11th Class Biology Plant Growth and Development / पौधे की वृद्धि और विकास Senescence And Death

Plant and their parts develop continuously from germination until death. The production of flowers, fruits and seeds in annuals and biennials leads to senescence. The latter part of the developmental process, which leads from maturity to the ultimate complete loss of organization and function is termed senescence. Several workers equate ageing and senescence as same process. Ageing is a sum total of changes in the total plant or its constituents while senescence represents degenerative and irreversible changes in a plant. The study of plant senescence is called phytogerontology. 

Types of senescence : Plant senescence is of four types- whole plant senescence, shoot senescence, sequential senescence and simultaneous senescence. The last three are also called organ senescence.    

(1) Whole plant senescence : It is found in monocarpic plants which flower and fruit only once in their life cycle. The plants may be annual (e.g., rice, wheat, gram, mustard etc.), biennials (e.g., cabbage, henbane) or perennials (e.g., certain bamboos). The plant dies soon after ripening of seeds.

(2) Shoot senescence : This type of senescence is found in certain perennial plants which possess underground perennating structures like rhizomes, bulbs, corm etc. The above ground part of the shoot dies each year after flowering and fruiting, but the underground part (stem and root) survives and puts out new shoots again next year. e.g., banana, gladiolus, ginger etc.

(3) Sequential senescence : This is found in many perennial plants in which the tips of main shoot and branches remain in a meristematic state and continue to produce new buds and leaves. The older leaves and lateral organs like branches show senescence and die. Sequential senescence is apparent in evergreen plants e.g., Eucalyptus, Pinus, Mango.

(4) Simultaneous or synchronous senescence : It is found is temperate deciduous trees such as elm and maple. These plants shed all their leaves in autumn and develop new leaves in spring.  Because of this shedding of leaves, autumn season is also called fall. e.g., Dalbergia, Elm, Mulberry, Poplar.

Theories of senescence

(1) Wear and tear : According to this theory, senescence occurs due to loss of activity and cells undergo wear and tear due to disintegration of organelles.

(2) Toxicity : It is viewed that senescence takes place due to accumulation of toxic and deleterious substances in all.

(3) Loss of metabolites : It is assumed that senescence leads to gradual depletion of essential metabolites in a cell.

(4) Genetic damage

Characteristics of ageing and senescence

(1) There is general decline in metabolic activities, decline in ATP synthesis and also decreased potency of chloroplast.

(2) Decrease in RNA and DNA

(3) Decrease in semipermeability of cytoplasmic membranes.

(4) Decrease in the capacity to repair and replace wornout cells.

(5) There may be accumulation of chromosomal aberrations and gene mutations with advancing age as a result of these changes protein synthesis becomes defective.

(6) Increased production of hydrolytic enzymes such as proteases and nucleases.

(7) Deteriorative change in cell organelles and membranes.

(8) Decrease in the internal content of auxin and cytokinins and increase in the production of abscisic acid or ethylene.

Importance of senescence : Biologically senescence and death have following advantages :

(1) It maintains efficiency since the old and inefficient organs are replaced by young efficient part like leaves, buds, flowers and fruits. etc.

(2) During senescence, the cellular breakdown results in release of many nutrients including amino acids, amides, nucleotides, simple sugars and minerals. The same are withdrawn from the senescing organs into the main trunk and later utilised in the growth and developed of new parts.

(3) Shoot senescence is a mechanism to help the plants perennate during the unfavourable periods.

(4) Simultaneous or synchronous leaf fall occurs in autumn prior to winter. It reduces transpiration, which is essential for survival in winter, when the soil in frozen and roots can not absorb water.

(5) Litter of fallen leaves and twigs is an important source of humus and mineral replenishment for the soil.