Differential Rice Grain Arsenic in Cultivars Associated with Soil and Water Management.

Reduction and methylation of arsenic (As) in rice are important processes that contribute to food quality and minimization of any potential risk from dietary exposure, primarily because of the considerably lower toxicity of methyl As compared to inorganic As species.  Twenty-one rice cultivars from the U.S. World Germplasm Collection, with variable susceptibility to straighthead and including both indica and japonica subspecies, were grown in two soils (a native soil of moderate As concentration and an adjacent high As soil utilized for screening of straighthead which has received As in the form of monosodium methylarsonate [MSMA] in alternating years) and under two water treatments (saturated and flooded).  The total arsenic concentration in milled rice grain was determined by ICP-MS following digestion by HNO₃/H₂O₂.  Arsenic species were quantified by HPLC-ICP-MS following extraction with 0.16 M TFA.  The total grain As concentration and speciation varied significantly among the cultivars.  Higher total grain As concentration was observed in the flooded soil compared to the saturated soil. Also, grain As concentration was greater in rice from the MSMA amended soil.  The dominant species of arsenic extracted from the grain were dimethylarsonate (DMA) and inorganic As (III), and the concentrations were highly influenced by soil-As concentration and water management.  Cultivar susceptibility to straighthead disease in the MSMA-amended soil was not related to grain-As concentration or speciation.  These results demonstrate the potential of plant breeding in combination with water management to produce rice with acceptably low grain-As concentrations and inorganic:organic As ratios.