Phase Change and Intra-Clonal Variability

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Abstract
There is considerable opportunity for intra-clonal variability due to nongenetic or epigenetic causes because of the well-known and striking age-related changes in developmental patterns in plants propagated from seed. It is well-known that a juvenile phase exists in the development of plants from seed, lasting up to 30-40 years in forest trees, during which flowering does not occur and cannot be induced by normal flower-initiating treatments. The ability to flower is achieved in time, however, and the plant is said to have attained the mature condition. This phenomenon has been referred to as a phase change by Brink (6). Changes in morphological and developmental characteristics, such as leaf cuticular characteristics (13), leaf shape and thickness, phyllotaxis, thominess, shoot orientation (46), and other physiological characteristics such as seasonal leaf retention, stem pigmentation (46), ability to form adventitious roots and buds (7, 46), partitioning of photosynthates into main stem vs. branches (30), disease resistance (W.J. Libby, Jr., personal communication) and cold resistance (27), are associated with the phase change. Phase-change-related characteristics are most obvious in woody, perennial plants but also have been demonstrated in herbaceous annuals and perennials (24, 54). Some of these characteristics are important horticulturally and the ability to control or manipulate them has important practical implications.

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