Optimum timing of gibberellin A_4/7 sprays to promote cone production in jack pine seedlings

Abstract

Three-year-old jack pine (Pinus banksiana Lamb.) seedlings in pots were sprayed with gibberellin A_4/7 solution during the 1989 growing season. Unsprayed and sprayed controls, and seedlings sprayed with gibberellin A_4/7 at three concentrations (200, 400 and 600 mgL^-1) were tested in each of five different treatment periods based on jack pine morphogenesis. Each spraying period consisted of five weekly applications between May and October. Trees produced significantly more pollen cones than the two controls when sprayed at 400 or 600 mgL^-1 during July and early August. Spraying at 600 mgL^-1 during May–June and June–July produced significantly more seed cones than the two controls, as did spraying at 400 or 600 mgL^-1 during July and early August. The best treatment for both pollen- and seed-cone production was weekly spraying at 400 or 600 mgL^-1 gibberellin A_4/7 from early July, which coincided with the end of terminal shoot elongation, through early August, which coincided with microsporophyll initiation in the pollen-cone buds and initiation of the potential seed-cone buds.