Physiology of Senescence and Abscission

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, A mravati

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. 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. Deciduous Senescence – In deciduous woody plants, all the leaves die but the bulk of the stem and root system remains viable. It is called deciduous senescence or simultaneous or synchronous senescence. E.g. Leaf fall in deciduous trees.

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. Causes of Senescence : Leaf senescence is accompanied by early loss in chlorophyll, RNA and enzymes. Cellular constituents are decreased due to slower synthesis or faster break down. Competition between vegetative and reproductive organs for nutrients. A senescence factor (a hormone) is produced in soybean fruits that move to leaves where it causes senescence.

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. Physiology of Senescence The following physiological changes occur during senescence : Photosynthesis stops. Chlorophyll degradation: The colour of leaf changes from green to yellow. Anthocyanin pigments accumulation in the leaves causing reddening in leaves.

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. Senescence Promoters Senescence is promoted by hormones such as abscisic acid and ethylene. The senescence accelerating ability of abscisic acid is well documented. The function of ABA as a promoter of flower tissue senescence including initiaton of colour fading or blueing has been established. The ABA content of aging leaves increases markedly as senescence is initiated. Ethylene plays a very important role in the senescence of certain plant parts, particularly fruit and petals and in the abscission process. It is an inducer in the senescence of flower tissue. Senescence Retardants: The primary plant hormones involved here are auxin, gibberellin and cytokinin.

15. Significance of Senescence The whole plant senescence occurs in monocarpic plants coinciding the seed setting and seed dispersal. Due to the formation of abscission layer, the older leaves tend to fall down so that the nutrients will be diverted to the next young leaf. The senescence process helps the mobilization of nutrients and of the vegetative parts of the plant into the fruits. Plants escape the influence of seasonal adversity by undergoing senescence of its organs. Leaf fall in deciduous trees reduces the rate of transpiration to survive under adverse 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

18. Environmental factors which strongly promote Abscission – Drought N deficiency

19. 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.

20. 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.

21. 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.

22. Significance of Abscission 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.