Anatomical aspects of abscission.

Anatomical aspects of abscission are reviewed mainly on the basis of experimental studies on Coleus, Gossypium, and Phaseolus. In Phaseolus histological studies of explants show that petiolar abscission is correlated with the formation of tyloses in the vessels proximal to the zone of separation, and that the abscission zone is much less well demarcated than in Coleus or Gossypium. Radioautographic studies of Phaseolus petiole explants indicate little initial difference in the distribution of nucleolar RNA and in nuclear and cytoplasmic protein in cells distal (toward the blade) or proximal (toward the stem) to the region of separation. However, in ethylene-treated explants an increase in nucleolar RNA and in nuclear and cytoplasmic protein is evident in cortical cells immediately proximal to the abscission zone, and binucleate cells commonly occur. Abscission occurs by dissolution of newly formed cell walls and disruption of the mother cell walls in the zone of recently divided cells. It is suggested that the experimental results can be explained on the basis of changes induced in levels of ethylene in the petiole, the experimental application of ethylene becoming effective in expediting abscission only after the endogenous ethylene level in explants has declined.