Abscission refers to the normal separation of senescent plant organs like leaves. It is a controlled process initiated during organ development with the formation of an abscission zone, a distinct layer of cells formed transversally across the petiole base. The abscission zone contains two layers - the abscission layer and protective layer. During abscission, the cells of the abscission layer separate due to dissolution of middle lamellae and cell walls by enzymes. Physiological changes also occur like chlorophyll degradation, decreased auxin, and increased ethylene production, which promote formation of cell wall degrading enzymes and abscission layer separation.
3. Abscission
• Abscission refers to the normal separation of a senescent plant organ
• It occurs to shed or separate the unnecessary plant parts or organs
• For instance, shedding of old leaves at the base of the petiole which
usually occurs during autumn
• It is a controlled process that, in most cases, is initiated during the
development of the organ by the formation of an abscission zone
4. • In a mature leaf, prior to abscission, a distinct zone of few layers of cells is formed
transversally across the petiole base. This zone is called Abscission zone
• Abscission zone has two layers :-
(i) abscission (or separation) layer, and
(ii) protective layer
Abscission Layer –
• The cells of abscission layer are thin walled parenchymatous and without the
deposition of suberin and lignin
• The vascular bundles in the abscission layer are also reduced in size
• The cells of the abscission layer separate from each other due to the dissolution
of middle lamella and primary wall of cellulose by the activity of pectinase and
cellulase enzymes
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6. Protective Layer –
• A corky layer containing suberin and lignin forms beneath the abscission layer
to protect the exposed surface after leaf fall from dessication and infection.
The process of abscission is initiated and proceeds as follows:
The parenchyma cells start dividing rapidly
They secrete a layer of suberin in the walls nearest the stem
The middle lamella, cell walls and the cells of the abscission zone dissolve
(enzymatic degradation)
Detachment of leaf – at this stage, the petiole remains attached to the stem
by vascular elements only. But due to its own weight and the wind force, the
leaf is detached from the stem
7. The broken vascular elements are soon plugged with tyloses and
gums in xylem and callose in the phloem to stop translocation of water
and food
In some woody plants, another protective layer called periderm is
developed by cork cambium below the first protective layer.
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9. Physiological changes associated with abscission :
Degradation of chlorophyll – During abscission, the colour of the leaves,
flowers and fruits changes due to degradation of chlorophyll and the
synthesis of anthocyanin pigment takes place
Auxin decreases – abscission starts when the amount of auxin begins to
decrease
Ethylene production increases – due to which formation of cell wall
degrading enzymes in the abscission zone takes place
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14. Formation of abscission layer – Leaf abscission takes place at the
base of the petiole. The site of abscission is internally marked by a
distinct zone called abscission zone. The zone is made up of one or
more layers of cells arranged transversely across the petiole base.
This is called abscission layer.
Dissolution of Middle Lamellae – The cells of the abscission layer
separate from each other due to the dissolution of middle lamellae
and the primary cellulose walls under the influence of the activity of
enzymes, pectinase and cellulase.
Physiological changes associated with
abscission (Contd.)
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18. 3. Simultaneous/Synchronous senescence
In perennial, polycarpic, deciduous trees all the leaves senesce and die and shed at
the same time, called the leaf fall, that occurs in a particular season usually in
autumn, e.g., Poplar, Maple, Mulberry, Dalbergia, etc.
