Induction of Cold Hardiness in Apple Tree Shoots with Ethephon, NAA, and Growth Retardants1

Abstract

Abstract Growth regulators were applied in early autumn to evaluate their effects on induction of cold hardiness in 2-year-old apple shoots, as determined by the conductivity test. Mid-October applications of 500 ppm (2-chloroethyl)phosphonic acid (ethephon), followed 11 days later by 100 ppm of naphthaleneacetic acid (NAA), increased cold hardiness up to 5°C by early November in 1974 and to a lesser extent in 1976. Two annual sprays of succinic acid-2,2-dimethylhydrazide (daminozide, SADH) in June increased cold hardiness slightly (2 to 3°C) in late fall of 1974, 1975, and 1976. However, in 1974 treated trees were no hardier than the controls later in the winter and fruit set on ethephon-treated trees was severely reduced. In most cases, the combined content of fructose, glucose, sucrose, and sorbitol in the 2-year-old wood was slightly higher in treatments that induced cold hardiness in November or December, 1974 than in the controls. Techniques are described for screening growth regulators for induction of cold hardiness and estimating relative cold protection.