ISHS Acta Horticulturae 56: Symposium on Juvenility in Woody Perennials
PROMOTION OF FLOWERING IN CONIFERS OF THE PINACEAE BY CERTAIN OF THE GIBBERELLINS |
| Authors: | R. P. Pharis, S. D. Ross, Robert L. Wample, John N. Owens |
| Abstract:
The problem of terminating juvenility and enhancing coning in conifers is re-examined in light of (a) earlier work with gibberellin A3 (GA3) and (b) more recent work with the less-polar gibberellins A4, A5, A7, A9 and A14. Promotion of early flowering by all of the above gibberellins (GAs) occurs in at least one Cupressaceae species and success with the polar GA3 is ubiquitous throughout the Cupressaceae and Taxodiaceae. However, only the less-polar GAs are generally effective in Pinaceace species. The differential efficacy of GAs may be related not only to familial differences in GA metabolism and physiology, but also to differences between families and species in patterns and timing of sexual differentiation. The possibility that endogenous levels of certain GAs are limiting factor(s) in the flowering of mature-phase Pinaceae and juvenile-phase Cupressaceae species appears to be warranted as a working hypothesis. However, other as yet unknown factors may limit flowering in very young juvenile-phase Pinaceae species. The early work of Kato et al., (1958) and others (reviewed by Pharis and Kuo (1976) and Pharis (1976)) using the hormone GA3 resulted in precocious flowering in numerous species of Cupressaceae and Taxodiaceae. A number of GAs in addition to GA3 (GA1, GA2, GA4, GA5, GA7, GA9, GA14, GA24) were also highly effective when tested on several species in the Cupressaceae (Pharis and Morf, 1967, 1968; Pharis et al., 1969; Pharis and Morf, unpublished data). Attempts have since been made, using GA3, to extend these successes to members of the Pinaceae, especially those species where breeding and selection programs are underway for trees possessing genetic superiority in growth rate, fiber length, stem form, etc. In general, these attempts have not been readily repeatable (reviewed by Pharis and Kuo (1976)), or were unsuccessful and are not reported in the literature. An exception is the experiment of Hashizume (1967) with Japanese larch (Larix leptolepis Gord.). Thus, in contrast to the Cupressaceae and Taxodiaceae where flowering is the general response to GA3 application, species in the Pinaceae have remained intractable with regard to promotion of flowering by treatment with this hormone. Possible reasons for this are reviewed by Pharis (1976). The varying response of species from different families probably | |
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