senescence is gradual. First it occurs in old leaves followed by new leaves then stem and finally root system. It is common in annuals (Figure . ).
. 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.
. 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 accelerates senescence but low temperature retards senescence.
• Senescence is rapid in dark than in light. • Water stress leads to accumulation of ABA leading to senescence. . Programmed cell death (PCD) Senescence is controlled by plants own genetic programme and death of the plant or plant part consequent to senescence is called Programmed Cell Death .
In short senescence of an individual cell is called PCD . The proteolytic enzymes involving PCD in plants are phytaspases and in animals are caspases . The nutrients and other substrates from senescing cells and tissues are remobilized and reallocated to other parts of the plant that survives. The protoplasts of developing xylem vessels and tracheids die and disappear at maturity to make them functionally efficient called abscission zone or abscission layer .
An abscission layer is greenish-grey in colour and is formed by rows of cells of to cells thick. The cells of abscission layer separate due to dissolution of middle lamella and primary wall of cells by the activity of enzymes pectinase and cellulase resulting in loosening of cells. Tyloses are also formed blocking the conducting