Unusual Accumulation of Silver in the Aleurone Layer of an Indian Rice (Oryza sativa) Landrace and Sustainable Extraction of the Metal

Uptake of noble metals by cereal plants is not reported in literature. Our study of 505 native rice landraces showed that nine of them accumulate silver at a high concentration when grown in the same soil. Among these, a medicinal rice landrace from West Bengal, Garib-sal was found to accumulate silver at an especially high concentration in the grains. Cultivation of Garib-sal rice in three successive years in Basudha farm in the rice growing period of June-October confirmed that for the same concentration of silver in the soil (∼0.15 mg/kg), Garib-sal accumulates it in the grains to the extent of ∼15 mg/kg. Laboratory experiments also demonstrated that silver uptake by Garib-sal is significantly greater than for other varieties grown on the same soil, and that the metal accumulates mostly in the grain. To detect the location of deposition of silver in the grains, secondary ion mass spectrometry was performed. The images reveal that the silver is concentrated in the aleuronic layer of the rice bran. Its concentration decreases in the subaleurone and becomes negligible in the endosperm. Accumulation of silver does not alter the grain morphology and chemical characteristics. The metal may be extracted from the bran after milling of the rice, thereby causing no loss of the foodstuff.