Senescence can be defined as the collective, progressive and degenerative processes that ultimately lead to termination of functional activities of an organ or organism. It may also be defined as a period between reproductive maturity and death of plants or plant parts.
Annual and biennial plants after the production of flowers, fruits and seeds die. Many trees and shrubs shed their leaves at the end of growing seasons. Plants growing in temperate zone shed their leaves before the arrival of winter.
In other parts of the world, plants shed their leaves during the harsh summer and grow during the wet period which is most favourable to the plant. In trees, before leaves die and shed their protein are hydrolysed to yield amino acids which are then exposed to storm. This is an important form of resource conservation.
Types of Senescence:
Senescence in plants is manifested in various forms which can be broadly grouped into the following categories:
Whole plant senescence:
This type of senescence is characteristic of monocarpic plant species which flower and fruit only once in their life cycle and as such, senescence of whole plant occurs in them. The process of senescence begins with the reproductive maturity and the whole plant dies after seed production.
A signal is sent out from the developing seed, progressively causing senescence of leaves, stems and the whole plant ultimately forcing the plant to die. Examples of this type include annual and biennial and some monocarpic perennials such as Bambusa, Agave etc.
In this type , the shoots die after flowering while the underground stems and roots survive and produce new shoots. Shoot senescence is noticed in many perennials such as Gladiolus, Zingiber, Curcuma, Musa, Chrysanthemum, Dahlia etc.
In this type , the lateral organs such as leaves and the fruits die much prior to the death of the whole plant. It may be of the following types.
Simultaneous or synchronous: This type of senescence is noticed in temperate deciduous trees like maple, shed all their leaves in autumn. This is also called seasonal senescence.
Sequential : It is a progressive senescence of lower older leaves while the new ones are added to the growing shoots in a sequence. Because the leaves have limited life span, here the senescence takes place in a sequential manner depending on the age of the leaf.
Physiology of senescence:
A number of structures and physiological changes occur in the cells of senescing organs. It leads to disruption of internal organization of the cells and organelles and enzymatic breakdown of starch, proteins, fats nucleic acid etc., into their simpler constituents.
The young leaves have higher rate of respiration, more synthesis of RNA and protein, and relatively high content of growth hormones. The young leaves transport solutes to other organs. With the approach of autumn and the onset of senescence, starch, protein, chlorophyll and RNA contents in the leaves decline.
It is possible due to the synthesis of hydrolytic enzymes in the ageing leaves which break down these products. These products are transported to the young growing parts where they are used in synthetic processes. The onset of these breakdown processes is stimulated by elevated temperatures, short days, nutrient deficiency and drought periods.
Due to the loss of chlorophyll, the leaves turn yellow. This is accompanied by synthesis and accumulation of anthocyanin pigment imparting red colouration to the leaves.
Molisch (1928) suggested that the senescence is due to nutrient depleting action of sexual reproduction. According to Molisch, development of flowers and fruits accelerate translocation of food from leaves to the reproductive parts due to which leaves rather get starved for nutrients. Senescence of fruits may occur when the nutrient supply of the plants become exhausted. It may be one of the reasons of the death of annual plants at the end of the reproductive phase.
Senescence involves a series of cytological and biochemical events. Some cell organelles are destroyed while a few others lose their efficiency. It shows the presence of several hydrolytic enzymes such as proteases, nucleases, lipases, and chlorophyll degrading enzymes. The synthesis of these enzymes is controlled by certain specific genes. Such senescence inducing genes are called senescence-associated genes and these genes code for specific hydrolytic enzymes.
Significance of senescence:
- Senescence in plants makes way for the young, functionally efficient organs and allows the plant to get rid of the old.
- It causes breakdown of the complex metabolites and cellular components into simpler ones such as sugars, amino acids, nucleotide, fatty acid etc., which are translocated from senescing leaves to young developing leaves, fruits, seeds etc.
- Synchronous or seasonal senescence of leaves of deciduous trees help plants to escape influence of seasonal adversities such as the excessive loss of water due to transpiration. This helps plants to survive under adverse conditions of water stress, temperature extremes etc.
- Sequential senescence of the basal leaves saves nutrients from being wasted since the older leaves are functionally inefficient and live at the cost of metabolites synthesized by new ones.
- Falling of leaves due to senescence adds to the humus content of the surface layer of soil and thereby makes the soil rich for germination and growth of new seedling.
- It ensures recovery and re-utilization of mineral elements and organic nutrients from older senescing organ to newly formed growing organs.