Let us make an in-depth study of the types and regulation of senescence process in plants.
Gradual Senescence:
In some plants, some portions of the plant remain in juvenile phase, while others may start senescing. The study of the process of Senescence in plants has become an important branch of botany, now known as Phytogerontology.
Now we will discuss some important features of these four phases of plant life:
1. Juvenility:
This phase is characterized by high degree of vegetative growth and anabolic activities. Different parts of the plant do not show any natural abscission. In some plants this phase extends for a very long or indefinite period.
Some plants show heterophylly or heteromorphic development, i.e., shape, size and arrangement of different vegetative parts vary in juvenile and mature plants. For example, in beans (Phaseolus vulgaris) the juvenile leaves are simple and opposite, while adult leaves are alternate and trifoliate.
Not only external morphology, sometimes the anatomical features also vary in juvenile and mature plants. For instance, adult or mature plants often show secondary growth. The young fern sporophytes have protostele and siphonostele, while adult plants show dictyosteles. Such heteromorphic development can be noticed in many other plants such as Eucalyptus, Acacia etc.
2. Maturity:
This phase as emerges from the juvenile stage shows gradual decline in vegetative growth. The vegetative growth is taken over by reproductive growth exhibited by the development of floral buds or other reproductive organs. The process of ageing or senescence is delayed in those plants where the completion of reproductive phase is prolonged or delayed. An interesting example of Agave Americana (Century plant) may be quoted here.
This plant has a life span of eight years under certain climatic conditions encouraging early completion of reproductive phase, but the same plant may survive for more than 100 years under such climate where induction of reproductive phase is delayed indefinitely. Whenever the plant flowers and produce fruits, it takes no more time to die.
3. Senescence (Ageing):
As written earlier also, senescence refers to those changes which lead sooner of later to the death of an organism or some part of it. What are those changes which are responsible for the process of ageing?
Some of these changes recognized so far are given briefly here:
(a) It is marked by general decline in metabolic activities, accompanied by a decline in ATP synthesis and chloroplast activities.
(b) Decrease in DNA and RNA
(c) Semi permeability of cytoplasmic membranes in the cells starts declining.
(d) Impairment of the process of formation of new cells to replace the old and dead cells.
(e) Due to anomalies in protein synthesis caused by senescence chromosomal aberrations and gene mutations may occur in the body cells which may be fatal.
Types of Senescence:
Four types of senescence have been recognized (Fig. 6.1):
(a) Overall Senescence (Monocarpism):
Here the whole body of the plant dies suddenly soon after it completes its life-cycle, e.g. annual plants.
(b) Top Senescence:
This is a partial senescence where the aerial parts of the plant die (senesce) but the underground parts remain unaffected.
(c) Deciduous Senescence:
In deciduous plants, leaves senesce and fall every year. During next growth season, new leaves emerge.
(d) Progressive Senescence (or Sequential Senescence):
Here different parts of the plant body senesce throughout the year, such as seen in the plants of Coleus.
Regulation of Senescence Process in Plants:
Senesce processes are tightly controlled processes in which the sequence of events is usually highly ordered until the terminal stages (death) are underway. Various factors which control the processes of senescence in different organs during life cycle of a flowering plant are given below.
It is evident from these details that at all stages a wide range of growth substances appears to be involved:
(a) Senescence of root: which also includes senescence of vascular tissues, root cap and hairs, is regulated by auxins and cytokinins.
(b) Senescence of cotyledons: is regulated by axis, shoot, light fruit, root and all hormones.
(c) Senescence of leaf: which also includes senescence of abscission zone, is regulated by fruit, root, light, all hormones and ethylene.
(d) Senescence of stem: which also includes senescence of vascular tissues, is regulated by auxins, cytokinins and sugars.
(e) Senescence of fruit: which also includes senescence of seeds is controlled by all hormones and ethylene.
(f) Senescence of flower: is regulated by pollination, ABA (Abscisic acid), cytokinins and ethylene.
(g) Senescence of apex: is regulated by fruits, day-length and gibberellins.
(h) Senescence of whole plant (monocarpism): is regulated by fruits, roots, day-length, auxins, ABA (Abscisic acid) and cytokinins.
4. Death:
This is terminal phase of any plant’s life. It results from senescence or ageing, and involves disintegration and death of the cells of the body. This all is brought about by large scale and long-lasting disorders in the metabolism. There could be several causes for such massive metabolic disorders. Plant dies giving way to its next generation to come and flourish.