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The below mentioned article will highlight the role of ethylene in the senescence and abscission process of plants.
The term abscission is used to describe the process involved in the shedding of plant structures, such as leaves, characterized by the degradation of cell walls at the point of weakening (i.e. abscission zone).
Present evidence suggests that cells surrounding the fracture line produce and secrete cell wall degrading enzymes which hydrolyze the central region of the wall, allowing the cells to separate and fracture to occur.
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In contrast, the less common process of ‘mechanical tearing’ involves the generation of large forces which tear apart an inherent weak bond of cells. The cells along the fracture line thus play a passive role in this process, e.g. flaking of bark in trees.
The more conspicuous examples of abscission such as the shedding of fruits, leaves, bud scales, floral structures and branch lets will be familiar, but it is important to realize that virtually any aerial part of a plant can be shed in this way. Structures may range in size from the complete shoot system of tumble-weeds down to the hairs shed from developing leaves.
However, in some developmental mutants the ability to abscise is apparently lost, for instance the tomato varieties “Joint less” and “Lateral Suppressor” have no floral abscission zone. The senescence of fruits and leaves usually precedes abscission and as a result it is generally believed that the former process is an essential prerequisite for the latter.
Role of Ethylene in Senescence and Abscission:
Ethylene, a gaseous hormone, appears to be a prime controlling agent in many aspects of plant senescence including the fading of flowers, the ripening of fruits, and the abscission of leaves. If an orchid flower goes un-pollinated it remains fresh for a long time, but very soon after it is pollinated it starts to fade. The reason for the post-pollination decline is that pollination initiates the production of ethylene, which then causes the senescence of the flower petals.
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It is difficult to determine whether ethylene is the actual trigger for senescence or whether it simply accelerates the process. The effects of ethylene can be negated by high concentration of CO2. Auxin and ethylene interact in many ways, since high auxin level triggers ethylene production, while high ethylene levels can cause induction of an enzyme, peroxidase,that inactivates IAA. Ethylene is formed from 1-amino propane – 1- carboxylic acid (ACC), a methionine metabolite.
The application of ethylene to many unripe fruits results in a marked rise in respiratory CO2 output called climacteric. Following this change, organic acids decline, intercellular pectin’s are degraded, and fruit becomes ripe.
Application of ethylene to leaves similarly triggers a new set of metabolic events leading to abscission; these include new cell divisions, forming an abscission layer of weak- walled cells, whose digestion by newly-formed cellulose enzyme brings about leaf fall (Fig. 6.2).
