Seasonal changes in dehydration tolerance of xylem ray parenchyma cells ofStylax obassia twigs that survive freezing temperatures by deep supercooling

Xylem ray parenchyma cells ofStylax obassia twigs undergo supercooling to −26 °C as the terminal temperature of the low temperature exotherm in summer and to −41 °C in winter upon freezing. During supercooling, no evidence of cell dehydration was recognized; cell walls were uncollapsed, no plasmolysis occurred, no ultrastructural changes in the plasma membranes or in cytoplasmic structures occurred, and high survival rates were maintained. Osmotic manipulation with hypertonic solutions of sorbitol did, however, cause dehydration of ray parenchyma cells, in which shrunken protoplasts and plasmolysis were observed. Freeze-fracture replicas showed that, in summer samples, osmotic dehydration produced aparticulate domains accompanied by fracture-jumps in the plasma membranes, which are known as a symptom of dehydration-induced injury, but no such effects were seen in winter samples. By contrast, in winter samples, osmotic dehydration produced conformational changes in the endoplasmic reticulum, which are known as a feature of dehydration tolerance, however, these changes were not seen in the summer samples. These results indicate that the ray parenchyma cells inS. obassia have the ability to tolerate dehydration in winter and lose this ability in summer. Such tolerance is similar to the dehydration tolerance in plant cells that adapt to freezing by extracellular freezing.