Senescence and Abscission: Do you know that plants have a fixed life span like all other living things, and they also grow old and die after completing the stipulated time? Plants also mature, which is characterised by cell organelle, physio-chemical, and cellular changes that occur during their lives and eventually contribute to the plant’s death. Ageing is the sum of changes in the total plant or its organs.

Senescence is the natural process of ageing in plants. Abscission, on the other hand, is the removal of a senescent plant part or organ from the plant body. People frequently mix up senescence and abscission, yet the two are separate processes. Let’s read on to explore more about how a plant goes through senescence and abscission.

Senescence Definition

The biological ageing of a living thing is referred to as senescence. It refers to a cell’s or an organism’s morphological characteristics and function deteriorating over time which ultimately leads to complete loss of organisation and functioning of the plant or its parts. The main feature of senescence is that metabolic processes become catabolic and finally irreversible, leading to death.

Senescence Types

Senescence can affect the entire plant or be restricted to specific plant organs such as leaves and flowers, fruits, cells, or cell organelles. According to Leopold (1961), depending on the plant part undergoing senescence there are four types of senescence patterns in plants:

1. Overall senescence: This occurs in monocarpic plants that only produce flowers and fruits once in their lives, such as rice, wheat, mustard, cabbage, bamboos, etc. Senescence and death of the entire plant occur in this type; the plant dies after producing flowers and fruits.

2. Shoot or top senescence: This condition affects perennial plants with underground structures such as bulbs, tubers, and rhizomes. The above-ground portion of the shoot dies each year after flowering, while the underground portion of the shoot persists and produces new shoots the next season, e.g., Banana, Ginger, Gladiolus, etc.

3. Deciduous senescence: The temperate deciduous trees lose all of their leaves in the autumn and grow new ones in the spring. Due to the deciduous senescence of leaves, they may survive and resist unfavorable seasons. Deciduous senescence is often known as simultaneous or synchronous senescence. E.g., Dalbergia, Elm, Mulberry, Poplar.

4. Progressive senescence: In many perennial plants, the tips of the main shoot and branches remain meristematic and continue to produce new buds and leaves. Senescence and death occur in older leaves and lateral organs such as branches. Evergreen trees, such as Eucalyptus, Pinus, and Mango, show sequential senescence. Sequential senescence is another name for progressive senescence. E.g., the leaf falls in a coconut tree.

Fig: Different types of Senescence in Plants

Causes of Senescence

The different causes of senescence are:

1. Early loss of chlorophyll, RNA, and enzymes is associated with leaf senescence.
2. Hormones like ethylene and abscisic acid play a role in senescence.
3. The number of cellular constituents is reduced because of slower synthesis or faster breakdown.
4. Soybean fruits release a senescence factor or a hormone that travels to the leaves and causes senescence.
5. Nutrient competition between vegetative and reproductive organs.
6. Flowering and leaf senescence is induced by short days and long nights.

Physiological Changes Associated with Senescence

During senescence, the following physiological changes occur. Below we have listed some of the Physiological Changes associated with Senescence:

1. Leaf cells undergo significant physiological and biochemical changes during senescence, including a major metabolic shift from anabolism to catabolism, resulting in nutrition redistribution to newly emerging organs.
2. There is an overall decrease in metabolic activity, ATP generation, and chloroplast potency.
3. The amount of RNA and DNA in the plant body is decreasing.
4. With increasing age, chromosomal aberrations and gene mutations may accumulate, and protein synthesis may become defective as a result of these alterations.
5. Production of hydrolytic enzymes such as proteases and nucleases has increased.
6. Cell organelles and membranes are degrading.
7. The levels of deteriorating hormones including ethylene and abscisic acid (ABA) are rising.
8. The degradation of chlorophyll takes place.

Role of Hormones in Senescence

Hormones like abscisic acid and ethylene promote senescence. The ability of abscisic acid to accelerate ageing is widely recognised. The role of ABA in flower tissue senescence, including the initiation of colour fading or blueing, has been established. As senescence progresses, the ABA content of aged leaves rises dramatically. Ethylene is essential for the senescence of various plant parts, especially fruit and petals, as well as the abscission process. It is a senescence inducer for flower tissue.

Ethylene promotes leaf, flower, and fruit abscission, while IAA prevents senescence and abscission. When the days get shorter in the autumn, IAA production drops and ethylene production rises, accelerating changes in the abscission zone cells.

Senescence retardants are substances that slow down the ageing process. Auxin, gibberellin, and cytokinin are the main plant hormones that delay senescence.

Importance of Senescence

The importance of Senescence are as follows:

1. In monocarpic plants, entire plant senescence occurs at the same time as seed set and seed dispersal.
2. It maintains functionality by replacing old and ineffective organs with young and efficient parts such as leaves, buds, flowers, and fruits, among other things.
3. Shoot senescence is a mechanism that allows plants to perennate during periods of low light.
4. When the cells break down during senescence, several nutrients are released, including amino acids, amides, nucleotides, simple sugars, and minerals.
5. Leaf fall occurs simultaneously or synchronously in the autumn, just before winter. It decreases transpiration, which is necessary for life in the winter when the temperature on earth is very low and the roots are unable to absorb water.

