and decreased SSR and TSW, which were ascribed to the significantly increased Se accumulation in rice grain. Conclusions This field study suggest that treatment with appropriate level of Se (0.5 μg/mL in this study) is an efficient way to reduce Hg accumulation in rice and increase rice yield and quality, thereafter to protect the health of the rice-dependent populations in Hg-contaminated area.