Explain 'senescence'

Explanation

Old age is called senescence in plants. Senescence refers to all collective, progressive and deteriorative processes which ultimately lead to complete loss of organization and function.
4 types of senescence:
1. Overall senescence
2. Top senescence
3. Deciduous senescence
4. Progressive senescence
Overall senescence:
This kind of senescence occurs in annual plants when entire plants gets affected and dies.
Eg: Wheat and Soybean.
It also occurs in few perennials also.
Eg: Agave and Bamboo.
Top senescence : It occurs in aerial parts of plants.
It is common in perennials, underground and root system remains viable. Eg: Banana and Gladiouls.
Deciduous senescence: Common in deciduous plants, occurs only in leaves of plants, bulk of the stem and roots system remains alive. Eg: Elm and Maple.
Progressive senescence:
1. This kind of senescence is gradual.
2. First it occurs in old leaves followed by new leaves then stem and finally root system.
3. It is common in annuals.
Physiology of sensecence:
1. Cells undergo changes in structure.
2. Vacuole of the cell acts as lysosome and secretes hydrolytic enzymes.
3. The starch content is decreased in the cells.
4. Photosynthesis is reduced due to loss of chlorophyll accompanied by synthesis and accumulation of anthocyanin pigments, therefore the leaf becomes red.
5. There is a marked decrease in protein content in the senescing organ.
6. RNA content of the leaf particularly rRNA level is decreased in the cells due to increased activity of the enzyme RNAase.
7. DNA in senescencing leaves degenrate by the increased activity of enzyme DNAase.
Factors affecting senescence:
1. ABA and ethylene accelerate senescence while auxin and cytokinin retard senescence.
2. Nitrogen deficiency increases senescence whereas nitrogen supply retrads senescence.
3. High temperature accelerates senesence but low temperature retards senescence.
4. Senescence is rapid in dark than in light.
5. Water stress leads to accumulation of ABA leading to secescence.
Programmed cell death:
1. Senescence is controlled by plants own gentic programme.
2. Death of the plant or plant part consequent to senescence is called programmed cell death.
3. In short senescence of an individual cell is calledPCD.