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Control of Senescence

The factors which control plant senescence and death are poorly understood. As mentioned earlier, in many cases, senescence can be delayed by removing flowers from the plants. There is some evidence to indicate that growth substances are involved in the control of senescence. Roots produce cytokinins, which appear to delay the senescence of leaves. Part of the action of cytokinins appears to involve the redistribution of biologically active materials from one part of the plant to the other. When one of a pair of leaves opposite each other of a bean plant is treated with cytokinin, it remains dark-green and healthy. The untreated leaf opposite it, on the other hand, turns completely yellow and senesces rapidly. This is the result of loss of materials from the untreated to the treated leaf.

As against the action of cytokinins, the phytohormone, ethylene, promotes senescence. Ethylene is produced by all parts of the plant and it exerts a number of effects. Ethylene promotes leaf abscission and ripening of fruits. The over-ripened rotten fruits release ethylene, which triggers ripening and subsequent rotting of other fruits in a barrel. Ethylene inhibits stem elongation and causes stems to lose their sensitivity to geotropic stimulation.

Role of Senescence in Plant Life

Leaf senescence and subsequent leaf fall (abscission) are of real importance for the survival of the plant. They occur at the end of the growing season, shortly before the adverse conditions of winter sets in. In many species of plants, leaves become a liability during the cold winder months, when water is frozen and unavailable. Yet water could still be lost from the leaves through stomata. Therefore it is advantageous for the plant to shed leaves during cold periods of the year. The leaf shedding also serves another purpose. Before the leaves die and are shed, their proteins are hydrolysed to yield amino acids, which are then exported from the leaves to the stems. Thus, controlled leaf abscission proves to be advantageous to the plant to conserve resources. The senescence and death of the entire plant which occurs after flowering and seed formation appears to be an adaptation for producing more offspring.

Thus the growth substances or phytohormones play an important role in regulating almost all aspects of plant growth and development. Some of the important effects of phytohormones are shown in the table below.

Effects of phytohormones on plant growth and development





Increased cell expansion and cell division; retarded leaf abscission, apical dominance; root initiating and fruit growth from unfertilised flowers.

Gibberellins Increased cell expansion; internode elongation; germination of some seeds and fruit growth.
Cytokinins Cell division; mobilisation of materials; retardation of senescence and organ formation.
Abscisic Acid Closing of stomata; dormancy of some seeds and buds, senescence.


Fruit ripening; formation of the hook at the apex of the etiolated dicot seedling and inhibition of stem elongation.

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