Flower-Bud Formation in Apple as Affected by Various Gibberellins

SummaryTwo-year-old potted Cox’s Orange Pippin trees were sprayed with GA3, GA4, GA7 or GA4+7 (500 ppm) at full bloom and two or four weeks thereafter. Shoot growth was not greatly affected by the treatments. When sprayed at full bloom, GA3 and especially GA, and GA4+7 markedly reduced flowering on spur buds, but GA4 had hardly any effect. In the two later treatments GA3 and GA4 had no effect, whereas GA舢 alone or combined with GA4, still showed distinct activity. Similar gibberellin treatments were applied to one-year-old potted Cox’s Orange Pippin trees either in the middle of May or at the end of July. Again, shoot-growth response was not pronounced. Flower-bud formation on the current year’s shoots was little or not at all affected by the early sprays but was reduced considerably by the late treatments, at least when GA, or GA4+, were applied. This reduction was mainly of flower-bud production on the top halves of the shoots. It is suggested that the gibberellins interfere during the early phases of bud development, long before the actual production of flower parts starts. The findings are discussed in relation to current hypotheses of flower-bud formation in apple.