LAUSR

Seed orchards play a critical role in reforestation, but often struggle to consistently produce enough seed. A mixture of the less polar gibberellins, GA4 and GA7, has been shown to promote flowering in Pinaceae species, but the responses vary significantly based on species, genotype and provenance, application method, timing and dosage. Previous studies with lodgepole pine (Pinus contorta Dougl. var. latifolia Engelm.) suggest that the response to exogenous GA4/7 varied among genotypes. Gibberellins also play a critical role in seed development and germination but it remains unclear how exogenous GA4/7 will affect seed quality. We used 25 lodgepole pine genotypes in a commercial seed orchard in Alberta, Canada to explore the impact of genotype and geographic origin on the efficacy and optimal timing of GA4/7 stem injections to promote female cone bud initiation, cones and seed production. Overall, we found that GA4/7 stem injections promoted female conelets in 40% of genotypes included in this study. For these 10 genotypes, we found that the latitude of their provenance was positively correlated with later effective application times. Of the remaining 15 genotypes, five were hypersensitive to exogenous GA4/7 showing needle discolouration and loss, and 10 genotypes showed no response. In general, stem-injected GA4/7 did not affect the number of seeds per cone while two genotypes showed a reduced seed yield and one genotype showed increased seed yield per cone. Seed weights were not affected by the stem-injected GA4/7, while the July application date resulted in higher germination rates compared to control trees.