Twelve-week-old Malus seedlings were induced to cold harden by exposure to low temperature and freezing environments. The effectiveness of induced acclimation by exposure to stimuli such as low temperature (3 to 5 °C), frequency of exposure to freezing temperatures (-3 °C), storage time before and after induction and the effects of different screening temperatures (-20, -30, and -40 °C) were investigated with seedlings grown in a greenhouse from open-pollinated `Golden Delicious' apple (Malus pumila (Mill.), `Antonovka' apple (M. baccata (L.) Borkh. × (M. pumila) and `Rescue' apple (M. baccata) × (M. pumila). Differentiation of the seedling populations with respect to cold hardiness was not achieved until after acclimation at cool temperatures (3 to 5 °C) for 6 weeks. Further population differentiation was achieved by exposure to one or more frosts (-3 °C). Once the acclimation response had been initiated the seedlings could be held for up to 11 days, under the same conditions, with no significant decrease in hardiness. Hardiness levels of acclimated and nonacclimated open pollinated seedlings coincided with known inherent hardiness responses for all three maternal cultivars evaluated. A binomial form of regrowth data collection, percent seedling survival, was determined to be the most efficient and most precise measure of evaluation. Induction of cold hardiness in very young seedlings and the use of a controlled freeze testing protocol should facilitate rapid screening of large progenies and improve the rate of progress in breeding for cold hardiness.
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