ARSENIC STRESS RESPONSE IN RICE: COMPARISON BETWEEN ORGANIC AND INORGANIC PESTICIDE

Arsenic (As) contaminations of soils, sediments and waters occur from both natural and anthropogenic sources. One of the major sources of As in soil is the excessive use of arsenical pesticides in agriculture. The use of inorganic pesticides was banned by the USEPA in the late eighties and early nineties, but vast areas of agricultural lands had already been contaminated. In contrast, organic arsenicals such as monomethylarsenic acid (MMA) and dimethylarsenic acid (DMA) are considered to be less toxic, and are still allowed to be used as herbicides in agricultural lands, orchards, golf courses etc. Symptoms of As stress in plants include growth inhibition, chlorosis, defoliation and water-deficiency stress. However, only sporadic information is available in the literature comparing the phytotoxic effects of various forms of arsenic on plant growth and metabolism. The objective of this study was to investigate the effect of high concentrations of sodium arsenate and DMA on germination and growth of rice plants. A greenhouse study is in progress using four different types of soils, chosen on the basis of their potential differences with respect to reactivity and potential phytoavailability of As. The soils were amended with sodium arsenate and DMA at two rates: 675 and 1500 mg/kg of As, representing the high end of As contamination, to simulate Superfund site conditions. Rice plants were grown for a period of 4 months, and harvested twice: at a young seedling stage (after 1 month) and a mature plant stage (after 4 months). Effect of sodium arsenate and DMA on germination and growth in rice is being measured in terms of germination percentage, plant biomass, root and shoot length, chlorophyll content and total protein content in plant tissues. In order to further elucidate the differences in the rate of uptake of organic and inorganic arsenicals, As uptake kinetic studies are being conducted in laboratory-grown rice seedlings. Eight-day old rice seedlings were exposed to DMA and sodium arsenate solution (675 and 1500 mg/kg As) for 30 minutes, 1 hr, 2 hr, 4 hr, and 8 hr periods. The differences in the rate and amount of As accumulation in the roots and shoots of the seedlings were measured using GFAAS. Results from this investigation will elucidate the differences in phytotoxicity induced by organic and inorganic As compounds.