Promotion of cone and seed production in grafted and seedling Douglas-fir seed orchards by application of gibberellin A4/7 mixture

Abstract

Both 4-year-old grafts and 6-year-old seedlings of Douglas-fir (Pseudotsugamenziesii (Mirb.) Franco) were induced to flower, and the production of seed cones and filled seeds was significantly enhanced by branch applications of gibberellin A4/7 (GA4/7) mixture.For 6-year-old seedlings, when pooled over all levels of the auxin, napthaleneacetic acid (NAA), seed cone bud differentiation, and number of filled seeds per treatment branch tended to be enhanced by GA3, GA5, and GA9, relative to controls. A more marked enhancement (although still not significant) in number of filled seeds per treatment branch was noted for GA3, GA5, and GA9 when applied in combination with GA4/7, relative to GA4/7 alone. The combination of GA9 + GA4/7 significantly increased the number of seed cone buds differentiated, relative to GA4/7 alone. Low levels of NAA, pooled over all GA treatments, significantly increased the number of seed cones per branch at maturity, and tended to increase both the number of seed cone buds differentiated and filled seeds per branch.Girdling, an effective adjunct treatment for promoting flowering, reduced filled seeds per cone for grafts, but had no effect on seedlings. Neither cone abortion nor seed set was affected by the hormone treatments, although there was a tendency in the seedlings for NAA to reduce cone abortion and increase filled seeds per cone. Seed from GA-induced cones germinated equally as well as that from controls. For grafts, GA treatments resulted in a 500% increase in per-branch yield of filled seeds; for seedlings, increases of 250 to 600% were noted.