Reassessment of the Role of Calcium in Development of Bitter Pit in Apple

Abstract

From an extensive evaluation of the literature on effects of orchard factors on bitter pit development, the following hypothesis has been derived. The primary factor causing bitter pit is high gibberellin (GA) levels late in the season, likely resulting from excessive root activity. The increased GA levels may cause increased permeability of cell membranes in the fruit close to vascular bundles, thereby resulting in increased sensitivity of the fruit cells to post-harvest water stress. Water stress, especially after harvest, may trigger the mechanism of bitter pit development if the primary factor, establishing susceptibility, prevails over factors that reduce susceptibility. Calcium (Ca²⁺) deficiency could be a secondary factor, increasing an existing risk of bitter pit development. Ca²⁺ may stabilise the cell membranes and reduce their permeability. However, high GA levels may hamper its movement to the fruit. Externally applied growth retardants, or ripening-related endogenous GA antagonists in the fruit flesh (e.g. ethylene and ABA), could aIso reduce susceptibility to bitter pit, independently of Ca²⁺, by antagonising the GA effect. The limitations of the present systems, which use Ca²⁺ content of the apple fruit to predict bitter pit, may reflect the secondary function of Ca²⁺ in bitter pit development.