Promotion of Flowering in the Pinaceae by Gibberellins

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Abstract
Gibberellins (GAs) were effective in promoting flowering in sexually mature (45‐year‐old scions) grafts of loblolly pine (Pinus taeda L.). Seed‐cone production was increased 12‐fold in field‐grown, grafted ramets by bi‐weekly, May‐September applications of 500 μg per branch of GA4/7. Gibberellin A3 was equally effective at 500 μg but not at 100 μg per branch, while GA5 was ineffective at either concentration. A second study using potted, less‐sexually mature (8‐to 10‐year‐old scions) grafts gave a reduced level of seed cones in response to GAs. However, even on these younger grafts GA4/7 was a significant promotive treatment, GA3 being considerably less effective. Branch girdling, tested as an adjunct treatment, was ineffective. It is now apparent that exogenous applications of GA4/7 are effective on a number of Pinaceae species, and their use to promote earlier and more abundant flowering in breeding orchards of grafted ramets for at least two species, loblolly pine and Douglas fir [Pseudotsuga menziesii (Mirb.) Franco] is practical.