Enhancement of shoot elongation in Douglas-fir by gibberellin A4/7 and its relation to the hormonal promotion of flowering

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Abstract
The enhanced stem elongation that frequently accompanies the GA4/7 promotion of flowering in Pinaceae species was investigated for Douglas-fir 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 wk) 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 yr old). These findings are consistent with a hypothesis that exogenous GA4/7 is 1st utilized for vegetative growth processes, increased flowering occurring only after a threshold concentration is reached.