The Relationship of Exotherms to Cold Injury in Apple Stem Tissues1,2

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Abstract
During controlled freezing as many as 4 exotherms were detected in apple twigs by differential thermal analysis (DTA). Three of the exotherms usually occurred at relatively high temperatures above the killing point of stem tissues. There was no apparent association between any of these exotherms and freezing injury to cambium, cortex, phloem, or vegetative buds. The temp of the fourth exotherm (D), however, coincided with the killing temp of xylem ray and pith parenchyma as hardiness changed during the season. DTA studies and hardiness tests performed on whole and dissected twigs indicated that the fourth exotherm arose from the xylem and pith tissues; that injury to xylem and pith occurred when twigs were frozen below the initiation temp of the fourth exotherm independent of cooling rate and that bud and bark tissues were considerably hardier than xylem in mid-winter while in early autumn and late spring xylem and pith were hardier than buds and bark. The results suggest that freezing injury to xylem and pith cells is caused by a specific freezing event, and that it is different from freezing injury to other stem tissues. Measurements of the fourth exotherm may provide a convenient way of studying freezing injury and resistance in xylem – a recurring problem in many fruit species.

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