Assessment of mercury accumulation in various tissues of rice plant (Oryza sativa L.) from a Ramsar site in India

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Dietary exposure to mercury through rice consumption is an issue of solemn global concern. The favourable bio-geochemical environment of paddy fields often makes them potential mercury cycling hotspots. India is one of the largest producers and consumers of rice globally. To date, studies focusing on mercury bioaccumulation in rice during the different stages of its growth cycle have not been studied in Indian agro-ecosystems.This inturn resulted in a knowledge gap in the global mercury assessment. Being a party to the Minamata Convention, there is an urgent need to unravel the scenario of mercury contamination in rice, one of the staple food crops across the world. In the present study, total mercury (THg) in the rhizosphere soil as well as its accumulation in different tissues of the rice plant were examined by a field investigation in Kuttanad – a unique Ramsar site in Kerala, India. For the purpose of the study five permanent plots were selected and monitored for the entire rice-growing season (from the 30th to the 120th day). Direct Mercury Analyzer (DMA – 80, Milestone, USA) was used for analyzing THg in rhizosphere soil and plant tissues. Mercury concentration in soils varied from 0.037 mg/kg to 0.117 mg/kg with a mean concentration of 0.088 mg/kg. The THg accumulation pattern in plant tissues followed the order: root > leaf > grain > husk > stem. The mean THg concentration in rice grains was 0.22 mg/kg. The low values for bioaccumulation factor (0.616) and translocation factor (0.425) revealed that a major portion of mercury accumulated by the plant is stored in the roots.