Arsenic speciation and its DNA fractionation in the rice plant Oryza sativa

Abstract

The transport of arsenic from soil through to edible crop is important when assessing the potential health risks from a food source. Samples of soil, irrigation water and the rice plant, Oryza sativa, were collected from an agricultural site in the Middle East. Total As, its speciation and DNA fractionation were evaluated using HPLC-ICP-MS in samples which included the root, stem, leaf and grain of the rice plant. Methodology was validated through the use of CRMs. The highest concentration of As was found in the root of the plant and the lowest in the grain. The concentrations found for As were: soil (aqua-regia extractable; 2.88 μg g⁻¹), irrigation water (0.58 μg L⁻¹), roots (8.28 μg g⁻¹), stem (4.00 μg g⁻¹), leaves (2.93 μg g⁻¹) and grain (1.02 μg g⁻¹). Levels of inorganic As^V and As^III were identified in the soil and plant material, while organo-arsenic species (DMA, MMA) were below the limit of detection. The ‘plant available’ levels of As in the soil (14%) were determined as part of a full, validated BCR three-step sequential extraction procedure. Since As can behave as a phosphate analogue, a method was developed for the extraction of vegetative DNA to determine the different forms of As associated with, or integrated within, the DNA fractions. Measurement of As in the DNA extracts were above the LOD (0.019 μg kg⁻¹) for the root, stem and leaf samples. The concentration of both weakly and strongly associated As with DNA obtained from the root, stem and leaf decreased with decreasing total As concentrations. A narrow, near-constant ratio for the strongly associated As value (As/total As DNA) in all root, stem and leaf DNA samples (41.3 ± 0.3%) was further evidence for the incorporation of As into the DNA.