STRONG SYNERGISTIC EFFECTS OF GIBBERELLINS WITH THE SYNTHETIC CYTOKININ N-(2-CHLORO-4-PYRIDYL)-N-PHENYLUREA ON PARTHENOSCARPIC FRUIT-SET AND SOME OTHER FRUIT CHARACTERISTICS OF APPLE

The induction of parthenocarpic fruit set was investigated using the a pple cvs. Golden Delicious and Jonagold. The gibberellins GA(3), GA(4) , GA(5) and GA(7) and the synthetic phenylurea-type cytokinin CPPU (N- (2-chloro-4-pyridyl)N-phenylurea), were applied alone and in combinati on to unpollinated flowers at the end of petal fall. Gibberellins indu ced only a marginal final set of parthenocarpic fruits. CPPU sprays we re more effective, particularly in the first year. When applied in com bination, CPPU and gibberellins had a positive synergistic effect on p arthenocarpic fruit set and fruit size, but a negative effect on flowe r induction the next year. After CPPU + GA sprays, percent fruit set w as similar, or greater, compared to natural pollinated trees. The part henocarpic fruits induced by CPPU + GA had an increased length to diam eter ratio. CPPU stimulated, and GA(4) and GA(7) reduced, the russetin g of the parthenocarpic fruits. The internal quality of the fruits was hardly affected, but Ca-deficiency symptoms occurred more frequently in parthenocarpic fruits.