Seasonal changes in the low-temperature behaviour of xylem ray parenchyma cells in red osier dogwood ( Cornus sericea L.) with respect to extracellular freezing and supercooling

Abstract

The low temperature behaviour of xylem ray parenchyma cells in red osier dogwood ( Cornus sericea L.) was examined by different thermal analysis (DTA), cryo-scanning electron microscopy, freeze-fracture replica electron microscopy and a survival assay (leakage of electrolytes). DTA provided a profile that is typical of extracellular freezing in xylem ray parenchyma cells in both winter and summer. Observations of xylem ray parenchyma cells by electron microscopy indicated, however, that low-temperature behaviour was different from that predicted by DTA. Electron microscopy revealed that, upon cooling at 0.1°C/min, the ray parenchyma cells in winter exhibited typical extracellular freezing, whereas cells in summer exhibited intracellular freezing below -15°C. Cooling at 1.25°C/day (<0.001°C/min) produced a slight collapse of the cell walls as a result of partial dehydration, but it did not inhibit the intracellular freezing in xylem ray parenchyma cells in summer. It is suggested that failure of DTA to reveal the low temperature exotherm (LTE) upon intracellular freezing was due to an overlap of temperature between the high temperature exotherm (HTE) and the LTE, in addition to a reduction in the LTE by the partial dehydration of cells. It is concluded that red osier dogwood has xylem ray parenchyma cells whose low-temperature behaviour changes from extracellular freezing in winter to supercooling in summer, possibly as a result of seasonal differences in permeability of the cell walls to water. This type of seasonal change in the low-temperature behaviour may produce a superior mechanism for the adaptation to freezing temperatures of cells of plants growing in cold regions, in which dehydration tolerance also changes seasonally.