Promotion of Flowering on Mature Picea sitchensis by Gibberellin and Environmental Treatments. The Influence of Timing and Hormonal Concentration

Male and female flowering were significantly promoted by gibberellin A_4/7+ A₃ mixtures applied beneath bark flaps on branches of 11-year-grafted mature scions of Picea sitchensis (Bong.) Carr. growing in the open in Scotland; 4 mg/ml gave more flowering than higher or lower concentrations of gibberellins and application in June was more effective than in July. Gibberellins also increased male flowering on mature scions when applied directly to the buds in a polythene house. A gibberellin A_4/7+ A₃ mixture produced greater numbers of strobili when applied to 2-year-grafted plants in May and June than when applied in July and August. Flowering of 5-year-grafted plants increased with increasing concentrations of a gibberellin A_4/7+ A₉ mixture up to 19.7 mg/ml above which flowering decreased. Modifying the environment by keeping in a polythene house enhanced the combined male and female flowering of 5-year-grafted mature scions above the amount on control plants outside, the effect being more marked in June and July than in August and September. Larger numbers of gibberellin applications or longer total lengths of time in the polythene house gave significantly greater amounts of combined male and female strobili.