Resumen
Aim of study: Seed production in forest tree species commonly takes a long time due to the length of the juvenile stage. Even though several treatments have been used to induce early flowering in conifer species, experience on their use in subtropical Pinus species is limited. This study aimed to evaluate the effect of GA4/7 dose (0, 1.27 and 2.54 mg cm-2) and application time (July-October), alone or in combination with partial stem girdling, on male and female strobili production in young Pinus patula Schiede ex Schltdl. et Cham. clones. Area of study: Nine clones with different flowering background of high-elevation Pinus patula growing in a six-year old seed orchard established in Central México (Aquixtla, Puebla) at 2,800 m elevation.Material and methods: Two independent flowering trials (FT1 and FT2) were carried out in the seed orchard during the 2009 and 2010 flowering cycles; similar factors were evaluated at both trials but time of application, clones tested, and experimental design used varied for each of them. Partial stem girdling was done at the base of the trunk and the GA4/7 solution was injected into the xylem above the point of girdling. The following spring, the percentage of trees with strobili and the number of strobili per tree were determined for both male and female structures.Main results: Significant differences (p = 0.05) among clones in flowering capacity were found at both trials. None of the treatments applied in FT1 resulted in an increase of strobili formation, most probably because they were applied too late in the growing season. In FT2, however, application of GA4/7 combined with partial stem girdling increased the percentage of trees with strobili and the number of strobili of both sexes, particularly when applied in early July. Partial stem girdling was more effective on promoting male strobili than female ones in gibberellin-treated grafts.Research highlights: Timing of GA4/7 application and stem girdling was important, with a significant interaction with clones tested in FT2 on production of male strobili but not on female strobili. Application of 1.27 mg cm-2 GA4/7 and stem girdling in July promoted the highest percentage of trees with strobili, and increased 25-fold the number of female strobili and 5-fold the number of male strobili per tree as compared to the control treatment. Thus, operational use of this induction treatment would be valuable to increase and accelerate seed production in Pinus patula seed orchards in the region.Keywords: early flowering; gibberellins; juvenile stage; seed orchard; seed production.Abbreviations: GA: Gibberellic acid; GA4: Gibberellic acid 4; GA7: Gibberellic acid 7; GA4/7: Mix of gibberellic acids 4 and 7; FT1: Flowering trial carried out in the 2009 season; FT2: Flowering trial carried out in the 2010 season.