List the types of senescence.

Explanation

(i) Type of senescence: Leoplod (1961) has recognised four types of sensecence.
(a) Overall senescence: This kind of senescence occurs in annual plants when entire plat 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 Gladiouls.
(c ) Deciduous senescence: It is common in deciduous plants and occurs only in leaves of plants, bulk of the stem and root system ramins alive. eg: Elm and Maple.
(d) Progressive senescence: This kind of senescence is gradual. First it occurs in old leaves followed by new leaves then stem and finally rooty system. It is common in annuals.
(ii) Physiology of Senescence
* Cells undergo changes in strucutre.
* 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 fhlorophyll 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 RNA ase.
* DNA molecules in senescencing leaves degenerate by the increased activeity of enzyme DNA ase.
(iii) Factors affecting Senescence:
* ABA and enthylene accelerate senescence while auxin and cytokinin retard senescence.
* Nitrogen dificiency increases senescence whereas nitrognen supply retards senescence.
*High temperatuer accelerates senescence but low temperature retards senescence.
* Water stress leads to accumulation of ABA leading to senescence.