THE CHEMICAL REGULATION OF THE DORMANCY PHASES OF BUD DEVELOPMENT

Literature pertaining to the development and regulation of dormancy in the buds of woody species is reviewed and interpreted as follows. Morphological observations, the effects of environmental factors, and other evidence support the concept that bud dormancy involves a cycle of 3 separate phases of development. Beginning at the developmental pattern of spring, the 3 phases are: (1) dormancy development leading to the dormant state; (2) release from dormancy leading to the non-dormant state; and (3) the initiation of spring burst leading again to spring development. The regulation of dormancy is, therefore, discussed in terms of the regulation of development of the apex within each phase and the regulation of transitions between phases. The principal existing theory of dormancy regulation implies that dormancy consists, in total, of merely the inhibition of spring development, and that regulation involves first the accumulation of an inhibitor then its disappearance. The conceptual basis of this inhibitor theoryis argued to be inadequate as is the experimental evidence; for the existence of a specific inhibitor and for a correlation between its concentration and dormancy induction or release. There is Little direct evidence on the mechanism of the regulation of bud development within any developmental phase. Circumstantial evidence suggests the developmental patterns arise from chemical patterns resulting from the interactions of classes of growth regulator such as auxin, kinin, and gibberellin. Some evidence exists concerning the regulation of the transitions between the phases of dormancy. A substance has recently been detected which may be a hormone regulating the initiation of dormancy development. The production of this substance may be photoperiodically determined. A role for gibberellins in the regulation of dormancy release has been postulated. RESEARCH on the hormonal regulation of plant development appears ready to tackle the critical problems of the regulatory mechanisms which establish and maintain developmental patterns in the meristematic zones of plants. Two classes of regulators, auxins and gibberellins, have been chemically defined; another class, kinins, is close to being so defined. Other types of regulators have been detected and clearly have roles in development, but already the auxins, gibberellins, and kinins have been shown to be capable, through their interactions, of regulating the development of cells and tissues and the initiation of bud or root meristems (Kefford and Goldacre, 1961). It is appropriate, as growth-regulator research moves into this area, to evaluate past attempts to investigate regulatory mechanisms within meristematic zones. It so happens that the regulation of 1 Paper invited by the Editorial Committee. Received for publication April 4, 1964. The authors were supported partially (H.S.), or completely (N.P.K.), by a National Science Foundation Grant to Professor A. W. Galston, who suggested the preparation of the paper and, together with Professors P. F. Wareing and L. M. Sussex, criticized early drafts. The authors are grateful to all. 2 Theresa Seessel Postdoctoral Research Fellow. Present address: Department of Botany, Birkbeck College, London University. 3 Fulbright Senior Research Fellow, on leave from the Division of Plant Industry, C.S.I.R.O., Canberra, Australia. one aspect of bud development has been studied for over 20 years, and in the course of these studies many pitfalls have been encountered, which are of general significance to the investigation of meristematic zones. These investigations have been on bud dormancy. In this paper an attempt is made to rationalize existing information on the dormancy of buds, particularly of woody species, and to assess the pertinence of experiments and theories to the regulation of their dormancy. Whilst there is much variation between species, and between the types of bud on one species, in the development of dormant buds, the available evidence appears capable of supporting some general principles to indicate what might be expected of a regulatory system. In our arguments we shall follow others such as Thimann (1960) and Haber (1963) and use the terms growth and development to represent distinct concepts. Growth describes only an irreversible increase in the volume of a cell, tissue, or organ, and is not concerned with the processes which happen to contribute to the volume change. In contrast, development is primarily concerned with the processes contributing to plant functions and their relative changes with time. Clearly, then, an inhibition of growth and concurrent positive developmental changes, within a bud for example, are compatible. It is this circumstance, which may be common, that is of principal interest in the development of a dormant bud.