Enhancement of cold hardiness in apple trees by paclobutrazol, thidiazuron and flurprimidol

Abstract

Two-year-old apple trees (cv. Spur McIntosh/MM.106) were treated once during September 1987 and again in September 1988 with the growth regulators paclobutrazol, thidiazuron or flurprimidol alone or in different combinations. Seasonal changes in electrolytic leakage at test stress temperatures of −15, −25 and −35 °C were recorded from October to May. During the 2-yr study, there was a well-defined acclimatization phase until early February followed by a deacclimatization phase during February–May. The treatment that produced greatest increases in apple twig hardiness was a mixture of paclobutrazol, thidiazuron and flurprimidol. In 93% of the test dates, this treatment gave a lower injury value than the control at the −25 °C stress temperature and 66% gave significantly (P < 0.05) lower injury levels. Chemical treatments increased the hardiness of apple wood throughout the fall, winter and spring periods without altering the pattern of seasonal changes in hardiness level. Multivariate analyses indicated that treated shoots were less sensitive than the controls to mild temperatures during the winter period.Key words: Apple, cold hardiness, growth regulators