Abscission is a phenomenon in which plants form a specific cellular layer called abscission layer in the petioles and pedicels. The formation of this layer is stimulated by the growth-inhibitor hormone abscisic acid, which is also known as stress hormone.
Abscission layer contains thin-walled parenchymatous cells. There are three steps of formation of this layer:
A. In the first stage, the quick degradation of chlorophyll molecules takes place along with the slow degradation of carotenoids.
B. The parenchymatous cells divide and form cork cells and synthesize and inject suberin and lignin under the abscission zone.
C. During detachment, these parenchymatous cells secrete enzymes to digest the middle lamella of the cells of the abscission zone.
The formation of this layer is stimulated by two conditions:
1. If the internal auxin concentration decreases. Auxins prevent the formation of this layer near the base of petioles and pedicels by counteracting ABA.
2. If the concentration of ABA increases or if ABA is externally applied to a particular part.
Florigen is a hypothetical hormone that stimulated a vegetative apex into a floral apex.
Cytokinins do not play a direct role in the leaf-fall.