Abstract
Most of xylem parenchyma cells of boreal hard wood species respond to subfreezing temperature by deep supercooling. Because xylem tissue cells that adapt by deep supercooling have the least resistance to subfreezing temperature among woody tissues, the limit of supercooling ability of xylem parenchyma cells is an essential factor for the survival of woody plants in cold areas. However, the deep supercooling mechanism of xylem parenchyma cell has not yet been clarified. In this study, accumulation of soluble sugars during seasonal cold acclimation in birch (Betula platyphylla var. japonica Kara) xylem tissue has been examined to discuss the influences of the intracellular sugars to the supercooling ability of xylem parenchyma cells. The supercooling ability of xylem parenchyma cells of birch seasonally changed. Fructose, glucose, sucrose, raffinose and stachyose were mainly accumulated in birch xylem parenchyma cells. Seasonal change in the amount of sucrose, raffinose and stachyose was positively correlated with the supercooling ability in xylem parenchyma cells. Although the proportion of each sugar composition was different between xylem parenchyma cells that undergo deep supercooling and bark parenchyma cells that undergo extracellular freezing, the major components of soluble sugars which accumulated in xylem cells were same with those of bark cells. However, the concentration of soluble sugars in xylem parenchyma cells might be higher than that of bark cells or cambial cells. It seems that the intracellular soluble sugars participate, at least in part, to the increase of supercooling ability of xylem parenchyma cells via increasing intracellular osmotic concentration. However, such a freezing temperature depression alone cannot explain high supercooling ability of xylem parenchyma cells, suggesting presence of other factors which may promote supercooling.
