Flowering on long and short shoots of Larix laricina in response to differential timing of GA(4/7) applications.

Abstract

In Larix, reproductive buds most often occur terminally on short shoots, but they can also differentiate in lateral positions on long shoots. The phenology of long and short shoots differs considerably, with short shoots breaking bud and expanding about 5 weeks before the start of long shoot extension. Foliar sprays of GA(4/7) were applied to 160 branches on 10 greenhouse-grown Larix laricina (Du Roi) K. Koch grafts either before (early) of after (late) the start of long shoot extension, or during both periods, to test whether the timing of GA(4/7) application affects flowering on short and long shoots. All three treatments induced flowering on both long and short shoots. The early GA(4/7) treatment led to slightly, but not significantly, more flowering on short shoots than on long shoots, whereas the late GA(4/7) treatment resulted in increased flowering on both types of shoots, but primarily on long shoots. Application of GA(4/7) during both periods did not result in increased flowering over the early or late treatment alone. Based on the fact that gibberellins are metabolized rapidly in conifers and our finding that GA(4/7) applied before shoot elongation, when the bud primordia were at a very early stage of development (detectable bud differentiation only occurred several weeks later), induced flowering on long shoots, we conclude that the early GA(4/7) treatment did not affect differentiation as it was occurring, but somehow predisposed the bud primordia to differentiate reproductively.