Promotion of cone and seed production in rooted ramets and seedlings of western hemlock by gibberellins and adjunct cultural treatments

Abstract

Over 100-fold increases in numbers of seed cones and 6- to 24-fold increases in pollen cones were achieved for 7- and 8-year-old rooted ramets of sexually mature western hemlock (Tsugaheterophylla (Raf.) Sarg.) from stem injections of gibberellin A4/7 (GA4/7) mixtures and calcium nitrate fertilization. Female, but not male, flowering was also promoted significantly in nitrate-fertilized 6-year-old seedlings, both by stem injections (47% flowering) and foliar sprays (40% flowering) of GA4/7. Flower induction in 3-year-old potted seedlings required water stress in addition to GA4/7 foliar sprays (30% initiated seed cones and 7% pollen cones). The cones produced developed normally and set sound seeds. For mature ramets, flower promotion was roughly proportional to the logarithm of GA4/7 + GA9 concentration from 25 to 400 mg•L−1; calcium nitrate fertilization, though ineffective when used alone, significantly enhanced this response. Treatments of 6- and 12-week durations, beginning 24 May, were equally effective in promoting both male and female flowering. However, production of seed cones was only slightly enhanced and that of pollen cones not at all when treatment was delayed until 7 July (6 weeks), indicating the need to start GA applications before cone-bud differentiation has been completed (late June for pollen cones and mid-July for seed cones).