Articles
EARLY FLOWERING OF CULTIVATED APPLE SEEDLINGS FORCED IN THE GREENHOUSE
Plants were manually defoliated twice and by 26 months after germination 93% of one progeny and 89% of the other had flowered.
Although all axillary buds did not grow in the greenhouse, the adult phase appeared to have begun at 10–14 months.
After selected plants were chilled and planted in the field all surviving buds grew and the phase change from juvenile to adult was precisely located.
It was 5–33% lower on the stem than previously thought, ranging from 1.83 to 3.07 m.
Other apple progenies from a breeding program for multiple disease resistance have been forced in the greenhouse after inoculation with Venturia inaequalis, Gymnosporangium juniperi-virginianae and Erwinia amylovora. Many plants exceeded 3 m in height at 18 months.
Seedlings of cultivated apple (Malus pumila Miller) rarely flower in the field before they are 3 years old and often not until they are aged 8 years or more.
This lengthy period before flowering is probably composed of a long juvenile period plus a shorter transition period (Zimmerman, 1973). It is probably during the transition period that flowering is induced precociously by such treatments as trunk ringing (Way, 1971) or treatment with growth regulators (see Zimmerman, 1973). Accelerated maturation of plants by favorable cultural practices (Visser, 1970) or a longer growing season (Jonkers, 1971) probably reduces the length of the juvenile phase.
Zimmerman (1971) obtained first flowering in the tea crab-apple (M. hupehensis (Pamp.) Rehd.) seedlings in 9.5 months from germination by growing them continuously under favorable greenhouse conditions.
He found that the transition to adult phase occurred at the 75th to 80th node, representing a height of 1.8 – 2.0 m (Zimmerman, 1973). The phase change probably occurred 7–10 months after germination.
I shall report results of using some of Zimmerman’s techniques on cultivated apple seedlings.
Seedlings from 2 controlled crosses of apple selections were screened for resistance to apple scab (caused by Venturia inaequalis (Cke.) Wint.) and then grown under optimum conditions in the greenhouse (Aldwinckle, 1975). Flowering commenced spontaneously on some plants 16 months after germination.
Three months later plants which had not flowered were manually defoliated.
By 20 months after germination 86% of one progeny and 68% of the other had flowered.
A second defoliation induced flowering in additional plants so that by 26 months after germination 93% and 89%, respectively, of each progeny had flowered.
Since all buds did not grow, initiation of the adult phase could not be determined exactly.
The lowest flower bud observed did,