Environment and genotype controls on mercury accumulation in rice (Oryza sativa L.) cultivated along a contamination gradient in Guizhou, China

Abstract

Accumulation of total Hg (THg) and methylmercury (MeHg) was investigated among 50 indica rice varieties cultivated in 3 sites in Guizhou, China, classified as highly-contaminated, moderately-contaminated, and background sites. Concentrations of soil and pore water (depth: 0–2cm) THg and MeHg were poor predictors of THg and MeHg concentrations in polished white rice and bran, with significantly higher accumulation in the moderately-contaminated site compared to the other 2 sites (ANOVA, p<0.0001). Results indicated other environmental factors played a critical role in the translocation of Hg species from paddy soil to rice grain. We hypothesized alkaline conditions at the highly-contaminated site (surface water pH 11, other 2 sites pH 7.6–8.0), mitigated the uptake of Hg species by decreasing the solubility of micronutrients needed for plant growth, or by reducing the bioavailability of Hg species. White rice MeHg concentrations were associated with genotype but not inorganic Hg(II) concentrations (IHg=THG–MeHg) (MeHg: p<0.001, IHG: p=0.44), while bran MeHg and IHg concentrations were not significantly associated with genotype (p>0.05 for both analyses), indicating there may be genetic markers for the translocation of MeHg from the caryopsis to the endosperm, i.e., from the maternal to the filial tissue. Lastly, calculation of daily MeHg ingestion rates confirmed international guidelines for MeHg exposure were exceeded at the moderately-contaminated site (background: 0.013±0.0052μgkg−1d−1, n=46; highly-contaminated: 0.066±0.034μgkg−1d−1, n=49; moderately-contaminated: 0.42±0.13μgkg−1d−1, n=49). However, MeHg exposure at this site may be decreased up to 69% by cultivating low Hg-accumulating rice varieties.