Speciation and distribution of arsenic and localization of nutrients in rice grains

Abstract

Summary• Arsenic (As) contamination of rice grains and the generally low concentration of micronutrients in rice have been recognized as a major concern for human health. Here, we investigated the speciation and localization of As and the distribution of (micro)nutrients in rice grains because these are key factors controlling bioavailability of nutrients and contaminants.• Bulk total and speciation analyses using high-pressure liquid chromatography (HPLC)-inductively coupled plasma mass spectrometry (ICP-MS) and X-ray absorption near-edge spectroscopy (XANES) was complemented by spatially resolved microspectroscopic techniques (µ-XANES, µ-X-ray fluorescence (µ-XRF) and particle induced X-ray emission (PIXE)) to investigate both speciation and distribution of As and localization of nutrients in situ.• The distribution of As and micronutrients varied between the various parts of the grains (husk, bran and endosperm) and was characterized by element-specific distribution patterns. The speciation of As in bran and endosperm was dominated by As(III)-thiol complexes.• The results indicate that the translocation from the maternal to filial tissues may be a bottleneck for As accumulation in the grain. Strong similarities between the distribution of iron (Fe), manganese (Mn) and phosphorus (P) and between zinc (Zn) and sulphur (S) may be indicative of complexation mechanisms in rice grains.

Arsenic Shoot-Grain Relationships in Field Grown Rice Cultivars

“…This would suggest that there is a large variation between the cultivars in P remobilisation from the shoots to the grain. In situ synchrotron X-ray absorption spectrometry and extraction/HPLC-ICP-MS analysis demonstrate that As speciation in rice grain is dominated by free arsenite (9,24), which is not a phosphate analogue, and by DMA. As speciation has not been determined robustly in rice shoots because extraction followed by HPLC-ICP-MS allows uncoordinated arsenate and arsenite to interconvert (10).…”

Selenium Characterization in the Global Rice Supply Chain

“…These were then mounted onto a metal plinth with adhesives to enable microtome thin sectioning. Procedures described in Lombi et al (35) were employed to ensure that the integrity of nonembedded samples was maintained during cutting. Two 70 µm thick sections, one from each of the spatial planes, were selected for in situ analysis.…”

Nutritionally Enhanced Staple Food Crops

“…Lombi et al (2009) detected high concentrations of Cu, Fe, Mn, and Zn in certain regions of the husk. However, the distribution of these nutrients varied considerably in other parts of the seed.…”

“…However, these reactions are subject to competitive exchange equilibrium with endogenous ligands and are usually considered liable to visualize only labile ions (McRae et al 2009). Spectroscopic methods include proton-induced X-ray emission (PIXE) that targets the embryo region (Mazzolini et al 1985), scanning and transmission electron microscopy (STEM) in combination with energy dispersive X-ray microanalysis (EDX) that focuses on aleurone and scutellum cells to provide subcellular information (Ockenden et al 2004;Lombi et al 2010), nanosecondary ion mass spectrometry (nano-SIMS) that visualizes the subcellular distribution but is limited to regions of only a few mm 2 (Moore et al 2010), and the X-ray fluorescence ((-XRF) method that provides elemental maps for various elements in whole grain sections (Lombi et al 2009;Takahashi et al 2009). The nondestructive m-XRF technique permits a three-dimensional reconstruction of accumulation patterns and can also distinguish between ionic valencies, critical for accumulation of toxic forms of various ions (Scheckel et al 2007).…”