The plants or their organs like all other living organisms have a certain span of life during which they develop, grow, attain maturity and after some time at the end die. But prior to death, distinctive but natural deteriorative processes occur in them in growth and synthetic activities.
These deteriorative processes that naturally terminate their functional life are collectively called as senescence and the plants or plant organs at this stage are called as senescent. These deteriorative processes may terminate in death either gradually or abruptly depending upon the plant. Senescence is a normal energy dependent developmental process which is controlled by plants own genetic programme and the death of the plant or plant part consequent to senescence is called as programmed cell death (PCD).
Senescence is not confined only to whole plant. It may be limited to a particular plant organ such as leaf and flowers or cells such as phloem and xylem or cell-organelles such as chloroplasts and mitochondria etc. Senescence is closely associated with the phenomenon of aging and both are sometimes considered as the same by many workers. But according to Medawar (1957), the term senescence should be used to refer to natural changes towards termination of life while ‘aging’ to refer to changes in time without reference to the natural development of death.
Leopold (1961) has recognised 4 types of senescence patterns in whole plant (Fig17.41) which are as follows:
1. Overall Senescence:
This type of senescence occurs in annuals where whole of the plant is affected and dies.
2. Top Senescence:
This is represented by perennial herbs where senescence occurs only in the above ground parts, the root system and underground system remaining viable.
3. Deciduous Senescence:
This type of senescence is less drastic and takes place in woody deciduous plants. Here senescence occurs in all the leaves simultaneously but the bulk of the stem and root system remains alive.
4. Progressive Senescence:
This is characterized by gradual progression of senescence and death of leaves from the base upwards as the plant grows. (The senescence of the entire plant after a single reproductive cycle is also known as monocarpic senescence) Senescence can best be studied in leaves or similar other organs of plants e.g., cotyledons, sepals, petals etc. or cell organelles like isolated chloroplasts.
i. Senescing cells and tissues are metabolically very active and an ordered series of cytological and biochemical events occur during senescence.
ii. Senescence is characterised by increased respiration, declining photosynthesis and an orderly disintegration of macromolecules.
iii. At the cellular level, chloroplasts are the first organelles to be disintegrated. Nuclei remain structurally and functionally intact until the last stage of senescence. Meanwhile, other cell organelles and bio-membranes also gradually deteriorate.
iv. Expression of senescence down-regulated genes (SDGs) decreases. Such genes encode proteins in photosynthesis and other biosynthetic processes. Concentration of growth promoting hormones especially cytokinins decline.
v. Expression of senescence associated genes (SAGs) increases. Such genes encode hydrolytic enzymes such as proteases, ribonucleases and lipases as well as enzymes involved in biosynthesis of deteriorative hormones such as abscisic acid (ABA) and ethylene.
vi. Some of the SAGs have secondary functions in senescence that are useful to plant. These genes encode enzymes that are involved in conversion and remobilization of nutrients and substrates from senescing tissues and their reallocation to other parts of the plant that survive (i.e., not senescing).
vii. Brilliant Colours are developed in leaves of many plants during senescence.
viii. This is due to degradation of chlorophylls, resulting in unmasking of more stable carotenoid pigments.
ix. Towards the end of senescence, the cells and tissues also lose respiratory control.
Several environmental factors especially those which suppress normal plant growth also tend to enhance the rate of senescence. These are deficiency of soil nutrients, high temperatures, water deficit, darkness etc. In many plants on the other hand, removal of flowers, fruits and vegetative growing points can markedly delay the senescence of leaves. The role of externally supplied cytokinins in delaying senescence especially in detached plant parts is also well established.