Dimethylarsenate (DMA) exposure influences germination rates, arsenic uptake and arsenic species formation in wheat

Abstract

The contamination of cereals with arsenic (As) is a global health and agronomic concern. This study compared the physiological response, As uptake and As speciation in the grains and above ground tissues of 20 wheat cultivars exposed to 5 mg As kg^-1 soil as either arsenate (As^V) or dimethylarsenate (DMA) under glasshouse conditions. Germination rates for the majority of cultivars exceeded 80% for the majority of cultivars when exposed to As^V, but fell significantly to 20-40% when exposed to DMA. For a number of cultivars, grain yields were 20-50% lower when plants were exposed to DMA compared to As^V. Grain As concentrations were between 0.6 and 1.6 μg As g^-1 grain across the twenty cultivars when exposed to As^V, whereas grain As concentrations were much higher (2.2-4.6 μg As g^-1 grain) when exposed to DMA. When plants were exposed to As^V, 100% of the As present in the grain was found as inorganic As while in plants exposed to DMA, 70-90% of As was present as DMA with the remainder found as inorganic As. DMA is believed to be incorporated by plants via silica (Si) acid channels and assessment of grain Si concentrations demonstrated that up to 40% less Si was accumulated in grains when plants were exposed to DMA. The decreased germination rates and grain yields in the presence of DMA is similar to the symptoms described for straight head disease in rice, which has been linked to DMA exposure. The results presented here indicate some analogous processes occur in wheat to those described in rice. We hypothesise that exposure to DMA may have inhibited Si-metabolism and translocation which resulted in both developmental impairment and possibly an increased susceptibility to soil pathogens.

Keywords: Arsenic-silica relationships; Cacodylic acid; Methylated arsenic species; Rice-wheat rotation.