Study on Accumulation Mechanism of Cadmium in Rice (Oriza sativa L.) by Micro-XRF Imaging and X-ray Absorption Fine Structure Analysis Utilizing Synchrotron Radiation

This paper reports on the distribution of Cd and essential elements in the stems of rice plants, 'Nipponbare' and 'Milyang 23', measured by synchrotron radiation micro-XRF imaging. X-ray absorption near edge structure (XANES) analysis was applied to estimate the chemical forms of Cd in the roots, stems and leaves, aiming to reveal the mechanisms of Cd transport in rice. The results show that the Cd concentration of 'Nipponbare' was found to be higher than that of 'Milyang 23' at the root, although there was no significant difference between these two cultivars at the shoot. In addition, the distribution of Cd and the essential elements inside the plant were clearly revealed by micro-XRF imaging. It was found that Cd was also localized to the bundle. The Cd K-edge XANES analysis indicated that Cd was bound to sulfur in the root for both rice plants. Furthermore, the content of Cd combining with S (Cd-S) in the stem of 'Nipponbare' became 1.5 times higher after exposing to Cd for a long time, while only a slight change in the stem of 'Milyang 23' was found. In contrast with the stem, the leaf of 'Nipponbare' contained a lower Cd-S content than that of the control after a long-term exposure, while that of 'Milyang 23' became higher. In conclusion, the difference in the chemical form of Cd at the shoots affected the different accumulation behaviors of Cd between 'Nipponbare' and 'Milyang 23'.