Studies of Frost Hardiness in Woody Plants. II. Effect of Temperature on Hardening

Abstract

The effect of temperature on hardening was studied at temperatures ranging from 0 to -20[degree] C using twigs of willow and poplar. In twigs which could withstand continuous freezing without injury, hardening at -3 to -5[degree] C was most effective in increasing the frost hardiness of the twigs. Below -20[degree] C, only negligible increase was observed either in frost hardiness or sugar content. The rate of starch to sugar conversion differed markedly in different twig tissues. The starch in xylem was more slowly converted to sugar than in the cortex. The optimum temperature for converting starch into sugar during frost-hardening was also found to be -3 to -5[degree] C. In addition, the greater the effectiveness of the hardening treatment, the greater the rate of conversion from starch to sugar. The frost hardiness of a twig was closely related to the sugar content of the twig, especially in the xylem.