Senescence and Abscission

Senescence

The natural process of aging in plants is senescence. It involves all the changes occurring in plants causing the death of tissues, cells and the entire plant body. This process takes place in various ways which affects the entire plant or a part of a plant.

Senescence takes place in flowers, leaves, stems, fruits and roots. However, commonly, it takes place at different intervals in different organs. In several perennial herbs such as alfalfa, the complete part over the ground system dies out every year, however, the crown and the root system stays viable largely. Leaves die in deciduous woody perennials, while most of the stem and the root tissues stay alive.

Sensescene is responsible for the deterioration which terminates the functional life of an entity or a structure. Even though meristems do not experience senescence and can even potentially be considered as immortal, all cells generated from them experience aging gradually leading to death.

Types of Senescence

Depending on its role in the aging of plants, senescence can be grouped into these categories-

• Shoot senescence – takes place in some perennial plants with underground structures such as rhizomes, tubers, bulbs. The shoot stays above the ground and dies each year after the process of flowering; however, the underground structure survives, producing new shoots in the upcoming season. For instance – ginger, banana, gladiolus, etc
• Whole plant senescence – takes place in monocarpic plants that produce fruits and flowers only once in its lifetime. For instance, wheat, cabbage, mustard, bamboos, etc. They have an impact on the complete plant. The plant tends to die after it produces fruits and flowers
• Sequential senescence – seen in most of the perennial plants, the tips of the main shoot and engirdling branches grow and produce new leaves and bulbs. However, older leaves and the lateral parts such as old branches experience senescence and die out. For instance, Eucalyptus, Pinus etc.
• Synchronous or simultaneous senescence – takes place in the deciduous trees such as the maple and elm. Here, the trees shed their leaves in autumn and give rise to new leaves in spring.

What causes Senescence?

• Process of leaf senescence comes along with early loss of chlorophyll, enzymes and RNA
• As a result of a faster break down or slower synthesis, there is a decrease in the cellular constituents
• Competition between reproductive and vegetative structures for nutrients
• Long-night and short-day conditions induce leaf senescence and flowering
• Senescence factor is secreted in fruits of soybean which passes to leaves causing senescence
• The process of senescence is also governed hormonally
• Loss of integrity in food storage cells of seeds and degradation of food reserves

Physiological Changes Occurring During Senescence

• Chlorophyll degradation
• Decrease in starch content
• Decrease of proteins and RNA
• Process of photosynthesis stops
• Enzyme DNase degrades DNA molecules
• Decrease in growth promoting hormone such as cytokinin
• Functioning of vacuoles as lysosomes and digestion of cellular matter
• Reddening of leaves as a result of accumulation of Anthocyanin pigments in leaves
• Increase in the content of deteriorative hormones such as abscisic acid and ethylene

Abscission

The process of Abscission is the normal separation of the senescent structure or plant. In simpler words, it is the shedding of flowers, leaves and fruits. For example – ripened fruits or old leaves. It takes place to separate or shed the unwanted plant structures or parts. The shedding of old leaves at the base of the petiole typically takes place at the time of autumn. For normal life of a plant, abscission is important. It seals off the vascular system preventing nutrient or water loss. It also safeguards plants from fungal or bacterial infections.

This process is distinct in shrubs and deciduous trees. Leaves of deciduous plants in autumn fall simultaneously imparting a naked appearance to plants. There is an eventual abscission of leaves in evergreen plants. Older leaves fall, new leaves continuously develop all through the year. But, in the majority of the herbaceous species, leaves are not shed even after it dies. Leaves are retained in several cases in withered dry conditions even after the whole shoot dies.

The process of abscission is a complicated physiological process. In the process of abscission, the colour of flowers, leaves and fruits alters as a result of the degradation of chlorophyll and the synthesis of the anthocyanin pigment.

It is at the base of the petiole that leaf abscission occurs. The site of abscission is marked internally by different zones referred to as abscission zones. Such a zone is composed of one or many layers of cells that are transversely organized across the base of the petiole. This is referred to as the abscission layer.

The abscission zone is brown or pale in colour. The cells of this abscission layer separate from one another as a result of dissolution of the middle lamellae and the primary cellulose walls in the influence of action of enzymes – cellulase and pectinase. The petiole stays attached to the stem at this phase only by the vascular elements.

Abscission is as result of the formation of cell wall degrading enzymes in the abscission zone as a result of production of ethylene.

Importance of Abscission

• The process helps to divert nutrients and water towards the younger leaves
• Aids in disseminating vegetative and fruits propagates
• As it is a self pruning process, damaged structures and fruits shed from the parent plant
• It is functional in removal of plant structures which comprise waste substances

Also see: Important Notes on Plant Growth and Development

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Differences Between Senescence and Abscission

Following are differences between senescence and abscission –

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