Brief Essay on Abscission
The shedding of old leaves, ripe fruits and old flowers by the plants is of common occurrence. This process of shedding of organs by a plant is called abscission. It is a complex physiological process generally associated with senescence, cessation of growth and maturity of organs. Abscission is restricted to a morphologically distinct region called abscission zone.
This is located at the point of attachment of organ to the mother plant. The cells of this zone remain small compact and densely filled with protoplasm. It lacks fibres and lignifications is absent or weak. Role of axing and ethylene are considered vital to this phenomenon. It is observed that auxins and cytokinins retard while ethylene and abscise acid promote abscission.
Due to abscission, leaf fall occurs, seasonally in all deciduous plants. The mechanism of leaf fall due to abscission is detailed in the following.
In winter or in summer seasons, the plants absorb minimum amount of water from the soil and evaporation rate is higher. The leaves of the deciduous trees fall down during this time of the year to counterbalance the water deficit in the plants. This occurs by formation of an abscission layer at the base of the petiole. It takes place due to cytological and chemical changes in cells along which leaf will separate.
This is called abscission zone. In this zone, two layers are formed abscission or separation layer and a protection layer. Here, structural changes facilitate the separation of the leaf. Beneath this , develops the protective layer which protects the exposed surface formed by falling of the leaf.
(a) Abscission (separation) layer: Several cellular processes may be associated with absciso on and it is not similar in all the species of plants. The actual separation may be preceded by cell division. The new cell walls may be degraded.
Cell enlargement may also take place. The separation of leaf along the abscission layer may be caused by three methods. In the first case the middle lamella is dissolved, in the second case the primary walls are dissolved and in the third, the entire wall is dissolved.
The dissolution of the middle lamella of the abscission layer occurs only in the tissues surrounding the vascular region. After the dissolution of the separation layer, the leaf is held by the vascular region only.
The formation of tyloses in vessels and callose in sieve tissue stops supply of water and nutrition to the plant. A little disturbance in the environment like wind may cause the break in the vascular tissues and leaf fall. Protective layer
As soon as the leaf falls, the protective layer develops on the site of abscission. It may be primary or secondary. It protects the exposed surface from dessication, injury and invasion of parasites.
The primary protective layer develops by depositing lignin and suberin on the walls and intercellular spaces. The secondary protective layer is typically due to protective layer in woody plants.
Auxin and ethylene are seen to be the best known agents in regulation of abscission. Auxin accelerates the formation of abscission zone when applied at the proximal side and retards when applied to the distal end. Ethylene normally drives abscission to completion.
The enzymes needed for cell wall degradation is peroxidase and phosphatase. Other chemical substances like gibberellic acid, cytokinin and abscisic add also influence abscission.