Rice & Organic Arsenic Species: Interactions with Silicon.

Rice (Oryza sativa) is a globally important food crop that is frequently contaminated by arsenic. Research has shown that silicic acid is an effective addition to minimize uptake of arsenite, but less is known about factors that minimize rice uptake of organic arsenic species, such as dimethylarsinic acid (DMA) and monomethylarsonic acid (MMA). These species are the most commonly encountered organic As species in rice, particularly in rice grown in the southern US. Silicic acid has also been hypothesized to play a role in uptake of DMA and MMA, but this has not been tested in long-term experiments. In hydroponic experiments, we exposed rice plants to either MMA or DMA and different concentrations of silicic acid throughout the life cycle of the crop. MMA exerted toxicity, regardless of silicon concentration, while DMA did not affect biomass. Silicon significantly affected DMA uptake, where increasing concentrations of silicic acid led to lower uptake of DMA in shoots, husk and grain. DMA also caused reduced yield, although this effect was alleviated by increasing silicon concentration. Notably, at the lowest silicon concentration, straighthead was observed and very few grains matured. These data suggest DMA is a causal agent of straighthead, but the negative effects of DMA can be alleviated by silicon addition.