Stem-specific cadmium accumulation in cadmium-tolerant Polygonum thunbergii

Polygonum thunbergii was selected among wild plants as a cadmium (Cd)-tolerant plant because of the high accumulation of Cd in the stems. However, the concentration of Cd in the leaves was low, which suggested the existence of an inhibitory mechanism of Cd translocation from stems to leaves. Observation of Cd distribution in stem-petiole-lamina in P. thunbergii plants that had absorbed Cd, using SEM-EPMA, revealed that Cd accumulation occurred mainly in the nodes, junctions of petioles, and vessels in stems, with minimal translocation in petioles. A detection method for tissue distribution of Cd was developed. Resin-fixed tissue tips were stained with a fluorescent probe BTC-5N and then sites of Cd location were detected with a fluorescence microscope. It was found that Cd in the stem tissues of P. thunbergii was distributed at specific sites of the cell walls of vessels and surrounding cells. The results were consistent with the findings of the investigation of Cd distribution in stem cells obtained by the cell fractionation method, which showed that most of Cd was present in the cell wall and cytoplasm. These results suggest that P. thunbergii accumulates Cd in specific cell walls of the stems and inhibits Cd-translocation to the leaves.