Fruit and leaf explants of ‘Pineapple’ orange were aged for 2 to 36 hr after excision and prior to treatment with ethylene or 2,4-dichlorophenoxyacetic acid (2,4-D). Abscission of fruit explants, aged for various durations, was accelerated by ethylene applied as early as 2 hr after excision but was delayed by 2,4-D administered within 26 hr after excision. Leaf explants did not exhibit an abscission response to ethylene up to 20 hr following excision, but they became responsive at 28 hr. The growth substance, 2,4-D, was effective in delaying abscission of leaf explants aged for 44 hr. Ethylene, 2-chloroethylphosphonic acid (Ethrel), and cycloheximide failed to accelerate abscission in young ‘Valencia’ orange fruit (3.1 to 3.8 cm diam.) while they greatly accelerated abscission in mature fruit. Citrus leaf, but not mature fruit explants, exhibit the 2-stage abscission phenomenon in response to auxin and ethylene. Results suggest the possibility of differential abscission promotion in mature vs immature ‘Valencia’ orange fruits.
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M. A. Ismail (1970) studied this question.