Accumulation of Arsenic in Leaves and Grain Is Affected By Variety and Soil Arsenic.

The arsenic (As) levels in rice grains and food products can reach toxic levels when produced under certain growing conditions found mostly in Asia.  The World Health Organization (WHO) recently set a CODEX limit of 0.2 ppm inorganic As in milled white rice, and lower limits are expected to be set for baby food products.  The aim of the present study was to identify genes and physiological mechanisms that limit the accumulation of As in rice grains.

A set of 15 rice accessions were selected from among 1763 diverse rice accessions for their production of grains exceptionally high or low in arsenic concentration, and were grown in soil amended with arsenic (monosodium methyl arsenate herbicide, MSMA) as well as in native or non-amended flooded field plots. Arsenic is toxic to plants as well as animals, and application of MSMA has been used for decades to select rice cultivars resistant to As-induced straighthead disease.  Association was found between grain-As and straighthead response, with none of the high grain-As accessions being resistant to straighthead.  This suggested that reduced As uptake and/or increased As detoxification in the plants might be contributing to both limited grain-As accumulation and straighthead resistance.  The leaf-As concentrations for all accessions were higher in As-amended soil than in native soil.  Only accessions resistant to straighthead set seed in the As-amended plots, and again, concentrations were increased with growth in As-amended soil versus native soil.  Several high grain-As accessions were found to have more As in their immature flag leaves than in their mature flag leaves, suggesting that As was being translocated out of the flag leaves during grain fill.  In contrast, all of the accessions selected for low grain-As continued to accumulate As in their flag leaves during grain fill.