Interaction between gibberellin A_4/7 and root-pruning on the reproductive and vegetative process in Douglas-fir. II. Effects on shoot elongation and its relationship to flowering

Abstract

Shoot elongation and female flowering response were assessed for gibberellin A 4/7 (GA 4/7 ) and root-pruning (RP) treatments in 9- and 10-year-old Douglas-fir (Pseudotsugamenziesii (Mirb.) Franco) seedlings of families with good- and poor-flowering histories. In families with a poor-flowering history, stem injections of GA 4/7 significantly enhanced elongation of third-whorl terminal shoots but produced no flowering response. In families with a good-flowering history, GA 4/7 treatment had no effect on shoot elongation but did result in a significant increase in seed-cone buds. In contrast, root-pruning significantly retarded shoot growth in families with both good- and poor-flowering histories and was also the single most effective treatment for enhancing flowering. Combined, GA 4/7 plus RP had a synergistic effect on flowering and GA 4/7 partially overcame the inhibition of shoot growth caused by RP alone. These results are consistent with a hypothesis that exogenous and endogenous gibberellins are used preferentially for vegetative growth processes, with increased flowering occurring only after a threshold concentration of effector gibberellins is reached.