Inorganic mercury accumulation in rice (Oryza sativa L.)

To investigate the source and process of inorganic mercury (IHg) accumulation in rice, we monitored the concentrations of IHg in tissues of rice plants (Oryza sativa L.) from four experimental plantation plots. Biweekly during the rice-growing season, tissues of rice plants, corresponding soil, precipitation, and irrigation water samples were collected. The sampling data support the following: (1) the atmosphere is the principal source of IHg to the aboveground parts of the rice plant; (2) both the atmosphere and soil contribute to IHg content in stalks, but the former source tends to be more important; and (3) soil is the major source of root IHg content. These observations and the fact that the gradually increasing concentration and mass of IHg in stalks and leaves during the rice-growing season suggested that atmospheric Hg could be absorbed by and incorporated into the aboveground parts of the rice plant and that limited or no Hg emission to the air or translocation to the soil occurred after deposition of atmospheric Hg. The root surface acted as a potential Hg barrier and consequently reduced the translocation of Hg ion mass through the root system to the aboveground parts. Accumulated IHg in aboveground parts of rice plants cannot be transported to seeds, which is completely different from the case of methylmercury.