Arsenic Accumulation in Rice (Oryza sativa): An Overview
Abstract
Accumulation of arsenic (As) in rice is highly variable across the globe and depends on a number of factors. Presently, there are no set standards on what can be considered as a regulatory concentration in rice, yet studies related to As toxicity to humans are abundant. This study is an overview of the range of As concentrations in rice grown globally and examples of the relevant contributing factors . Based on this overview, As concentration in rice can range from 0.01 to 0.8 mg kg-1, and some of the factors that can cause it are growing conditions, varieties, soils and water quality.
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Introduction
Rice (Oryza sativa) is one of the major staple food crops in the world, with daily consumption of up to 0.5 kg (dry weight) per adult per day in Asian countries (FAO 2002). In the United States alone, approximately 4.1 million metric tons of rice was consumed in 2014/2015 (Statista 2015). Rice in the U.S. is primarily grown in California and the southern states of Arkansas, Texas, Louisiana, and Mississippi, with approximately 50 and 20 % of total rice production coming from Arkansas and California, respectively (Zavala and Duxbury 2008). Of all grain crops, rice accumulates the highest amount of arsenic (As), largely because of the high availability of arsenic under anaerobic (reduced oxygen) soil conditions (Marin et al. 1993; Williams et al. 2007 a). In addition, flooded (paddy) rice is more efficient in As uptake compared to other small grains like wheat and barley. This is attributed to (i) arsenite (As O z-) mobilization and its enhanced bioavailability to rice plants under x y anaerobic conditions (Xu et al. 2008), and (ii) the ability of arsenite to share the highly efficient silicon (Si) uptake pathway in rice (Ma et al. 2006; Zhao et al. 2009) (Fig. 1). It has been shown that by maintaining a steady aerobic state throughout the entire growing season was a highly effective way to reduce As accumulation in rice (Xu et al., 2008), however, this is not a practical solution. The same mechanisms that sequester Si in rice can also transport and incorporate arsenic into the plant because the metalloid readily accumulates when rice is grown in arsenic rich flooded soils. Among the non-hyperaccumulators of As, rice ranked highest in the xylem transport (Zhao et al. 2009).
The toxicity of As will depend on its chemical speciation. Methylated As compounds are considered less toxic than the inorganic forms (Schoof et al. 1999). While the methylated species (dimethylarsinic acid) can account up to 80% of the total As, it is predominantly present in the grain; whereas, inorganic As is the predominant species in rice straw (Abedin et al. 2002).While the pathway of As methylation in microorganisms has been well studied, the genes responsible for As methylation in rice and other higher plants have not been identified (Zhao et al. 2009).
