Fruit set and June drop in Cox's Orange Pippin apple as affected by pollination and treatment with a mixture of gibberellins A4 and A7

Single spray applications of 100 ppm of a mixture of the gibberellins A4 and A7 (GA4 + 7) were made on fruiting spurs of the apple cultivar Cox's Orange Pippin to investigate the effect on set, June drop and growth of fruits, as well as on shoot development and flower-bud formation on the bourses. Applications were made from 1 to 50 days after full bloom following partial hand-pollination of flowers, i.e. two stigmas per flower pollinated. In another experiment applications were made from 1 to 20 days after full bloom following complete (five stigmas per flower) or partial (two stigmas per flower) hand-pollination, after open pollination, or on emasculated flowers. GA4 + 7 only temporarily increased fruit set after open pollination or after effemination of flowers, and then only after application 1 day after full bloom. GA4 + 7 did not affect the very high fruit set after complete or partial hand-pollination. Both latter pollination treatments induced an equally high fruit set. GA4 + 7 reduced June drop significantly whenever fruits were left after first drop, except after early applications following open pollination. GA4 + 7 was effective in June-drop reduction up to 40 days after full bloom, i.e. until the onset of the June drop. Fruit size was not clearly affected by GA4 + 7. The smaller fruits obtained in some cases after GA treatment could be explained by assuming that maturity was reached by fruits that would have abscised without an exogenous GA4 + 7 supply. GA4 + 7 also increased seed abortion. Fruit length was increased by GA4 + 7 only for applications made up to 20 days after full bloom. GA4 + 7 stimulated bourse-shoot development to some extent. Flower-bud formation on the bourses was not clearly affected by GA4 + 7, but was markedly influenced by the presence of fruits. That GA4 + 7 reduced June drop so much in spite of a slight promotive effect on bourse-shoot growth and a slight abortive action on seeds suggests that these gibberellins may be specific stimuli for apple-fruit growth after actual fruit set is achieved.