Abscission of Leaves in Plants (With Diagram)

In this article, we will discuss about the abscission of leaves in plants.

Introduction to Abscission of Leaves:

Detachment of the older (rather senescent) leaves or leaf fall is a common phenomenon in plants and is called as abscission of leaves. Abscission is quite distinctive in deciduous trees and shrubs of temperate regions in autumn when all the leaves of such plants fall at about the same time giving the plants a naked appearance, the new leaves developing in the subsequent spring.

In evergreen plants there is gradual abscission of leaves, the older leaves fall while new leaves are developed continuously throughout the year. In most of the herbaceous spe­cies, however, the leaves are not shed even after they die and in many cases are retained in withered dry condition even after the whole shoot is dead.

Leaf abscission takes place at the base of the petiole which is internally marked by a dis­tinct zone of few layers of thin-walled cells arranged transversally across the petiolar base. This zone it called as the abscission zone or abscission layer (Fig. 17.42 A). The cells of the abscission layer separate from each other due to the dissolution of middle lamella and also the primary cellulose walls under the influence of the increased activity of the enzymes pectinase and cellulose.

At this stage the petiole remains attached to the stem only by vascular ele­ments and very soon due to its own weight and pressure of wind is detached from the stem. The broken vascular elements are soon plugged with tyloses or gums.

Mechanism of Abscission:

The young leaves remain attached to the stem and do not absciss till they become old. However, if the blade or lamina portion of a young leaf is cut, the debladed petiolar stump soon abscises. In case auxin (IAA) in lanolin paste is applied to the cut end of petiole of such a young leaf the abscission of the petiolar stump is greatly suppressed.

The intact young leaf does not absciss because its lamina portion contains auxin synthesized by it. These experiments have led to the belief that auxin has controlling influence in the abscission of leaves. This belief is further strengthened by the fact that endogenous auxin concentration in leaves falls considerably at the time of normal abscission.

In yet other experiments it has been found that if auxin is applied to the distal side (blade side) of the abscission zone of the debladed petiole of young leaf, the abscission of the petiolar stump is prevented. On the other hand, if the auxin is applied to the proximal side (stem side) of the abscission zone of the debladed petiole, abscission is accelerated (Fig. 17.42 B).

Normally, the auxin level of the stem side of the abscission zone is probably maintained due to basipetal transport of auxin from the stem tip while the source of the auxin on the blade side of the abscission zone is the blade or lamina of leaf itself. The above-mentioned experiments have led to the establishment of auxin gradient hypothesis according to which it is not the presence or absence of auxin but relative concentration of auxin on two sides of the abscis­sion zone which controls abscission.

A higher concentration of auxin on stem side of the abscission zone or nearly equal concentration of auxin on both its sides will promote abscis­sion while higher concentration of auxin on the blade side of the abscission zone will retard abscission. The mechanism by which auxin controls abscission is not clearly understood. Besides auxin, other growth hormones especially ethylene may also play important role in abscission.

Recent researches have now shown that the relative concentration of auxin on two sides of the abscission layer has regulatory influence on the production of ethylene that stimulates leaf abscission. At the time of abscission, concentration of auxin in the laminar region de­creases with simultaneous increase in ethylene production.

This also increases sensitivity of cells of abscission zone to ethylene which now synthesize cell wall degrading enzymes such as celluloses and pectinases. Activity of these enzymes results in cell wall loosening and cells separation ultimately leading to leaf abscission. (Although ABA is certainly involved in senescence of leaves but it is not concerned in the abscission of leaves. It is probable that through its promotion of senescence, ABA might increase production of ethylene that may stimulate abscission).