Characteristics of winter hardiness in extremely hardy twigs of woody plants

Lethal intracellular freezing rarely occurs in extremely hardy twigs, such as white birch, poplar and willow, especially when they are prefrozen to —15 or —20°C. These prefrozen twigs survive subsequent rapid cooling in air at — 150°C, and immersion in liquid nitrogen. The winter hardiness of these twigs remains stable throughout winter. Negligible changes in hardiness occur with environmental temperature fluctuations. In these twigs and in most of the hardy twig cortex, water is easily frozen out extracellularly at temperatures as high as —15 to — 30°C. Below this range the intensity of cold and the cooling rate does not seem to cause injury to these twigs. The general applicability of hypotheses proposed by Russian workers and by Weiser on hardening and injury below —30°C is discussed. Two hypotheses on freezing injury in twigs of woody plants below — 30°C, a second supercooling point and the vital water exotherm, have been respectively proposed by Tumanov and Krasavtsev (1) and by Weiser (2). A rapid decrease in hardiness after thawing in well hardened twigs, and rapid fluctuations in winter hardiness associated with environmental temperature change have been discusse