Effect of 2,5-norbornadiene on abscission and ethylene production in citrus leaf explants

Citrus ( Citrus sinensis L. Osbeck) leaf explants completely abscise within 48 h when exposed to saturating amounts of ethylene at 25°C. When 2,5-norbornadiene was added, 2000 μl 1⁻¹ reduced abscission of explants also exposed to 2 μl 1⁻¹ of ethylene to the level of the control, and 8000 μl 1⁻¹ reduced abscission in explants exposed to 10 μl 1⁻¹ of ethylene to the level of the control, but abscission was complete when 1 000 μl 1⁻¹ of ethylene was used in the presence of 8 000 μl 1⁻¹ of 2,5-norbornadiene. When explants were exposed to 2 μl 1⁻¹ of ethylene, 2000 μl 1⁻¹ of 2,5-norbornadiene prevented abscission if applied up to 10 h after exposure to ethylene. After 18 h, applied 2,5-norbornadiene had little effect on abscission at 48 h. A Lineweaver-Burk plot gave a 1/2 maximum value of 0.12 μl 1⁻¹ for ethylene on abscission, 2,5-Norbornadiene gave competitive kinetics with respect to ethylene with a K₁ value of approximately 120 μl 1⁻¹ of 2,5-norbornadiene. The presence of 2,5norbornadiene stimulated ethylene production, which progressively increased as the 2,5-norbornadiene concentration was increased from 250 to 8 000 μl 1⁻¹ 2,5-Norbornadiene also suppressed the induction of cellulase and polygalacturonase by ethylene. Together, 2,5-norbornadiene and 2,4-dichlorophenoxyacetic acid were more effective than either alone in reducing abscission. 2,5-Norbornadiene also was effective in preventing the reduction of indole-3-acetic acid transport induced by ethylene.