Abscission Definition

Abscission is the process of leaves, twigs, and flowers detaching from the parent plant. It is a complex physiological controlled process that involves the removal of plant parts after they’ve completed their development or function, or in response to adverse conditions. The colour of the leaves, flowers, and fruits changes during abscission when chlorophyll is degraded, and anthocyanin pigment is synthesised.

Fig: Abscission

Abscission Mechanism

Abscission occurs at the point where the petiole or peduncle connects to the stem. Internally, an abscission zone separates the abscission site from the rest of the body. One or more layers of cells are placed transversely across the petiole base to form this zone. This is known as the abscission layer. The colour of the abscission zone is pale or brown. Under the action of enzymes, pectinase, and cellulase, the cells of the abscission layer separate from one another due to the disintegration of middle lamellae and main cellulose walls.

The petiole is only connected to the stem by vascular components at this point. The leaf, however, is separated from the stem by its weight and the force of the wind. Tyloses or gums are quickly inserted into the fractured vascular components. The formation of a corky layer protects the plant from pathogen invasion and excessive water loss during wound healing in cells close to the breaking point. During the healing process, suberin and lignin are produced.

Drought and nitrogen deficit are two environmental variables that encourage abscission. Auxin is produced in growing leaf blades and has a powerful anti-senescence and anti-abscission effect. When the amount of auxin in the body starts to decline, abscission begins. Senescence and abscission are also delayed by cytokinins, and gibberellins released from the roots. Because of ethylene synthesis, abscission is produced by the creation of cell walls destroying enzymes in the abscission zone.

Fig: Leaf Abscission

Hormone Influencing Abscission

Abscission is influenced by all naturally occurring hormones. Abscission is slowed by auxins and cytokinins but accelerated by abscisic acid and ethylene. Auxins appear to have a key function in the abscission process.

Abscission Significance

The significance of Abscission is given below:

1. It helps in the water and nutrient transfer to the immature leaves.
2. The shedding of vegetative portions such as gemmae or plantules helps in vegetative proliferation in lower plants.
3. It is a self-pruning process in which fruits and injured organs are dropped from the parent plant.
4. It aids in the dispersal of fruits and vegetative propagation.
5. Abscission is a process that removes waste elements from plant parts.

Differences Between Senescence and Abscission

The differences between Senescence and Abscission are as follows:

Properties	Senescence	Abscission
Definition	It’s a process that leads to a plant’s death overtime.	A plant does not die because of it. It just removes the old and undesirable plant parts.
Relationship to ageing	Senescence is a term that is commonly used to describe the ageing process in plants.	Abscission is irrelevant to the ageing process.
Nutrients in plants	Senescence helps in the movement of nutrients to other parts of the plant body.	Abscission reduces the loss of water and nutrients.
Effects on Microorganisms	Plants cannot be protected from infection by senescence.	Abscission has a significant impact while safeguarding the plants against fungal infections.
Death and protection	Senescence can cause a plant’s death, either partially or completely.	Abscission protects plants from near-death situations.

Summary

Senescence is the natural process of ageing in plants. Abscission, on the other hand, is the removal of a senescent plant part or organ from the plant body. Senescence can affect the entire plant or be restricted to specific plant organs such as leaves and flowers, fruits, cells, or cell organelles. According to Leopold (1961), depending on the plant part undergoing senescence, there are four types of senescence.

The different types of senescence are overall senescence, shoot or top senescence, deciduous senescence, and progressive senescence. The senescence process helps in the mobilisation of nutrients and plant vegetative parts into fruits. Abscission occurs at the point where the petiole or peduncle connects to the stem. Abscission is a self-pruning process in which fruits and injured organs are dropped from the parent plant.

FAQs on Senescence and Abscission

Q.1. What is senescence in plants?
Ans: The gradual deterioration of cell function is referred to as senescence. Senescence is a phase of the ageing process that occurs after a certain amount of time has passed.

Q.2. Which hormone promotes abscission and senescence of leaves?
Ans: Hormones like abscisic acid and ethylene promote senescence and hormones like abscisic acid and ethylene accelerate abscission.

Q.3. What are the advantages of senescence?
Ans: The advantages of senescence are:
1. In monocarpic plants, entire plant senescence occurs at the same time as seed set and seed dispersal.
2. Older leaves tend to fall due to the formation of the abscission layer, allowing nutrients to be diverted to the next new leaf.

Q.4. What is an abscission zone?
Ans: The region at the base of a leaf, flower, fruit, or other plant parts that contain specialised cells that produce enzymes that cause that portion to separate from the rest of the plant.

Q.5. Write two significances of abscission.
Ans: The two significances of abscission are:
1. It is a self-pruning process in which fruits and injured organs are dropped from the parent plant.
2. It is a process that removes waste elements from plant parts.

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