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ISHS Acta Horticulturae 636: XXVI International Horticultural Congress: Key Processes in the Growth and Cropping of Deciduous Fruit and Nut Trees

FRUIT AND SHOOT GROWTH FOLLOWING COMBINED GIRDLING AND THINNING OF 'ROYAL GALA' APPLE TREES

Authors:   J.J.B. Pretorius, S.J.E. Wand, K.I. Theron
Keywords:   carbohydrate allocation, crop load, fruit size, source-sink competition, yield
Abstract:
Fruit size of ‘Royal Gala’ apples is generally unsatisfactory in the Western Cape region of South Africa. Orchard practices aimed at increasing fruit size generally lower either the vegetative sink strength (e.g. girdling) or the total reproductive sink strength (e.g. thinning), thereby stimulating individual fruit growth. The interactive effects of these practices on tree physiology are poorly studied. Full-bearing ‘Royal Gala’ apple trees growing in the Elgin district, South Africa, were girdled twice (2 and 5 weeks after full bloom) and subjected to five degrees of hand thinning (2-3 weeks after full bloom) in a factorial randomised block experimental design. Fruit diameter and extension growth of one-year-old and bourse shoots, were measured throughout the season, and final fruit size and yield determined. Girdling decreased bourse and extension shoot growth, and consistently led to increased fruit growth rates. Thinning resulted in more shoot growth than no thinning. However moderate thinning (to one fruit per cluster) stimulated more shoot growth than stronger thinning (to one fruit every second cluster) and complete fruit removal. Fruit size was significantly improved by thinning to one fruit per cluster or one fruit every second cluster. No interaction was found between girdling and thinning except for a particularly strong girdling-induced decrease in extension shoot growth in defruited trees. The best results were obtained for girdled trees thinned to one fruit every second cluster (169 g per fruit compared to 129 g for trees not girdled or thinned). Yield per tree and per centimeter trunk cross-sectional area was higher for girdled treatments, thus ensuring a higher revenue for the total crop on girdled trees.
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