Defining the distribution of arsenic species and plant nutrients in rice (Oryza sativa L.) from the root to the grain

The transport mechanisms of As from contaminated soil or irrigation water into roots and subsequently into grain, and the As species distribution—a toxicity determinant, is critical for assessing health risks imposed by As. However, the commonly-employed extraction of plant material with trifluoroacetic acid (TFA) has not proven successful in preserving inorganic As species. Synchrotron-based spectroscopic techniques are useful for discerning elemental distributions and chemical speciation of elements in situ. Here, we both characterize the mineral phases of Fe coatings on rice roots, and quantify plant nutrients and As species in situ on roots and grain samples. Arsenic in rice grains was present in bran layers as oxidized As (69–88% as As(V)ᵢ and 12–31% as DMA) and in the germ as a mixture of As(V)ᵢ and As(III)ᵢ, but was non-detected from the endosperm, which is consistent with previous findings. The extent of Fe coatings on rice roots was variable and, when present, consisted of lepidocrocite (γ-FeOOH), goethite (α-FeOOH) and ferrihydrite (Fe(OH)₃·nH₂O). Arsenic was co-located with root Fe coatings, but our findings indicate that Fe is not a direct interceptor of As uptake, and is rather a bulk scavenger mostly near the air-water interface. On whole root mounts with Fe plaque, arsenic was present as mixed species of As(V)ᵢ (44–66%) and As(III)ᵢ (34–56%). Within a root cross-section, oxidized As species were dominant in the xylem (86% as As(V)ᵢ and 14% as DMA) whereas mostly reduced species (71% as As(III)ᵢ, 29% as AsGlu₃) resided within a vacuole adjacent to the xylem. This finding contrasts the prevailing view that As(V)ᵢ is rapidly reduced in roots and transported to shoots as As(III)ᵢ, and points to the importance of interspecies differences in As-uptake dynamics.