Freezing behavior of xylem ray parenchyma cells in softwood species with differences in the organization of cell walls

By cryo-scanning electron microscopy we examined the effects of the organization of the cell walls of xylem ray parenchyma cells on freezing behavior, namely, the capacity for supercooling and extracellular freezing, in various softwood species. Distinct differences in organization of the cell wall were associated with differences in freezing behavior. Xylem ray parenchyma cells with thin, unlignified primary walls in the entire region (all cells in Sciadopitys verticillata and immature cells in Pinus densiflora) or in most of the region (mature cells in P. densiflora and all cells in P. pariflora var. pentaphylla) responded to freezing conditions by extracellular freezing, whereas xylem ray parenchyma cells with thick, lignified primary walls (all cells in Crytomeria japonica) or secondary walls (all cells in Larix leptolepis) in most regions responded to freezing by supercooling. The freezing behavior of xylem ray parenchyma cells in L. leptolepis changed seasonally from supercooling in summer to extracellular freezing in winter, even though no detectable changes in the organization of cell walls were apparent. These results in the examined softwood species indicate that freezing behavior of xylem ray parenchyma cells changes in parallel not only with clear differences in the organization of cell walls but also with subtle sub-electron-microscopic differences, probably, in the structure of the cell wall.