When silver ‘grows’ in paddy fields

A rice variety originally from West Bengal is able to accumulate the metal in its grain, IIT researchers find

The rice is put to the test in the field.

It is a rice variety with a silver touch, literally. Garib-sal, one of 505 types of rice plants tested by scientists, is capable of absorbing silver found naturally in soil and accumulating it in the grain to unusually high levels of 15 mg per kg.

The rice was able to accumulate high quantities of silver even when the soil contained only about 0.15 mg per kg.

The unusual accumulation of silver in the grain and other parts of the plant, researchers say, throws open the possibility of commercial extraction of the metal through farming.

The maximum concentration of silver in the plant is in the grains. Silver accumulation is largely in the bran of the rice grain, and once polished, the silver in the grain is reduced significantly.

Polishing grain is crucial

It is not, however, for consumption as food. “We do not advocate consumption of the unpolished rice as staple food. If the rice is polished very well then it may not lead to silver toxicity,” says Prof. T. Pradeep from the Department of Chemistry, Indian Institute of Technology Madras, who authored the research.

Silver is not known to accumulate in the reproductive tissues of any cereal, and in agricultural crops the amount of silver that gets accumulated is less than 1 mg per kg of dry weight of the plant.

Researchers at IIT Madras stumbled upon the rice variety while screening for different metal ions in the 505 rice varieties. Only nine showed high silver accumulation, with Garib-sal the highest.

The rice varieties are maintained by Dr. Debal Deb, head, Centre of Interdisciplinary Studies, Kolkata, as part of rice variety conservation efforts. Garib-sal used to be grown by farmers in Purulia, West Bengal. The researchers tested Garib-sal’s ability to accumulate silver even when grown in soils with very low silver concentration. Even when the soil contains only about 0.01 mg of silver per kg, the rice plant was able to concentrate 0.20 mg of silver per kg in the grains.

“The rice variety has the ability to accumulate silver about 100 times more than any other rice,” says Prof. T. Pradeep. The variety was cultivated in the farm for three successive years in soil containing about 0.15 mg per kg and the uptake and accumulation of the noble metal was nearly the same.

Garib-sal accumulated 50 times more silver than another type in control tests.