Flooded rice stores more arsenic, research finds
[BEIJING] Scientists have found that flooded rice accumulates more arsenic than rice grown under aerobic conditions, suggesting a practical way to prevent the toxic element from entering food chain.
Arsenic poisoning — linked to cancer and other diseases — is a significant public health problem worldwide, particularly in Bangladesh and India, where the compound is present in groundwater and taken up by rice plants in flooded paddies.
In a study published in this month's issue of Environmental Science & Technology, Fang-Jie Zhao from the UK-based Rothamsted Research and colleagues compared the arsenic levels between flooded rice — the most conventional farming practice in Southern countries — and aerobic rice planting in which rice roots are not soaked in water.
The researchers found that for the same variety of rice, arsenic accumulation in rice shoots and grain was 10–15-fold higher in flooded than in aerobically grown rice.
Flooding of soil led to a rapid mobilisation of arsenic in the soil solution, which would be absorbed by the rice roots and eventually taken up by the grain, say the authors.
The scientists point out that a new aerobic farming method, initially developed to conserve precious irrigation water, may have the added benefit of producing less arsenic-polluted rice.
"Under field conditions, aerobic rice is likely to produce a lower yield because flooding helps control soil nematode worms and pathogens. However, new aerobic rice cultivars are being developed, which may have a good yield potential," Zhao told SciDev.Net.
He adds that in the experiment, the zinc, copper, manganese and magnesium concentrations in the rice were higher in aerobic conditions than in flooded conditions, indicating that aerobically planted rice could also be more nutritious.
Huang Yizong, a scientist at the Research Centre for Eco-Environmental Sciences at the Chinese Academy of Sciences, says the results are encouraging. But he adds that the researchers should experiment with more types of rice to show that higher arsenic uptake in flooded conditions is not limited to certain varieties.
Zhao also co-authored a study on the mechanism of rice's arsenic uptake, published in the Proceedings of National Academy of Sciences (PNAS) last month (22 July).
The study identified two plant proteins that primarily transport arsenic into the plant. One protein is the port of entry for arsenic while the other controls flow from the roots to the stalk and grain.
The researchers also found that silicic acid — containing silicon, hydrogen and oxygen — interferes with arsenite uptake, and suggested that increasing the available forms of silicon in the soil may decrease the transfer of arsenic from soil and irrigation water to rice.
*Full paper available to users in developing countries
Environmental Science and Technology doi: 10.1021/es800324u
Proceedings of National Academy of Sciences doi: 10.1073/pnas.0802361105