Abstract
Freezing adaptation and the injurious mechanisms of biology are diverse. Upon freezing of extracellular water in biological system, chilling sensitive plant cells are subjected to intracellular freezing due to lack of barrier property against penetration of extracellular ice through both cell walls and chilling-injured plasma membranes. Similar property of cell walls and plasma membranes which allow for extracellular ice penetration exists in central tissues of dormant bud in cold-hardy boreal trees. Thus, in such dormant buds, extraorgan freezing occurs to avoid formation of extracellular ice in adjacent with the central tissue cells. In the most biological cells, on the other hand, plasma membranes as well as cell walls become barrier against penetration of extracellular ice producing extracellular freezing. In extracellular freezing, there are two types of mechanisms to produce injury. One type is injury caused by direct effects of freezing-induced concentrated solutions. Such type of injury, furthermore, may divide into two cases, injury which occurs even during very fast non-equilibrium freezing (at cooling rates faster than minute level) and injury which occurs by very long time (more than day level) at equilibrium freezing. The former case is injury during cryopreservation in freezing sensitive cells and latter case is injury during long time exposure to freezing under high subzero temperatures even in cold-hardy plants. Exposure of cells in such concentrated solutions produced diverse kinds of ultrastructural changes in plasma membranes which closely related on occurrence of injury. Second type of injury by extracellular freezing is produced by physical effect due to cellular shrinkage by dehydration as well as due to cellular deformation by growth of extracellular ice crystals. Such extracellular freezing-induced physical effects produced membrane fusion in plasma membranes as a serious cause of injury. The most useful protective mechanism to prevent membrane fusions by such physical effects of extracellular freezing is accumulation of compartible solutes including cryoprotectants in case of cryopreservation. Upon formation of extracellular ice, some of cells in trees that develop thick and rigid cell walls are not affected by the presence of extracellular ice and keep liquid state of intracellular water by supercooling to very low temperatures for long time periods without dehydration. Such cells keeping supercooling accumulate supercooling-promoting diverse kinds of polyphenols.
