Variations in Cold Resistance among Apple Cultivars during Deacclimation

Abstract

Coleman, W. K. 1985. Variations in cold resistance among apple cultivars during deacclimation.— J. exp. Bot. 36: 1159-1171. One-year-old vegetative twig samples from mature, bearing trees of nine apple cultivars were monitored over two years for their dormancy intensity and relative cold hardiness levels during the winter/spring deacclimation period. The apple cultivars exhibited a consistent response during the dehardening process which included a higher initiation temperature for the low temperature exotherm (LT2) and the development of an intermediate freezing exotherm (LT2). Imperial Red Mac/Antonovka was the hardiest cultivar during the two-year period while Imperial Red Mac/M.lll was the most tender. Cortland/Beautiful Arcade and Rogers Red Mac/M.lll varied considerably in their relative hardiness responses from year to year. Mid-winter hardiness levels were significantly and positively correlated with dormancy intensity in the nine cultivars. However, this relationship did not exist when the hardiness indices for late winter or early spring were compared with dormancy intensity. An intensive correlation and path analysis of the response of four cultivars (Jersey Mac/M.lll, Vista Bella/M.lll, Spur Mac/M.lll and Rogers Red Mac/M.lll) to previous maximum/minimum air temperatures indicated that past maximum temperature primarily affected LT2 while past minimum temperature affected LT,. When linear regression equations were fitted to the data, the mean air temperature of 0°C coincided with LT, values of —18 °C and LT2 values of — 36 °C to — 38 °C for all four cultivars. Correlation analyses between % moisture content and LT,/LT2 for the four cultivars were often positive but generally non-significant. Injury in living cells slightly preceded the initiation temperature of LT, and supports the idea that membrane destabilization may be an important and immediate precursor to intracellular freezing.