Abstract
The enhanced stem elongation that frequently accompanies the gibberellin A4/7 (GA4/7) promotion of flowering in Pinaceae species was investigated for Douglas-fir (Pseudotsugamenziesii (Mirb.) Franco) vegetative propagules in relation to their physiological age, GA4/7 concentration, and treatment timing. Maximum growth enhancement, but marginal promotion of flowering (males and females), occurred when GA4/7 was applied in early spring before vegetative buds had begun to swell. In contrast, shoot elongation was only slightly enhanced when GA4/7 treatment was timed (vegetative bud burst ±4.5 weeks) to bracket the critical period for effecting differentiation of axillary primordia into cone buds. Also, the GA4/7 concentration optimal for flowering was supraoptimal for shoot elongation, and the growth response to GA4/7 decreased, whereas the flowering response increased, with increasing physiological age of the propagules (scions 4–45 years old). These findings are consistent with a hypothesis that exogenous GA4/7 is first utilized for vegetative growth processes, increased flowering occurring only after a threshold concentration is reached.
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