PLANT PHYSIOLOGY , Vol 103, Issue 3 753-761, Copyright © 1993 by American Society of Plant Biologists
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ENVIRONMENTAL AND STRESS PHYSIOLOGY |
Ultrastructural Evidence That Intracellular Ice Formation and Possibly Cavitation Are the Sources of Freezing Injury in Supercooling Wood Tissue of Cornus florida L
Z. Ristic and E. N. Ashworth
Center for Plant Environmental Stress Physiology, Department of Horticulture, Purdue University, West Lafayette, Indiana 47907
Although cellular injury in some woody plants has been correlated with
freezing of supercooled water, there is no direct evidence that
intracellular ice formation is responsible for the injury. In this study we
tested the hypothesis that injury to xylem ray parenchyma cells in
supercooling tissues is caused by intracellular ice formation. The
ultrastructure of freezing-stress response in xylem ray parenchyma cells of
flowering dogwood (Cornus florida L.) was determined in tissue prepared by
freeze substitution. Wood tissue was collected in the winter, spring, and
summer of 1992. Specimens were cooled from 0 to -60[deg]C at a rate of
5[deg]C h-1. Freezing stress did not affect the structural organization of
wood tissue, but xylem ray parenchyma cells suffered severe injury in the
form of intracellular ice crystals. The temperatures at which the ice
crystals were first observed depended on the season in which the tissue was
collected. Intracellular ice formation was observed at -20, -10, and
-5[deg]C in winter, spring, and summer, respectively. Another type of
freezing injury was manifested by fragmented protoplasm with
indistinguishable plasma membranes and damaged cell ultrastructure but no
evidence of intracellular ice. Intracellular cavitation may be a source of
freezing injury in xylem ray parenchyma cells of flowering dogwood.
