Relationship of Flowering Time, Rate of Seed Germination, and Time of Leaf Budbreak and Usefulness in Selecting for Late-flowering Apples

The relationship between dormancy of seeds and buds of apple (Malus × domestica Borkh.) might provide breeders with an early opportunity to select for delayed development. Seeds of late-flowering genotypes require much longer exposure to chilling temperatures than those of early flowering" genotypes, and they germinate over a much longer period. In three progenies that exhibit much variation for the two traits, seed germination time was correlated with time of leafing-out of the resulting seedlings, and could be used to select for delayed budbreak. However, selection would be ineffective when little genetic variation for seed germination and budbreak is present. Leafing-out ratings in the nursery in the 2nd year were highly correlated with those in the 3rd year, indicating that selection for late leafing in the nursery during the 2nd year would be more effective than selection based on seed dormancy, especially in progenies exhibiting little genetic variability for this trait. Breeders can effectively use both relationships by first eliminating early germinating seeds and then eliminating early leafing seedlings. Spring frost is a major factor that limits the areas in which temperate zone deciduous tree fruits can be grown and is re- sponsible for much of the year-to-year fluctuations in yield. To minimize frost injury, fruit growers often invest considerable sums of money in wind machines, sprinkler-irrigation, and heat- ers. Trees are most sensitive to frost when in full bloom, and since the probability of frost is largely a function of calendar date, late-flowering selections often avoid injury. In apple, genotypes are known that vary in flowering date by >30 days. The latest blooming genotypes, cider apples from Europe (Mur- awski, 1967; Tydeman, 1958), bloom 3 weeks later than 'De- licious'. These cultivars have higher chilling requirements and higher heat requirements in the post-rest phase (Gianfaga and Mehlenbacher, 1985; Swartz and Powell, 1981). Their use is currently limited because of their unattractive appearance, poor fruit quality, and susceptibility to fire blight (Erwinia amylovora (Burrill) Winslow) (Aldwinkle et al., 1976). However, they represent very valuable germplasm that breeders can use in the development of late-flowerin g selections with improved fruit quality and disease resistance. This has been one of the objec- tives of the apple breeding program at Rutgers Univ. since 1955. As part of this continuing effort, crosses were made in 1984 between late-flowering genotypes and selections resistant to ap- ple scab and fire blight. Date of leafing-out of juvenile seedlings was shown by Tydeman (1964) to be highly correlated with flowering date in the adult trees, and elimination of early leafing seedlings in the nursery during the second season of growth (15 months after seed germination) was very effective in delaying bloom date in the seedling population (Murawski, 1967). A similar strategy has been used at Rutgers. During stratification, it was noted that seeds of the 1984 crosses involving late-bloom- ing parents, compared with crosses between selections flower- ing at the normal time, not only germinated later but they also germinated over a much longer time. Previous studies have