Promotion of flowering in Engelmann-white spruce seed orchards by GA 4/7 stem injection: effects of site, clonal fecundity, girdling and NAA, and treatment carry-over effects

Abstract

Stem injections of gibberellin A 4/7 (GA 4/7) and girdling were tested in factorial combination on good- and poor-flowering clones of Engelmann-white spruce in two seed orchards on the same site with different levels of past cone production. A second study compared one vs. two injections of GA 4/7 (same total dosage), the latter with and without the auxin naphthaleneacetic acid (NAA). In both years and both orchards, GA 4/7 alone promoted abundant seed and pollen cones, but treatment was relatively more effective on good-flowering clones and in the more productive orchard. Still, treatment improved the orchard parental balance in that without treatment, 80% of the seed cones were produced by only 10–17% of the clones, compared to 43% of the clones with GA 4/7. Flowering response to GA 4/7 was independent of number of injections and addition of NAA. Girdling was ineffective by itself in flowering promotion but enhanced the female response to GA 4/7 in the less-productive orchard and the male response by poor-flowering clones. Gibberellin A 4/7 treatment in 1988 had a slight adverse carry-over effect on tree condition and level of flowering in 1990, although this may be a consequence of heavy cone-bearing in 1989. Gibberellin A 4/7 stem injection is practical, but the optimal retreatment interval necessary for vegetative recovery and sustained abundant cone production remains to be established.