AN EVALUATION OF THE EFFECT OF PLANT GROWTH REGULATORS ON COLD HARDINESS IN APPLE TREES

Abstract

Two-year-old apple trees (cv. Spur Mac/M.106) were treated once during September 1985 and again in 1986 with the plant growth regulators thidiazuron and EL-500® in the presence or absence of the antitranspirant Vapor Gard®. Seasonal changes in the low temperature exotherms of twig samples, percent moisture, relative dormancy intensity, percent injury levels at stress temperatures of −15, −25 and −35 °C and daily maximum/minimum air temperatures were recorded. On a seasonal basis during the 1986–1987 season, the hardiness level of the trees increased until mid-February and then decreased regardless of treatment. During the acclimation period of September 1986 to mid-February 1987, a significant multiple regression equation was obtained between percent injury at a stress temperature of −35 °C and the following independent variables: percent shoot moisture, previous mean maximum and mean minimum air temperatures. During the deacclimation period of late February to May 1987, a significant regression equation was obtained between percent injury at a stress temperature of −35 °C and the independent variables, percent shoot moisture and previous mean maximum air temperatures. On a daily basis, fluctuating cold hardiness levels were correlated with prior chemical treatment, prior air temperatures, percent shoot moisture levels and low temperature exotherm. The pronounced fluctuations in percent injury at a stress temperature of −35 °C appeared to be primarily related to the previous mean maximum 3-d air temperatures when the multiple regression equations were examined for the major contributing variables. Thidiazuron in the presence or absence of Vapor Gard® was effective in increasing cold hardiness on a number of occasions and appears to warrant further evaluation.Key words: Malus × domestica, apple, cold hardiness, thidiazuron, EL-500®