Micro X-ray fluorescence imaging and micro X-ray absorption spectroscopy of cadmium hyper-accumulating plant, Arabidopsis halleri ssp. gemmifera, using high-energy synchrotron radiation

Monochromatic high-energy X-rays (37 keV) focused into a microbeam (3.8 × 1.3 μm²) with a Kirkpatrick-Baez mirror allows us for the first time to reveal the striking sub-cellular distribution and the chemical form of cadmium in the cadmium hyperaccumulating plant, Arabidopsis halleri ssp. gemmifera, by detecting the Cd Kα line. Micro X-ray fluorescence analysis (μ-XRF) and micro X-ray absorption near-edge structure analysis (μ-XANES) were used for the nondestructive analysis of the Cd distribution and chemical form of Cd at a cellular level in the plant in order to investigate the Cd accumulation mechanism. It was clearly observed by two-dimensional μ-XRF imaging that Cd was highly accumulated in a part of the trichomes, epidermal hairs existing on the surface of the leaves. The bases of the trichomes contained Cd, Zn and Mn at high concentrations and their distribution patterns were similar to each other. These elements were located near the surface of the trichome in the form of a ring. The μ-XANES analysis revealed that the majority of the Cd exists in the divalent state and bound to the O and/or N ligands. This study has demonstrated the potential of the Cd K-edge μ-XANES spectroscopy, which was applied here for the first time to plant samples.