The arsenic contamination in rice


Arsenic (As) is a heavy metal that is well-known as a poison and a carcinogen. Its average concentration in the soil ranges from 5 to 6 mg/kg which is generally related to rock type and industrial activity.
Arsenic contamination of paddy soils is widespread and elevated arsenic levels in rice grains is now a hot issue in many parts of the world. Martha Rose Shulman wrote in the New York Times (15 April 2013) that “it is clear that the levels of inorganic arsenic in rice and rice products are high, and that we and especially children, babies and pregnant women should limit our intake of rice and rice products.”


According to Bogdan and Schenk (2012) in their recent study published in the highly respected Journal of Plant Nutrition and Soil Science (Wiley-VCH Verlag GmbH), flooded rice may contain high arsenic concentration compared to other grain crops. In fact, aside from arsenic-contaminated drinking water, rice is the largest food dietary source of inorganic arsenic. This is because the reducing environment in flooded rice fields causes the dissolution of arsenic and thus increases its availability to the rice plant. Meharg (2004) added that under paddy field conditions, inorganic arsenic introduced into the soil is inter-converted between the reduced inorganic species arsenite (the dominant type) and the oxidized species arsenate. Moreover, arsenite is taken up into the root by the highly efficient Si pathway and arsenate can be taken up via the phosphate transport system.


Bogdan and Schenk (2012) observed among other things that continuous arsenic supply in the soil resulted in doubling of arsenic concentration in rice shoot and grains. They also found that arsenic was mobilized from the root and shoot to the rice grains where it accumulated.

References
Bogdan K. and M.K. Schenk. 2012. Arsenic mobilization in rice (Oryza sativa) and its accumulation in the grains. J. Plant Nutr. Soil Sci. 175: 135-141.
Meharg A.A. 2004. Arsenic in rice-understanding a new disaster for Aouth-East Asia. Trends in Plant Science 9: 415-417.

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