Senescence
Plant
life comprises some sequential events, viz:
germination, juvenile stage, maturation, old
age and death. Old age is called senescence
in plants. Senescence refers to all
collective, progressive and deteriorative processes which ultimately lead to
complete loss of organization and function. Unlike animals, plants continuously
form new organs and older organs undergo a highly regulated senescence program
to maximize nutrient export.
Leopold (1961) has recognised four types of senescence:
i.
Overall senescence
ii.
Top senescence
iii. Deciduous
senescence
iv. Progressive
senescence
The branch of botany which deals with ageing, abscission and
senescence is called Phytogerontology
a.
Overall
senescence: This kind of senescence
occurs in annual plants when entire plant gets affected and dies. Example:
Wheat and Soybean. It also occurs in few perennials also. Example: Agave and Bamboo.
b.
Top
senescence: It occurs in aerial parts of
plants. It is common in perennials, underground and root system remains viable.
Example: Banana and Gladiolus.
c.
Deciduous
senescence: It is common in deciduous
plants and occurs only in leaves of plants, bulk of the stem and root system
remains alive. Example: 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 root system. It is common in annuals (Figure 15.28).
·
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.
·
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.
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 to conduct water for transport. In aquatic plants,
aerenchyma is normally formed in different parts of the plant such as roots and
stems which encloses large air spaces that are created through PCD. In the
development of unisexual flowers, male and female flowers are present in
earlier stages, but only one of these two completes its development while other
aborts through PCD (Figure 15.29).
Abscission
is a physiological process of shedding of organs like leaves, flowers, fruits
and seeds from the parent plant body. When these parts are removed the plant
seals off its vascular system to prevent loss of water and nutrients. Final
stage of senescence is abscission. In temperate regions all the leaves of
deciduous plants fall in autumn and give rise to naked appearance, then the new
leaves are developed in the subsequent spring season. But in evergreen plants
there is gradual abscission of leaves, the older leaves fall while new leaves
are developed continuously throughout the year.
Leaf
abscission takes place at the base of petiole which is marked internally by a
distinct zone of few layers of thin walled cells arranged transversely. This
zone is called abscission zone or
abscission layer. An abscission layer is greenish-grey in colour and is
formed by rows of cells of 2 to 15 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 vessels. Degrading of
chlorophyll occur leading to the change in the colour of leaves, leaf
detachment from the plant and leaf fall. After abscission, outer layer of cells
becomes suberized by the development of periderm (Figure 15.30).
All
naturally occurring hormones influence the process of abscission. Auxins and
cytokinins retard abscission, while abscisic acid (ABA) and ethylene induce it.
1.
Abscission separates dead parts of the plant, like
old leaves and ripe fruits.
2.
It helps in dispersal of fruits and continuing the
life cycle of the plant.
3.
Abscission of leaves in deciduous plants helps in
water conservation during summer.
4.
In lower plants, shedding of vegetative parts like
gemmae or plantlets help in vegetative reproduction.
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