1. Types of Senescence: Leopold (1961) has recognised four types of senescence.
(a) Overall senescence: This kind of senescence occurs in annual plants when entire plant gets affected and dies,
eg: Wheat and Soybean. It also occurs in few perennials also,
eg: Agave and Bamboo.
(b) Top senescence: It occurs in aerial parts of plants. It is common in perennials, underground and root system remains viable,
eg: Banana and Gladiolus.
(c) Deciduous senescence: It is common in deciduous plants and occurs only in leaves of plants, bulk of the stem and root system remains alive,
eg: Elm and Maple.
(d) Progressive senescence: This kind of senescence is gradual. First it occurs in old leaves followed by new leaves f then stem and finally root system. It is common in annuals.
2. Physiology of Senescence:
- Cells undergo changes in structure.
- Vacuole of the cell acts as lysosome and secretes hydrolytic enzymes.
- The starch content is decreased in the cells.
- Photosynthesis is reduced due to loss of chlorophyll accompanied by synthesis and accumulation of anthocyanin pigments, therefore the leaf becomes red.
- There is a marked decrease in protein content in the senescing organ.
- RNA content of the leaf particularly rRNA level is decreased in the cells due to increased activity of the enzyme RNAase.
- DNA molecules in senescencing leaves degenerate by the increased activity of enzyme DNAase.
3. Factors affecting Senescence:
- ABA and ethylene accelerate senescence while auxin and cytokinin retard senescence.
- Nitrogen deficiency increases senescence whereas nitrogen supply retards, senescence. High temperature senescence but low retards senescence.
- Senescence is rapid in dark than in light.
- Water stress leads to accumulation of ABA leading to senescence.
