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CONTROL OF PHASE CHANGE IN WOODY PLANTS
Control achievable with hormonal, nutritional, environmental, grafting and other treatments is also reviewed.
From the experimental and observational evidence available it appears that both hormonal and nutritional factors can be involved in control of phase change but that a specific juvenile or aging hormone is not involved.
Hormonal control of assimilate partitioning in the apical region is presented as a possible mechanism by which phase change could be mediated.
In all woody plants there is a juvenile phase, lasting up to 30 to 40 years in certain forest trees, during which flowering does not occur.
In time, however, the ability to flower is achieved and the tree is said to have attained the adult condition.
This phenomenon has been referred to as phase change by Brink (1962) and is of considerable theoretical interest and practical importance.
Associated with the change from the juvenile to the adult phase are progressive changes in physiological and morphological characteristics such as leaf shape and thickness, season leaf retention, phyllotaxy, thorniness, pigmentation, and the ability to form adventitious roots.
Most of these characteristics change gradually during the period preceding the adult phase and usually no distinct change in any one characteristic is apparent at the time the ability to flower is attained.
Although a juvenile phase occurs in herbaceous plants it is much shorter in duration and differences in morphology are less distinct except in a few cases such as Brussels sprouts (Brassica oleracea L. var. gemmifera).
An important characteristic of the phase change phenomenon is that once the adult phase is attained, it is relatively stable and reversion to the juvenile condition does not occur as a result of asexual propagation such as cuttage or graftage although reversion can be induced by other means (see below). In some plants such as Hedera helix L. and H. canariensis L. the juvenile phase can also be maintained in a stable morphological condition by cuttage propagation.
Stoutemyer and Britt (1965) have shown that callus cultures derived from stem tissues of juvenile and adult Hedera have characteristic and relatively stable growth rates over long periods of time indicating that the cellular metabolic characteristics of the 2 phases are quite different and quite stable.
This stability of characteristics associated with the juvenile-to-adult phase change is in contrast with other changes such as reduced growth rate and type of branching which also occur as the plant grows older.
Reduced growth rate of old plants can be reversed if an aged shoot is grafted onto a young seedling or if a cutting of an aged shoot is rooted.
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