Objective  Capability of Oryza sativa L. in accumulating and transforming methylmercury (MeHg) at different growth stages was studied.

  Method  Rice plants without MeHg exposure (as control) and 4 treatments groups with a MeHg exposure at seedling, tillering, jointing, or heading stages were cultivated in a hydroponic setting. Total Hg and MeHg concentrations in tissues of the plants were determined, and percentages of MeHg to total Hg in straws and grains calculated.

  Result  Compared with control, the MeHg exposure increased the MeHg content in rice tissues at all growth stages, but the accumulation significantly varied according to both the growth stage of the exposure and the tissue tested. The total Hg and MeHg contents in all parts of rice at the seedlings or tillering stage were lower than those at the jointing or heading stage. It indicated greater accumulation tended to occur approaching the end of plant growth. And at the seedling and tillering stage, more than 60% of the Hg in the grains was in the form of MeHg, as opposed to more than 80% at the jointing and heading stages. Thus, MeHg was more highly accumulated, or less likely be transformed, in the grains at the heading stage than in the early stage of rice growth.

  Conclusion  As a rice plant maturing, the MeHg accumulation increased and the transformation lessened. Hence, exposure of rice to MeHg would present less severe a safety risk at early than late stage of growth.