Abscission in Plants: Leaf Abscission mechanism and steps, Physiology of Abscission, Role of Plant Hormones (Auxin, Ethylene and Abcisic Acid)

What is Abscission?

Abscission can be defined as shedding of leaves, fruits and flowers in plants due to physiological and anatomical changes.
Shedding of older leaves from the stem or branch without causing any injury to the stem is called as Leaf Fall or Abscission.
Abscission can be Simultaneously (Deciduous plants) or Periodic (Perennial plants)

MECHANISM OF ABSCISSION

STEPS INVOLVED IN ABSCISSION:

1.Abscission Zone:

The location where abscission occurs is called as Abscission zone that is determined in early stage of plant development. It comprises of Separation layer and Protective layer.

2.Differentiation of Abscission zone:

Differentiation of this zone is by set of genes that are involved in this process.

3.Activation of process:

Phytohormones also plays important role in abscission in plants. One of the phytohormone that is ETHYLENE is found to be effective in abscission in plants.

4.Cell Detachment:

Ethylene leads to cell detachment
The cells of the abscission zone then began to detach from each other in this zone due to secretion of cellulases, Pectinases, Polygalacturonases, Hydrolases that modify the cell wall, dissolve middle lamina and primary cell wall, and result in cell detachment. Vascular bundles are reduced in diameter.
At this time leaf is only attach to stem via reduced Vascular bundles which soon get detached by pressure of wind

5.Formation of Protective layer:

This Protective layer secretes a protective chemical called Suberin that forms a protective layer and prevent injury to living tissue of stem.
Ultimate abscission of leaf from stem left a leaf scar. The scar on the stem left by the leaf will be sealed by this layer, which will prevent desiccation and block microbial invasion.
The broken vascular element get plugged by Tyloses ang Gums in Xylem and Callose in Phloem to stop translocation of food and water.