By - Avinash Darsimbe Assistant Professor Department of Botany Shri Shivaji Science College, Amravati Physiology of Senescence and Abscission B.Sc. III (Sem - V) BOTANY : PLANT PHYSIOLOGY AND ECOLOGY Sant Gadge Baba Amravati University,Amravati

1. B.Sc. III (Sem- V)
BOTANY PLANT PHYSIOLOGY AND ECOLOGY
Unit - III: Metabolism and growth
3.3 Physiology of Senescence and Abscission
By
Avinash Darsimbe
Assistant Professor
Department of Botany
Shri Shivaji Science College, Amravati

2. Like human beings, plants a1so grow old and undergo aging and then they
die. Aging is the sum total of changes in the total plant or its organs. During
aging, the plants undergo chemical and structural changes. Aging leads to
senescence and later phase of development that u1timately terminates to death.

3. Senescence
Senescence is the process of aging in plants. It refers to all changes that occur in a
plant and cause death of cells, tissues and whole plant body. In other words, we can say that
senescence is a natural process that slowly causes the death of a living entity. Senescence can
occur in different ways, it can affect the whole plant or a single part of the plant.

4. Leaf Senescence

5. Types of Senescence
Senescence can be divided into the following categories on the basis of their role in the aging
of a plant.

6. 1. Whole plant or Overall senescence:
It occurs in monocarpic plants which produce
flower and fruits only once in their entire life, e.g. rice, wheat, mustard,
cabbage, bamboos etc. It affects the whole plant; the plant dies after
producing flowers and fruits.

7. 2. Shoot or Top senescence:
It occurs in certain perennial plants with
underground structures like bulbs, tubers and rhizomes. The shoot which
remains above the ground dies each year after flowering but the
underground part survives and produces new shoots in the next season, e.g.
banana, ginger, gladiolus etc.

8. 3.DeciduousSenescence–
In deciduous woody plants, all
the bulk of the stem and root system remains
deciduous senescence or simultaneous or
E.g.Leaffallindeciduoustrees.

9. 4. Sequential senescence:
It is found in most of the perennial plants.
The tips of the main shoot and surrounding branches keep growing and
producing new bubs and leaves but the older leaves and lateral parts like
old branches undergo senescence and die, e.g. Pinus, Eucalyptus.

10. CausesofSenescence:
1.Leafsenescenceisaccompaniedbyearlylossin
enzymes.
2.Cellularconstituentsaredecreasedduetoslower
down.
3.Competitionbetweenvegetativeand
4.Asenescencefactor(ahormone)isproducedin
toleaveswhereitcausessenescence.

11. 5. Short-day and long-night conditions induce
flowering and leaf senescence.
6. Degradation of food reserves and loss of
integrity in food storage cells of seeds.
7. Senescence is also hormonally controlled.

12. PhysiologyofSenescence
Thefollowingphysiologicalchangesoccurduring
senescence:
1.Photosynthesis stops.
2.Chlorophyll degradation:The colour ofleafchanges
3.Anthocyanin pigmentsaccumulation intheleaves

13. 4. The vacuoles function as lysosomes and digest the cellular
materials.
5. The starch content decreased.
RNA and proteins are decreased.
6. DNA molecules are degraded by the enzyme DNase.
7. Growth promoting hormones such as cytokinin decrease.
8. The deteriorative hormones such as ethylene and abscisic acid (ABA) content are
increased.

14. SenescencePromoters
Senescence ispromoted by hormones suchas abscisic acidandethylene. The
acidiswelldocumented. The functionofABA asa promoter offlowertissue
fadingor blueing hasbeen established. The ABA content ofaging leaves increases
Ethylene plays avery important roleinthe senescence ofcertain plant parts,
abscission process. It isaninducer inthe senescence offlowertissue. Senescence
hormones involved hereare auxin, gibberellin andcytokinin.

15. SignificanceofSenescence
1.Thewholeplantsenescence occursin monocarpicplants
seeddispersal.
2.Duetotheformationofabscission layer,theolderleavestend
nutrients willbedivertedtothenextyoungleaf.
3.Thesenescence processhelpsthemobilizationofnutrients
theplantintothefruits.
4.Plantsescapetheinfluenceofseasonaladversityby
Leaffallindeciduoustreesreducestherateoftranspirationto
conditions.

16. Abscission
Abscission refers to the normal separation of a senescent plant part or organ, e.g. old leaves or
ripe fruits. It occurs to shed or separate the unnecessary plant parts or organs. For instance, shedding of
old leaves at the base of the petiole which usually occurs during autumn. Abscission is very important
for the normal life of a plant. This process seals off the vascular system to prevent water or nutrients
loss and also protects the plant from the bacterial or fungal infections.

17. Leaf Senescence and Abscision

19. Environmental factors which strongly promote Abscission –
1. Drought
2. N deficiency

20. Physiological Changes -
 Degradation of Chlorophyll - During abscission, the colour of the leaves, flowers and
fruits changes due to degradation of chlorophyll and the synthesis of anthocyanin
pigment takes place.
 Auxin Decreases- Abscission starts when the amount of auxin begins to decrease.
 Ethylene Production Increases - due to which formation of cell wall degrading enzymes
in the abscission zone takes place.

21.  Formation of Abscission Layer - Leaf abscission takes place at the base of the petiole.
The site of abscission is internally marked by a distinct zone called abscission zone. This
zone is made up of one or more layers of cells arranged transversely across the petiole
base. This is called abscission layer.
 Dissolution of Middle Lamellae - The cells of the abscission layer separate from each
other due to the dissolution of middle lamellae and the primary cellulose walls under
the influence of the activity of enzymes, pectinase and cellulase.

22.  Detachment of Leaf - At this stage, the petiole remains attached to the stem by vascular
elements only. But due to its own weight and the wind force, the leaf is detached from
the stem.

23. Significance of Abscission
1. It helps in diverting water and nutrients to the young leaves.
2. It is a self pruning process through which fruits and injured
organs are shed from the parent plant.
3. It helps in disseminating fruits and vegetative propagates.
4. Abscission serves as function in removing plant parts
containing waste materials.