Effect of Cyclanilide, Ethephon, Auxin Transport Inhibitors, and Temperature on Whole Plant Defoliation

Whole plant defoliation studies were performed using dark red kidney bean (Phaseolus vulgaris L.) to compare the effect of cyclanilide [1-(2,4-dichlorophenylaminocarbonyl)-cyclopropane carboxylic acid], ethephon (2-chloroethylphosphonic acid), auxin transport inhibitors, and temperature on leaf abscission. Ethephon induced bean leaf defoliation, but ethephon combined with cyclanilide induced greater defoliation than ethephon alone. The response was dose dependent, as 0.067 kg/ha ethephon alone did not induce defoliation, but when combined with an equal rate of cyclanilide induced 63% defoliation five days after treatment (DAT). In addition, combining auxin transport inhibitors at 0.140 kg a.i./ha with 0.067 kg a.i./ha ethephon induced 50 to 52% defoliation, 5 DAT. Ethephon-induced defoliation was more temperature sensitive than the combination of cyclanilide with ethephon. Ethephon alone (0.067 kg a.i./ha) induced 26% defoliation 5 DAT at the highest tested temperature (30/26°C, day/night), but cyclanilide combined with ethephon induced 75 to 85% defoliation at all tested temperatures (except at 16/14°C, the lowest). These results indicate that cyclanilide is an ethephon synergist because it enhanced ethephon activity, even at low temperatures. These results support the model that ethylene released by ethephon induces abscission, and suggests that inhibition of auxin transport or auxin signaling will increase the effect of ethylene on the process of leaf abscission. Because of the similarities to the activity to 1-N-naphthylphthalamic acid (NPA) and 2,3,4-triiodobenzoic acid (TIBA) in vivo, cyclanilide may act as an auxin transport inhibitor.