Effect of Foliar-Applied Gibberellin A3 on Male and Female Strobilus Production and Cone and Seed Quality in Western Redcedar (Thuja plicata Donn)

Abstract

Abstract Western redcedar (Thuja plicata Donn) foliar-applied gibberellin A3 (GA3) induction trials were performed over a 4-year period at a number of different seed orchards in coastal British Columbia. The effects of GA3 timing, concentration, and frequency on male and female strobilus production, as well as timing on seed quality, were studied. Male and female strobili were induced over the complete span of shoot elongation from May to August, indicative of a less-precise induction period than species in the Pinaceae family. Female strobilus production was correlated with shoot increment, such that maximum cone production was associated with maximum shoot elongation. In addition, seed quality decreased with decreasing shoot increments in August. For operational efficiency, a one-time foliar application of 200 mg/l GA3 is sufficient for adequate female strobilus production. To increase the female-to-male strobilus ratio, a two-time foliar application of GA3 mid-May and mid-July, concentrating on vigorous shoots, is recommended. Results are discussed in relation to seed orchard management techniques that may potentially influence selfing rates.