ABSCISSION: THE ROLE OF SENESCENE

Indoleacetic acid IAA, Coumarin, and the cytokinins 6- furfurylaminopurine, N6-benzyladenine, and 6-benzylamino-9, 2- tetrahydropyranyl- 9H-purine SD 8339 inhibited the abscission of bean Phaseolus vulgaris L. var. Red Kidney petiole explants. IAA, coumarin, and SD 8339 inhibited senescence measured as the loss of chlorophyll, ribonucleic acid RNA, and protein from the pulvinus. Stimulation of RNA synthesis by ethylene in 22-hour-old explants was inhibited when explant senescence was retarded by IAA or by cytokinin SD 8339. Ethylene stimulated abscission without affecting the degradative processes associated with senescence. Increasing petiole length stimulated the abscission of explants, but addition of sucrose or ethylene masked the effect, suggesting that longer petioles were supplying either more carbohydrates or more ethylene to the abscission zone. Actinomycin D and cycloheximide inhibited degradative changes in the pulvinus, suggesting that catabolic enzymes are newly synthesized after excision of explants.