Fruit Breeding, Vol. I, Tree and Tropical Fruits (1996)
edited by Jules Janick, James N. Moore

Jules Janick, James N. Cummins, Susan K. Brown, and Minou Hemmat

pp. 13-14, Juvenility

Apple trees undergo different phases of development between seed germination and the adult fruiting tree. In the juvenile phase, when no flowers are produced, plants may differ considerably from mature adult trees. The leaves are smaller and usually more finely serrate, and the shoots are thinner and are often produced at right or obtuse angles to the main stem. Bud break is early and leaf fall is late. The onset of the adult phase is marked by the development of flower buds. Between these two phases there is a transition phase when the lower part of the plant is still juvenile and the upper part is adult. Cuttings from most adult apple trees used as scion cultivars are extremely difficult, if not impossible, to root. Cuttings from young seedlings root readily but rooting is more difficult to achieve as the seedlings age, suggesting that the transition is gradual. The duration of the juvenile phase vanes from 3 to 10 or wore years, depending on the genotype of the seedlings and the cultural practices.

Ways of shortening the duration of the juvenile phase have been investigated (Kemmer 1953; Murawski 1955; Visser 1964). Methods attempted to induce adult trees to flower include shoot pruning, root pruning, and bark ringing. However, any method that restricts growth in the very young seedling does not shorten the duration of the juvenile phase but in fact lengthens it and delays the onset of fruiting. Cultural methods that check the growth of seedlings are effective in hastening flowering only when a certain stage of development has been reached. Way (1971) was able to induce seedlings to flower by bark ringing when they were 4 years old, but 3-year-old seedlings did not respond.

Other cultural methods can effectively shorten the juvenile phase. One is to grow the seedling as fast and vigorously as possible. This can best be achieved by avoiding any check to growth in the early stages of development and, where this is not possible, to keep disturbance to a minimum. Seedlings have flowered in the greenhouse in 1 year by keeping them in a continuous state of growth (Aldwinckle et a1 1976). Seedlings grown under optimum conditions in the greenhouse, closely planted, watered, and fed, may reach 3 m in height in the first season compared with 1 m for seedlings planted out in the field. These may be planted directly into permanent field locations. The nursery should be avoided because it delays the process. Seedlings should continue to grow freely and, since the seedlings first come out of the juvenile condition at the top of the seedling, they should be left unpruned until they flower and fruit (Brown 1964; Zimmerman 1971). Tydeman and Alston (1965) have shown that the juvenile phase can be considerably shortened by budding seedlings onto dwarfing rootstocks. Buds were taken from the upper part of the main shoot in late summer of the second year's growth and worked on the dwarfing rootstocks 'M.9' and 'M.27', grown as closely planted cordons with all lateral growth pruned back annually in late summer. Nine years after germination 88% of 902 budded seedlings had fruited compared with 49% of those on their own rots. Virus-free rootstocks must be used to avoid infection.

Juvenility poses a different sort of problem for the rootstock breeder. In the stoolbed, juvenile plants are usually quite thorny, root very easily, and exhibit earlier bud break and later leaf fall. Evaluations in the rootstock program at Cornell University (Geneva Station) are usually made in the third season, when these juvenile characteristics have somewhat abated.

Fruit Breeding, Vol. I, Tree and Tropical Fruits
edited by Jules Janick, James N. Moore

Richard L. Bell, Harvey A. Quamme, Richard E. C. Layne, and Robert M. Skirvin

p. 466, Precocity

Besides the importance of a short juvenile period to ensure early selection of the fruit and shortening of the generation time (Hansche and Beres 1980), there is a positive correlation between juvenile period and precocity of seedling selections when propagated on rootstocks (Visser and De Vries 1970). Because the length of the juvenile period is heritable and under predominantly additive genetic control (Zielinski 1963; Visser 1976; Zimmerman 1976; Zhejiang Agricultural University 1978; Bell and Zimmerman 1990), selection for precocity is feasible. The selection of parents to use in breeding for precocity may be based on the length of the vegetative period (Visser 1967) or on progeny performance. In a population that included parents of interspecific hybrid origin. Bell and Zimmerman (1990) concluded that the genotype of the individual was more important than species pedigree in determining mean juvenile period of offspring. Studies in China indicated that clones and seedlings of P. pyrifolia were more precocious than P. X bretschneideri and P. communis (Zhejiang Agricultural University 1978).

Seedlings exhibiting juvenile growth may be identified by the presence of thorns, irregular leaf margins, and wide-angled branches (Fritzsche 1948) and it is possible that absence of these features might be used for early selection of seedlings with a short juvenile phase. Vigor, measured as stem diameter, has not been found to be a reliable predictor of juvenile period (Zimmerman 1977; Shen et al. 1982).

Li et al. (1981) introduced the concept of the juvenile span, defined as the amount of linear stem growth to the first flower. They found that the measurement correlated highly with juvenile period and was heritable.