Variability of Grain Arsenic Concentration and Speciation in Rice (Oryza sativa L.)
Pillai, Tushara Raghvan
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Arsenic is not an essential element and can be toxic to both plants and animals in high concentration. There is a demonstrated association between soil arsenic (As) and the occurrence of straighthead (a physiological disorder in rice characterized by panicle sterility and yield loss); however, the relationship between grain-As accumulation and straighthead susceptibility in rice is not yet fully understood. The objective of the current study was to evaluate a set of diverse rice cultivars, including both indica and japonica subspecies, for total grain-As (TGAs) and As-species concentrations in 2004, 2005, and 2007, on a native (moderate As-concentration) paddy soil and an adjacent monosodium monomethylarsonate (MSMA) amended soil. Cultivars were evaluated under both continuously flooded and intermittently flooded (saturated) field conditions. The genotypic differences in the occurrence of straighthead, total grain-As (TGAs) and As-species concentrations, and their relationships with plant growth parameters, e.g., heading date, plant height, and yield were assessed. The cultivars exhibited a considerable range in both TGAs and grain-As species concentrations. In 2004 and 2005, twenty-one rice cultivars replicated on native soil under continuous flooding showed significant differences in TGAs and As-species concentrations by genotype and year. In 2005, heading was generally delayed in the rice cultivars, resulting in reduced yields that were likely associated with unusually high temperatures and prolonged exposure to stresses in the field, including prolonged flooding and associated soil-As induced stresses. Lower grain-As concentrations were generally associated with early maturing and high yielding genotypes, but with some exceptions. Total grain-As concentrations were not correlated to straighthead susceptibility suggesting that high As concentration in rice grain might not be a direct cause of the genotype-dependent panicle sterility associated with MSMA in soil. The rice cultivars grown on the MSMA-flooded treatment could be effectively differentiated for their relative straighthead susceptibility, with scores ranging from 1 to 8 for the most resistant to the most susceptible genotypes, respectively. In general, traits such as low grain-iAsIII concentration,early maturity, and high yield were correlated with straighthead resistance. In the MSMA-flooded treatment, very high grain-As accumulation resulted in elevated rice-grain dimethyl-AsV (DMAsV) concentration, whereas, the concentration of the more harmful inorganic-AsIII species was less affected. The TGAs and As-species concentrations were considerably higher in continuously--flooded soil than the intermittently-flooded soil. The variations in TGAs and grain-DMAsV concentrations were more highly influenced by water regime than by genotype, whereas, grain-iAIII concentrations were more highly genotype dependent. In the native soil with intermittent flooding, the concentrations of grain-DMAsV and the less desirable grain-iAIII concentrations were lowest. The study concluded that for attaining lower As accumulation in the rice grain both genotype selection and water management are potentially useful approaches.