Abstract
Coptis chinensis Franch. is one of the most important medicinal plants globally. However, this species contains relatively high concentrations of chromium (Cr) which potentially detrimental to human health. It is important to understand Cr localization and speciation in order to evaluate its accumulation and transportation mechanisms and minimize Cr transfer to humans. As little previous work in this area has been carried out, we utilized synchrotron radiation microscopic X-ray fluorescence (SR-μXRF) and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) to spatially locate Cr, X-ray absorption near-edge spectroscopy (XANES) to analyze Cr speciation, and inductively coupled plasma mass spectrometry (ICP-MS) to detect Cr subcellular concentration. Micromapping results showed that Cr was distributed predominantly within the vascular cylinder, the periderm and some outer cortex, and the cortex and some vascular bundles in root, rhizome, and petiole, respectively. XANES data showed that Cr(VI) can be reduced to Cr(III) when grown with Cr(VI), and yielded a novel conclusion that this plant contain elemental chromium. ICP-MS data showed that Cr was primarily compartmentalized in cell walls in all tissues. The new insights on Cr accumulation in C. chinensis Franch. provide a theoretical basis for the evaluation of Cr in other medicinal plants.
Highlights
Cr occurs in several oxidation states that range between −2 and +6, its trivalent [Cr(III)] and hexavalent chromate [Cr(VI)] forms are the most common and stable in natural[12]
In order to investigate the spatial distribution of Cr in C. chinensis Franch., we carried out XRF microscan mapping of plant roots and rhizomes
Our μ-XRF maps of root cross-sections for the Cr10d group are presented in Fig. 1A; these results show that Cr is mainly localized in the central part of the root in these samples, a region that corresponds to the vascular cylinder
Summary
Cr occurs in several oxidation states that range between −2 and +6, its trivalent [Cr(III)] and hexavalent chromate [Cr(VI)] forms are the most common and stable in natural[12]. It is important to determine both the localization and speciation characteristics of the heavy metals in plants in order to understand their accumulation and transportation mechanisms[28,29] as these are regulated by various physiological processes including transport of metals across the plasma membranes of root cells, detoxification, and sequestration of metals in cell walls or vacuoles, xylem loading, and translocation from roots to shoots[30] Once it is absorbed by roots, Cr is initially retained largely within the roots and later transported throughout the plant by carrier ions[19]. X-ray absorption near-edge spectroscopy (XANES) is an element-specific method that can be used to analyze the in vivo ligand environments of metals within plants[39] We utilized these analytical approaches to generate basic information on Cr accumulation and transportation mechanisms by exploring of Cr localization and speciation in C. chinensis Franch. Based on the above research, we could provide reliable reference data on the mechanisms of Cr accumulated by medicinal plants and on solving the high level of Cr in C. chinensis Franch. in the agricultural environment, so that further mitigates potential transfer to humans
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