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Physiological studies of bitter pit in apple

Witney, Guy Wentworth.

Thesis, Ph.D., Virginia Polytechnic Institute and State University, 1989, 107 pages.

1989

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PHYSIOLOGICAL STUDIES OF BITTER PIT IN APPLE.

In a series of related experiments some aspects of the histology and physiology of the disorder better pit in apple (Malus domestica Borkh.) were studied.

 A two year field study was conducted to induce consistent bitter pit development in spur type 'Delicious' (D) and 'Golden Delicious' (GD) apple fruit.  Multiple spray treatments of CaCl2 and MgCl2, combined with paper bag fruit covers, were applied and subsequent bitter pit development examined.  The main effect of bags in both cultivars was increased pit development, decreased Ca in fruit and increased fruit K.  CaCl2 sprays resulted in less pit development, increased Ca in fruit, and less fruit Mg.  MgCl2 sprays resulted in increased bitter pit, decreased fruit Ca, and increased fruit Mg.  Overall, field treatments provided a wide range of bitter pit incidence after storage, from 100% (bag and MgCl2 spray combined) to 3% (CaCl2 spray alone) in both D and GD.

 The cellular morphology of pitted apple fruit from field trees was examined using transmission and scanning electron microscopy.  The overall tissue morphology of both cultivars was similar, but in pitted tissues differences were observed in tannin localization, starch hydrolysis, and cell wall morphology.  Cation levels in the tissues were examined using X-ray microanalysis.  High Mg levels were localized in pit cells, while K levels were similar in both healthy and pitted cells.  Ca levels in both tissue types were too low to be detected by this method.

 Using 'Golden Delicious' fruit from the field study, the relationship between pyruvate kinase activity, fruit cation concentration and bitter pit was investigated.  Pyruvate kinase activity during early fruit growth was higher in fruit which developed 100% bitter pit after storage (MgCl2 spray + bag), than in fruit that developed 3% bitter pit (CaCl2 spray).  Fruit with a high bitter pit incidence had a lower Ca: Mg + K ratio than fruit with a low level of the disorder.  There was a strong positive correlation between enzyme activity early in the season and bitter pit incidence after storage.  An assay for pyruvate kinase may be valuable for early prediction of postharvest bitter pit development.

 Finally, the qualitative electrophoretic patterns of soluble fruit proteins from each treatment were examined starting early in the season and continuing until termination of fruit storage.  Patterns from all treatments were almost identical throughout the season.