Seasonal changes in the freezing behavior of xylem ray parenchyma cells infour boreal hardwood species

The freezing behavior of xylem ray parenchyma cells in several boreal hardw ood species, namely, Betula platyphylla, Populus canadensis, P. sieboldii, and Salix sachalinensis, was examined by differential thermal analysis (DTA ), cryo-scanning electron microscopy (Cryo-SEM), and freeze-fracture replic a electron microscopy. Although DTA profiles of samples harvested in summer and in winter suggested that the xylem ray parenchyma cells in all four sp ecies responded to freezing stress by extracellular freezing, Cryo-SEM show ed clearly that the xylem ray parenchyma cells in all these species respond ed to freezing stress by shallow supercooling in summer and by extracellula r freezing in winter. It is suggested that DTA failed to reveal the true fr eezing behavior of xylem ray parenchyma cells because of an overlap of temp erature ranges between the high-temperature exotherm and the low-temperatur e exotherm and/or because of the limited extent of the LTE. The seasonal ch anges in freezing behavior of xylem ray parenchyma cells in all these borea l species, which are results of seasonal cold acclimation, support the hypo thesis that a gradual shift of freezing behavior in xylem ray parenchyma ce lls from shallow supercooling in hardwood species that grow in tropical zon es to extracellular freezing in hardwood species that grow in cold areas mi ght be a result of the evolutionary adaptation of hardwood species to cold climates. (C) 1999 Academic Press.