Abscission, is a process plants let go of parts of themselves and fruit. Abscission is a self-defense mechanism and a reproduction one. This article will discuss the scope of abscission which the plant kingdom goes through.
What is Abscission?
In simple terms, abscission means the intentional loss of various organs or parts of a plant that is no longer required during the developmental stages of the plant. It could be flowers or certain parts of flowers after the pollination season gets over and fruition will begin. It can also be the loss of leaves that have aged (senescent leaves) and are now obsolete for the plant’s survival.
Therefore, abscission is an important physiological process, cardinal to the growth, development, and aging process of a plant.
Why do plants go through Abscission?
Abscission promotes the removal of old, or damaged parts of a plant such as leafs or branches. Abscission helps the plant to make space for the formation of new leafs and branches. For example, oak trees start shedding their old, damaged leaves in the autumn season to facilitate the formation of new leaf buds. The old leaves turn brown, get detached from the plant, and fall.
Abscission is also important for the reproductive success of plants and to ensure efficient seed and fruit dispersal. Intentional shedding of seeds and fruits after they reach a certain maturity facilitates further propagation by external agents like insects, animals, winds, water, and so on.
Plants may also use abscission to their benefit to conserve and efficiently use available resources. For example, a plant may have limited resources that are not sufficient to allow all the fruits it has born to get matured. In such a case, the plant may choose to abscise a few fruits even before they mature (also called ‘fruit drop’) to conserve resources and help the other fruits to reach maturity.
Another classical example when a plant uses abscission to conserve resources is to abscise damaged leaves to reduce water loss. The plant may choose to shed the damaged leaf which is contributing less to the plant’s survival through photosynthesis but using up plant resources and adding to water loss by transpiration.
Abscission also plays a very crucial role in plant defense against pests and insects. Premature and intentional leaf abscission is seen in plants suffering from gall aphids. By abscising the leaves suffering from aphid galls, the plant can diminish a significant portion of the pest population. This is because 98% of aphids in galls die after abscission. This is a classic example of selective abscission of leaves for defense against pests.
Process of Abscission
The process of abscission occurs in three steps – resorption, formation of the protective layer, and detachment.
- Resorption – in the resorption process, all the chlorophyll molecules in the organ are degraded to reabsorb most of the nutrients. This is an important step in recycling important elements like Nitrogen which is very important for plant survival and growth and is often a limiting factor. Hence, after all the elements have been extracted, they are transported to other parts of the plant. The loss of chlorophyll is the primary cause of the loss of color of leaves in the autumn season.
- Formation of protective layer – a distinct abscission zone forms (usually a cork layer) to differentiate healthy cells from those that will undergo abscission. Several layers of parenchyma cells are added to either side of the abscission zone which produces and injects suberin and lignin. These two chemicals play a major role in creating a robust waterproof sheath for the healthy cells on the other side of the zone after the detachment is done.
- Detachment – the detachment can occur by two primary processes – self-digestion of the middle lamella of the cell wall or a by water imbibition. The plant cells on the healthier side of the abscission zone can secrete self-digesting enzymes that digest the middle lamella and detach the cell from the other side of the cell wall. On the other hand, a large amount of water imbibition can occur at the abscission zone that swells the cell and ultimately bursts the cell leading to detachment.
What causes Abscission?
Abscission can be triggered by a number of different factors ranging from environmental, biological, or intrinsic factors of the plant. Lack of chlorophyll, high sunlight, UV radiation, pests, high salinity, cool temperatures beyond the optimal range can also cause the abscission of various parts of plants. These trigger metabolic dysfunction as the homeostasis is disrupted because of a lack of optimal conditions. However, hormones play a cardinal role in regulating abscission and the most important of them are auxin, ethylene, abscisic acid.
However, as discussed above, abscission can also be totally self-programmed by the plant for self-defense, resource conservation, reproductive success, or any other reason.
Abscission and pruning
Regular pruning in horticulture is important to fasten the process of abscission or rather facilitate the removal of dead, damaged, and pest-infected plant organs. Regular pruning is therefore helpful for the plant to grow new leafs, flowers, fruits and buds to replace the abscised ones. Pruning can be extremely beneficial during infections when the infection is proceeding faster than the plant can abscise the organ.