ISHS Acta Horticulturae 179: V International Symposium on Growth Regulators in Fruit Production
RUSSETING AND CRACKING OF APPLE FRUIT AND THEIR CONTROL WITH PLANT GROWTH REGULATORS |
| Authors: | D.R. Taylor, J.N. Knight |
| DOI: | 10.17660/ActaHortic.1986.179.139 |
| Abstract:
Russeting and cracking of apple fruit skin often results in the downgrading of a considerable proportion of a crop. With the current differential in price between Class I and Class II fruit, such downgrading can seriously reduce profitability of fruit production. In addition, enhanced rotting, and possible water loss, of russeted fruit in store can cause further economic loss. In accordance with results from several other countries, excellent control of russeting has been achieved in Cox, Discovery and Golden Delicious using the gibberellin mixture of A4 and A7. Our results have indicated that GA4/7, at a concentration of between 2.5 and 10 mg/ l depending on cultivar, should be applied 3 or 4 times at 10-day intervals starting during the period first-flower to petal-fall to achieve satisfactory control. Of the many factors that lead to russeting of apple fruit, growth stresses during early fruitlet development are believed to be a major cause. The alleviation of stress within the fruitlet is thought to be the primary effect of GA4/7 treatment. Effects of gibberellin sprays on the rheology of fruitlet skin and fruitlet anatomy were studied. Sections of skin measuring 7 x 2.5 mm were stretched according to the Instron technique using a tensiometer applying a load of 20 g. Treatment with GA4/7 increased the plasticity of the fruitlet skin by 25%, whereas elasticity was unaffected. Sections of treated and untreated fruitlets demonstrate clearly that gibberellin causes enlargement of epidermal and hypodermal cells: cell longitudinal sectional area (measured seven days after final spray) in untreated skin was 73 x 1350 μ compared to 84 x 1350 μ in GA4/7-treated skin. The number of cells per unit fruit surface area was reduced from 7014 to 5293 by GA4/7 treatment, also indicating greater cell size. Cuticular morphology may also be affected; in untreated fruit, a greater occurrence of deep flanges protruding between epidermal cells suggests areas of weakness where cracking can arise and give rise to russet development. Uptake and translocation of exogenously-applied gibberellin was studied with a view to furthering the understanding of the mode-of-action of GA4/7 in controlling russeting, and to define the best strategy for commercial application. In one experiment, 0.3 μCi of 14C-labelled GA3 was applied to one primary rosette leaf of flowering spurs to study distribution within a spur, while in a second experiment 0.1 μCi of the labelled gibberellin was applied to one side of the surface of fruitlets to examine the localised | |
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