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The theory that fruit abscission may be the result of a reduction in metabolites available to the young fruit was addressed by this study. Shade cloth or terbacil, a photosynthetic inhibitor, were applied to limbs and whole trees to examine the influence of treatment and time of application on fruit nonstructural carbohydrates and abscission. 'Stayman' apple limbs shaded with 92% shade material from 5-15, 10-20, 15-25, 20-30, and 25-35 days after full bloom (April 22) had lower fruit retention than unshaded controls on 11 June. On 18 June, fruit diameter was greater on limbs shaded between 5-25 days after full bloom (DAFB) than on unshaded limbs. At 15, 20, 25, and 30 DAFB, fruit from limbs shaded for 10 days had lower total nonstructural carbohydrates (TNC), total sugars, and reducing sugars (% dry wt) than fruit from limbs shaded for 0 or 5 days. Terbacil (3-tert-butyl-5-chloro-6-methyluracil) was applied at 0, 50, 100, and 200 ppm to whole nine-year-old 'Redchief Delicious' apple trees at 15 DAFB. Terbacil markedly inhibited Pn; recovery occurred by 9 and 26 DAA for the 50 and 100 ppm rates, respectively. Phytotoxicity prevented the determination of Pn in the 200 ppm treated trees. Fruit dry weight, TNC, total sugars, and reducing sugars (% dry wt and mg/fruit) declined with increasing rates of terbacil. Total fruit abscission was observed 12 DAA for the 100 and 200 ppm treatments, while the 0 and 50 ppm applications retained 4.6 and 1.4 fruit per cm² limb cross sectional area (LCSA) at 35 DAA, respectively. Compared to the control, 50 ppm terbacil decreased fruit number and weight at harvest, but increased fruit weight. Terbacil at 75 ppm and 92% shade material were applied to whole, 3-year-old 'Redchief Delicious' trees at 18, 23, and 28 DAFB. Generally, fruit dry weight, total sugars, and reducing sugars were lowered by both shade and terbacil treatments. In most cases, fruit from shaded trees were lower in dry weight and measured nonstructural carbohydrates than fruit from terbacil-treated trees after 5 or 10 days of treatment. Shading for 5 or 10 days resulted in total fruit drop. Terbacil at 75 ppm resulted in 0.8 as opposed to 2.9 fruit per cm² LCSA on the controls at 54 DAFB. M.S.

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