Senescence processes in leaf abscission.

There is a large body of evidence which correlates the development of some phases of senescence with the ability of petioles to experience abscission. We have suggested that the change-over from stage 1 to stage 2 in the aging of bean petiole explants may be a reflection of initial stages of senescence in the pulvinar tissue. The abscission-inhibiting effect of auxin in interpretable as a retardation of pulvinar senescence. Senescence of cells in the separation layer has not been unequivocally established, and it seems unlikely that separation is itself a consequence of cellular senescence in the separation zone. More probably, senescence plays a role in the preparatory phases of abscission, that is, in the development of a condition of responsiveness to ethylene. In bean explants, ethylene responsiveness for abscission is associated with an ethylene-stimulated production of ethylene in the pulvinar tissues.