Promotion of flowering in the Pinaceae by gibberellins

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Abstract
Significant male and female flowering (cone bud production) by girdled branches of 6‐year‐old Douglas fir (Pseudotsuga menziesii (Mirb. Franco) seedlings was promoted by applications (mid‐April to June) of 1.6 or 3.2 mg per branch (in total) of certain non‐polar gibberellins (GA's). Girdling alone was ineffective. When tested alone, a mixture of GA4/7 was most effective. GA9 less so, while GA5 and the more polar GA3 were essentially ineffective. For female cone buds GA4/7+ GA9 were synergistically effective, but for male cone buds GA4/7 alone was best. The auxin naphthaleneacetic acid (NAA) was not tested alone, but at low dosage (0.175 mg/branch in total) NAA enhanced the flowering efficacy of GA's for both sexes; at a high dosage (0.875 mg/branch in total) male cone bud production was further enhanced, but only at the expense of females. For female flowering the best treatment (90% frequency of flowering 6.8 cone buds/branch), was GA4/7+ GA9+ low NAA; for male flowering, it was GA4/7+ high NAA (30% frequency and 4.2 cone buds/branch. Frequency of flowering for controls was 18% and 0%, average number of cone buds/branch was 0.9 and 0, for females and males, respectively. The successful treatments did not affect promordia initiation, rather they caused the differentiation of previously initiated, but undetermined, lateral primordia into cone and latent buds at the expense of vegetative bud differentiation.The lack of success reported by earlier workers in promoting flowering in Pinaceae species by GA's appears to be the unfortunate result of selecting GA3 for initial testing. The practical implications of this early and enhanced flowering by non‐polar GA's seedlings of a commercially important conifer are discussed in relation to accelerating the processes of tree improvement.