Physiology of senescence and abscission B.Sc.Part III SEm 5th Botany

1. Mr.D.L.Gavande
Assistant Professor
Department of Botany
Shri Shivaji Science and arts College,
Chikhli.
Physiology of Senescence and
Abscission

2. Senescence
 Senescence has been defined as the deterioration that
ends the functional life of an organism or an organ.
 Although meristems do not undergo senescence and
can perhaps be considered potentially immortal, all of
the cells produced from these undergo aging, which
eventually leads to death.
 Senescence occurs in leaves, flowers, fruits, stems,
and roots, but it commonly occurs at different times
in these organs.
 In many perennial herbs, such as alfalfa, essentially
the entire above ground system dies each year, but
the crown and root systems remain largely viable.

3.  In deciduous woody perennials, the leaves die,
but much of the root and stem tissues remain
alive.
 Another pattern of senescence is seen in several
herbaceous annual species such as beans,
tomatoes and the cereal grains.
 Here there is a progressive senescence of the
leaves from the older to the younger, followed by
death of both the stem and roots after flowering.
 Considered in this way, senescence may be
thought of as a final stage of differentiation
which itself is continuous from juvenility to
death.

4. Abscission
 Soon after senescence occurs, some plant organs are lost
by abscission.
 The physiology of this process is best understood in the
case of leaves, flowers and fruits.
 The longevity of flowers ranges from three or four hours
in Hibiscus trionum and Calandrinia compressa, upto
three or four months in Plialaenopsis scilleriana.
 Leaves are generally longer lived.
 Those of most herbaceous annuals are never shed and live
as long as the plant, those of deciduous woody
dicotyledons remain intact one growing season, and the
needles of certain gymnosperms are retained from about
one year to seven or eight years, depending upon the
species and growth conditions.
 Most fruits are lost during the season in which they are
formed.

5.  Abscission is delayed as long as organ is physiologically
active and occur if the normal metabolic processes are
completed or are interrupted by injury or disease.
 Leaf abscission is usually stimulated by the younger
leaves above, and one of the causative factors supplied
by these is an auxin.
 Abscisic acid stimulates abscission of leaves in a number
of plants.
 It seems that acid and the other abscisins play important
roles in the loss of leaves, of fruit and of flowers.
 Abscission benefits plants in various ways.
 Injury or disease often causes the abscission of the
affected organ.
 The plant thus rids itself of a useless part or of a
potential infection source.

6. Abscission of Leaves
 The leaves are periodically detached
from the perennial plants.
 During this phenomenon leaves are
separated from the stem without
causing any injury to the living tissues
in stem and the newly exposed surface
is also protected from desiccation and
infection.
 This phenomenon of the separation of
leaves from the stem takes place in a
particular region of the plant, known
as abscission region or abscission
zone.
 The phenomenon as a whole known as
abscission of leaves and the separating
leaf may be said to abscised.
 The abscission zone consists of a
separation layer through which the
actual break occurs, and the protective
layer.

7.  In simple dicotyledonous leaves
the abscission zone occurs
within the petiole or at its base.
 In compound leaves the
abscission zones occur in the
petiole of the leaf as a whole
and at the base of individual
leaflets.
 Within the abscission zone, a
few days or even weeks before
actual leaf fall, a separation
layer develops which aids
structurally in the leaf fall.
 When the tissues, found
beneath the separation layer
are exposed after the leaf fall,
they are protected from
desiccation and infection by
means of one or more
protective layers.

8.  At least one of these protective layers lies within
the abscission zone.
 These protective layers are of two types-a
primary protective layer and the secondary
protective layer or periderm.
 From the view point of its structure, the
abscission zone is the weakest part of the petiole.
 As soon as the leaf becomes mature the
abscission zone becomes evident.
 A shallow depression develops externally and the
colour of the epidermis changes in this zone.
 The vascular bundles in abscission zone are
reduced in diameter.
 The collenchyma is lacking and the sclerenchyma
becomes weak or altogether absent.

9.  The parenchyma cells of this zone possess denser
cytoplasm.
 The process of the separation of the leaf commonly
starts from the peripheral region of the petiole and
proceeds towards the middle of the petiole.
 The separation layer remains in continuation
through the parenchyma cells in the vascular
bundles, whereas the xylem and phloem elements
and other non-living cells have been broken
mechanically.
 At least one of these protective layers lies within the
abscission zone.
 These protective layers are of two types-a primary
protective layer and the secondary protective layer or
periderm.
 From the view point of its structure, the abscission
zone is the weakest part of the petiole.
 As soon as the leaf becomes mature the abscission
zone becomes evident.

10.  A shallow depression develops externally and the
colour of the epidermis changes in this zone.
 The vascular bundles in abscission zone are reduced
in diameter.
 The collenchyma is lacking and the sclerenchyma
becomes weak or altogether absent. The
parenchyma cells of this zone possess denser
cytoplasm.
 The process of the separation of the leaf commonly
starts from the peripheral region of the petiole and
proceeds towards the middle of the petiole.
 The separation layer remains in continuation
through the parenchyma cells in the vascular
bundles, whereas the xylem and phloem elements
and other non-living cells have been broken
mechanically.

11.  Shortly after the leaf fall the protective
layers develop on the exposed surface.
 The protective layers may be of the both
primary and secondary origin. Sometimes
they are only of secondary origin.
 The secondary protective layer is typical
periderm. At the region of separation, a
leaf scar is formed.
 The scar is formed because of the
deposition of the substances, which
protect the new surface from injuries,
infection and loss of water.
 These substances are found beneath the
separation layer in the cells and referred
as suberin and lignin.
 In some plants the periderm develops
just after leaf fall.
 This newly developed periderm beneath
the protective layer remains in
continuation with the periderm of the
stem.