Optimal conditions for inducing coning of container-grown Picea sitchensis grafts: effects of applying different quantities of GA4/7, timing and duration of heat and drought treatment, and girdling

Abstract

In cool, wet conditions outdoors, gibberellins A4 and A7 (GA4/7) applied to container-grown Sitka spruce (Picea sitchensis (Bong.) Carr.) did not stimulate flowering, even when three applications each of 100 mg were applied in combination with girdling. By contrast, GA4/7 in combination with heat and drought treatment in the polythene house significantly stimulated pollen and seed cone production and the optimal application rate of 2 mg per application was defined; girdling further increased the flowering when applied with a heat treatment. The optimal time and duration of treatment in the polythene house was 2 months during May and June, corresponding to the period of shoot elongation and preceding bud differentiation as assessed anatomically. Shorter treatments were also effective provided they were started before the completion of shoot elongation. Injection of GA4/7 into container-grown grafts in a polythene house is a simple, efficient and consistently successful technique of cone induction and could be used on a large-scale in tree-improvement programmes. Two cycles of grafts could be treated consecutively in the same season in the polythene house.