THE CHILLING REQUIREMENT IN APPLE AND ITS ROLE IN REGULATING TIME OF FLOWERING IN SPRING IN COLD-WINTER CLIMATES.

Bloom is delayed in spring in many temperate zone woody plants that have received insufficient chilling during the winter. This reaction to insufficient chilling is commonly seen in warmer climates. In colder climates, though, late blooming cultivars of fruit trees are also known. We have tested the hypothesis that late season flowering in Malus cultivars growing in cold-winter climates is due to insufficient chilling. The proposal that insufficient chilling occurs in such a climate may at first sound ridiculous, but it must be remembered that little physiological chilling occurs at temperature below 0°C. In our area, relatively little physiological chilling occurs during the 2–3 coldest winter months. We have found that the late bloomers invariably require substantially more chilling than early and intermediate bloomers before bud burst can occur. A model is proposed to explain bloom dates on the basis of chilling requirements.

Abscisic acid (ABA) relationships in high, intermediate and low chilling cultivars were examined. ABA was found to decline in all 3 chilling classes during the dormant season, the decline being most rapid in low chillers and least rapid in high chillers.