ISHS Acta Horticulturae 168: II Symposium on Winter Hardiness in Woody Perennials
AVOIDANCE OF FREEZING INJURY IN WOODY PLANTS BY DEEP SUPERCOOLING |
| Author: | H.A. Quamme |
| Abstract:
The type of freezing observed most often in hardy woody plants is that of extracellular freezing, but hardy xylem ray and axial parenchyma, floral tissues, phloem and buds of some, but not all woody plants and some seeds do not exhibit typical extracellular freezing. Water is retained in the cells of these tissues which does not freeze, but supercools. Barriers exist which isolate the cells from external nucleation (heterogeneous nucleation) and prevent water loss to surrounding ice. The entrapped water remains stably supercooled to the homogeneous nucleation point, the temperature at which water freezes in absence of nucleators (termed deep supercooling). Freezing of deep supercooled water produces a low temperature (LT) exotherm which can be detected by differential thermal analyses or calorimetry. The barriers which prevent heterogeneous nucleation are not well understood, but in peach flower buds, some conifer buds and seeds freezing occurs in preferred sites which act as sinks for ice formation. Dry regions which are created by water withdrawal to preferred freezing sites prevent ice spread. The presence of starch in xylem and phloem tissues may be responsible for water retention and deep supercooling in these tissues. Cell wall and membrane structure may also contribute to isolation of deep supercooled water. In Prunus flower buds, vascular differentiation is important in maintaining the deep supercooled state. Deep supercooling depends on stage of acclimation. In peach, deep supercooling of the flower bud appears to be dependent on water content and much of the seasonal variation can be explained by water flow to and from the flower primordium within the flower bud during freezing and thawing. Deep supercooling occurs widely in woody plants and has been reported in 33 families of angiosperms and 1 family of gymnosperms. Generally, the distribution of species which deep supercool appears to be limited to lower latitudes where the probability of the air temperature falling below the homogeneous nucleation point of supercooled water within the cells is low. Species which survive by tolerating extracellular freezing are distributed into colder regions than plants which deep supercool. Exotherm analysis is useful for measuring cold hardiness of deep supercooling plants and has application in breeding programs. | |
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