Cellular-level in planta analysis of radial movement of minerals in a konara oak ( Quercus serrata Murray) trunk

After the Fukushima Daiichi Nuclear Power Plant accident, radiocesium, one of the main radioactive materials, has been accumulated inside konara oak trunks. Radiocesium has been thought to move radially through the trunk, but it has not been scientifically vindicated because the mechanism of the radial movement of minerals has not yet been experimentally determined. In this study, mineral radial movement was investigated in konara oak trunks of standing trees. A stable isotope cesium (Cs) solution was injected as a tracer into the outer sapwood of standing konara oak tree trunks. A trunk part was subsequently freeze-fixed with liquid nitrogen and subjected to Cs distribution analysis using cryo-scanning electron microscopy/energy-dispersive X-ray spectroscopy. By comparing normal samples that included living cells and freeze–thaw treated samples that contained no living cells, it was concluded that the injected Cs has been moving through the sapwood by the combination of rapid symplasmic movement by virtue of the living xylem parenchyma cells and slow apoplastic diffusion. Conversely, the Cs solution did not diffuse from the sapwood to the heartwood, implying that Cs is exuded from the living parenchyma cells to the apoplast, i.e., cell walls or adjoined dead parenchyma cells in the layer at the sapwood–heartwood boundary, and then diffused into the heartwood. By integrating the results of this study and our previous results obtained on Japanese cedar, we conclude that the mechanism of the radial movement of minerals through the sapwood seems to be a universal characteristic of tree species. In contrast, since mineral concentrations varied among tree species, the movement mechanism across the sapwood–heartwood boundary can differ among tree species.