Physiological Factors Affecting Biennial Bearing in Tree Fruit: The Role of Seeds in Apple

Abstract

The evidence for several hypotheses regarding the mechanism(s) controlling biennial bearing in apple (Malus×domestica Borkh.) are reviewed, citing relevant evidence from work with citrus (Citrus sp.) species and pear (Pyrus communis L.). The view that flowering is inhibited by withdrawal of nutrients, primarily carbohydrates, by apple fruit is questionable, given the effects of seed development in inhibiting flowering in facultatively parthenocarpic (normally seedless) apple cultivars. The hypothesis that seeds inhibit flowering by exporting hormones, chiefly gibberellins (GAs), is an attractive one, given a) the effects of application of GAs in inhibiting flowering and b) the high concentrations of GAs in seeds. However, an alternative hypothesis, namely that seeds compete with apices for hormones that are required for flowering, is equally tenable.