Silicon reduces arsenic accumulation in rice.

Scientists in the UK and Japan have worked out why rice accumulates so much arsenic from soil and suggest ways to mitigate the problem.

Rice generally contains much more arsenic than other crops and can pose a potential health risk to people who eat a lot of rice.

Studies of soil chemistry reveal that rice grown under normal flooded paddy conditions accumulates over 10-fold more arsenic than under non-flooded (aerobic) conditions. This suggests arsenite, the reduced form of arsenic, may be the prevalent form in paddy fields. Previously, most research had focused on arsenate, the oxidised form that is present in aerobic soils, explains environmental scientist Fangjie Zhao, of the UK's Rothamsted Research.

Arsenite is an uncharged neutral molecule under most soil conditions, with a similar molecular size to that of silicon. Now, researchers led by Zhao have identified two transporters of silicon that also allow the uptake of arsenite (PNAS doi:I0.I073: pnas.0802361105).

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The researchers suggest arsenic uptake by rice may be reduced by either supplying silicon to paddy fields or by managing the water supply to minimise the mobilisation of arsenite in the soil.

Changing cultivation methods for a crop is not easy because rice is typically grown under flooded conditions to help suppress diseases and nematodes, points out Zhao. However, new cultivation methods are being developed to conserve water, such as growing the crop on raised beds supplied by water channels. 'We are also testing if draining water during certain stages of rice growth could have a good effect in decreasing arsenic accumulation in rice grain,' said Zhao.

Applying silicon fertilisers is most likely to help boost yield and decrease arsenic accumulation. Some wastes, including basic slag from steel, aluminium or cement production, have high silicon contents, and may potentially be used but need to be tested, said Zhao.

To develop a rice that accumulates less arsenic, the researchers also want to work out how arsenite is moved from rice stalk and leaves to the grain, as well as finding rice variants that favour silicon uptake but discriminate against arsenite uptake, said Zhao.

Arsenic poisoning is a big public health problem in Bangladesh and India, as arsenic-contaminated groundwater is used to irrigate rice crops.