Cellular level in planta analysis of radial movement of artificially injected caesium in Cryptomeria japonica xylem
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Ray parenchyma cells represent the main pathway for mineral radial movement from sapwood towards intermediate wood, based on direct cryo-scanning electron microscopy/energy dispersive X-ray spectroscopy analysis.
Ray parenchyma cells are considered to be associated with the radial movement of some elements from sapwood to heartwood in tree trunks. However, this function has not yet been experimentally confirmed. In this study, we analysed the radial movement of caesium (Cs), as a tracer of minerals, through the xylem at the cellular level in standing trees. A solution containing a stable Cs isotope diluted in aqueous acid fuchsin was injected into the outer sapwood of Japanese cedar trees for 3 h, 1, or 4 days. After the injection, the trunk (including the injection points) was freeze-fixed with liquid nitrogen. Subsequently, the trees were felled. The red colour from acid fuchsin was detected in the outer sapwood and did not move passively in the radial direction with free water. For detailed analysis of the Cs distribution in the radial direction (from the red-coloured sapwood towards the pith), we performed cryo-scanning electron microscopy/energy-dispersive X-ray spectroscopy point analysis of the frozen-hydrated samples to analyse the Cs distribution by detecting the characteristic Cs-Lα X-ray peak. In the outer sapwood, Cs was detected in all cell structures at all injection periods. In the inner xylem tissue, Cs was detected in the cell wall and lumen of ray parenchyma cells at 4-day injection, but not at 3-h and 1-day injection. These results suggest that ray parenchyma cells represent the main pathway whereby Cs can move radially from sapwood towards intermediate wood in standing Japanese cedar trees.
KeywordsCaesium Cryo-scanning electron microscopy/energy dispersive X-ray spectroscopy Cryptomeria japonica Frozen-hydrate Mineral Parenchyma cell
We thank Dr. Chunhua Zhang of Forestry and Forest Products Research Institute for assistance in sampling. This work was supported by Japan Society for the Promotion of Science, JSPS KAKENHI Grant Numbers 25292110, 16H04936, and 18H02258.
Compliance with ethical standards
Conflict of interest
The authors declare that they have no conflict of interest.
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