A profile of heavy metals in rice (Oryza sativa ssp. indica) landraces

Uptake of metals in rice has been reported in the context of human health and nutrition. In particular, low levels of heavy metals like Fe, Zn, Cu and Mn in rice and other cereals are considered to be desirable for good preventive nutrition. According to FAO and WHO, the daily recommended intake of Fe, Zn, Cu, and Mn for an average adult is 15, 12, 1.5–3.0 mg and 2–5 mg respectively. Zn deficiency in diets entails growth retardation, immune dysfunctions and cognitive impairment. Iron deficiency limits oxygen delivery to cells, resulting in fatigue and decreased immunity. Deficiency of Cu causes low immune competence, while Mn deficiency may cause hardening of veins. While Fe, Cu, Zn and Mn in cereals have nutritional value, certain other heavy metals (e.g. arsenic and mercury) in cereals are highly toxic, and pose serious health hazards, including cancer and disruption of the central nervous system. Several traditional rice varieties are considered in folk medicine to have high nutritive and therapeutic value. Some of these land races are known in indigenous cultures to cure anaemia in women during and after pregnancy, and many of them (e.g. Kalabhat, Navara, Norungan, etc.) are now known to contain high levels of iron. Thousands of folk rice landraces are yet to be screened for their micronutrients content and therapeutic potential. Examination of heavy metal content in different varieties of rice is therefore necessary to understand the dynamics of nutrient uptake in rice from different types of soil, containing heavy metals of known beneficial or deleterious effects on human health. Profiling of metals in different rice landraces would also uncover the poorly known nutritive and/or therapeutic properties of different rice landraces. This study is a contribution to generate a database of metal content of a large number of landraces of Oryza sativa ssp. indica, accessed from different regions of India and Bangladesh. From the accession of 1030 folk rice landraces, grown every year on Basudha farm located in Rayagada district of Odisha (www.cintdis.org/basudha), we selected 130 rice landraces for profiling of heavy metals. Each rice sample was collected from the core samples of panicles, harvested in 2014 and 2015, and analysed in the Department of Chemistry, Indian Institute of TechnologyMadras, Chennai. Ten samples of each landrace were tested. Each sample, weighing 500 mg, was dissolved in 5 ml HNO3, 2 ml H2O2 and 1 ml water, and digested in an Anton Paar Multiwave 3000TM microwave digester for 15 min at 800 W. After cooling, the solution was further diluted 10 times for analysis of different metal ions using an inductively coupled plasma mass spectrometer (ICPMS). After digestion, the solution was examined for silver ion concentration, using Perkin Elmer ICPMS (model NexION 300XTM). XPS