Physiological Processes Associated with Abscission

In this article we will discuss about the physiological processes associated with abscission.

During the autumn leaves become yellow, black and pale and red in colour. Similarly, seasonal defoliations and the different shedding habits of the deciduous and the evergreen plants are of common occurrence. The plants may be facultative deciduous which conform to the intermediate pattern.

The change in the colour of leaves, which are to be shed, is generally because of an increase in anthocyanin pigmentation and reduction in chlorophyll pigments. This process of shedding of the organs such as leaves, flowers, fruits is called abscission. It is a highly complex physiological phenomenon which is generally associated with the senescent changes with cessation of growth and maturity of an organ.

The process of abscission is very important to the plant, especially for perennials, as it helps in diverting water and nutrients to the young leaves instead of old ones. Similarly it is also a ‘self-pruning’ device through which developing young fruits and injured organs are shed from the parent plant.

Thus it reduces competition and promises protection against infection by several pathogens. In addition, it helps in disseminating fruits and vegetative propagules. Further, by removing plant parts containing waste materials, abscission serves an excretory function.

The abscission is restricted to a morphologically distinct region called abscission zone. The abscission zone is located at the point of attachment of an organ to the parent plant and is similar in leaf, flower and fruit, etc. This zone is made up of one or more layers of cells which during leaf development undergo transverse divisions to form an abscission zone across the petiole.

The zone is pale or brown in colour and usually there are one to two zones in a leaf. Gymnocladus, a legume, is known to have ninety abscission zones in a leaf. The cells of this zone are small, compact and densely filled up with protoplasm.

In this zone fibres are lacking and lignification is absent or weak. When a particular organ is to undergo abscission, a layer of cells develops in the distal portion of the abscission zone (Fig. 27-1).

These cells exhibit high metabolic activity and constitute the abscission or separation layer. When the leaf is to fall, the middle lamella and the cellulose walls of the cells forming the abscission layer undergo dissolution and consequently the petiole remains attached to the stem only by the vascular elements. Because of the wind or the gravity pull, the leaf is separated from the parent plant.

As a result of abscission a small scar is left behind. In fact, during the development of the abscission layer, a series of cell divisions occur in the proximal region of the abscission layer. Thus, there is formation of protective layer across the area left by the abscission. This layer provides protection and acts as a defensive measure against infections. The cells in this layer may become lignified or suberized.

Several factors promote or quicken abscission and these include extremes of temperature, rapid water deficiency or lodging, increased respiration or oxygen uptake, short photoperiods id several biotic factors (e.g. attack by bugs, mites, fungi) as well as chemical compounds like defoliants, ethylene, etc.

In fact, process of abscission is an excellent example of an outcome of several correlative phenomena occurring elsewhere and manifesting in removal of an organ. It is a common experience that when the apical bud is debladed, petiole abscission is deferred.

Evidently some substance is produced in the buds which was involved in the activity of the process. The application of IAA could replace the effect. Similarly, auxin concentration in the blade is higher than that of the petiole while in the old leaves nearing abscission, the leaves had hormone level equal to that of petiole.

This has led several workers to propose auxin-gradient theory to explain abscission. Not all the available evidences support this theory. Recent studies have suggested that process of abscission is an active process and not just a passive falling of the leaves or other organs. In fact, abscission is suppressed by a deficiency of oxygen or of carbohydrates.

Further, abscission is retarded—by inhibitors of RNA and protein synthesis and promoted by ethylene. Anatomical studies mentioned earlier have brought out, that cell wall break down is important for the separation of the organs.

Separation usually results from dissolution of cementing substances between one or more cell layers, although more extensive cell lysis, including complete digestion of thin primary walls may occur. The precise anatomical development of abscission varies, depending on species.

It has also been demonstrated that pectin methyl esterase (PME), and cellulase were necessary for abscission. However, the exact sequence of enzymatic changes associated with anatomical and cytological changes in the abscission zone is still wanting.

In Coleus the extent of methylation of pectin increases, causing increase in the amount of soluble pectins in the middle lamellae and walls of cells in the petiole that abutt the abscission zone. There is also degradation of pectic materials and loss of Ca²⁺ and Mg from the middle lamellae because of the rise in pectinase.

In Phaseolus, cellulase increase has been shown in the abscission zone and also in the cells closely associated with this zone. One view is that cellulase is synthesized de novo since application of cycloheximide inhibits abscission. Four major isozymes of cellulase are seen in Phaseolus and only one of them increase just before abscission indicating the specific nature of the abscission cellulase.

Some of the naturally occurring amino acids have been reported to accelerate abscission. Recent works have revealed that other enzymes like acid phosphatase may also be involved which cause destruction of membranes, RNA, sugar phosphates, structural proteins, etc.

In the process of experimental studies, several stages in the process of abscission have been identified and it is also reported that there is increase in the acid phosphatases and dehydrogenases which is associated with the stage-II of the abscission. Some workers have investigated the role of peroxidase as well.

Auxins have been reported to play a central role in this phenomenon. When applied to debladed leaves, it prevents abscission. It is generally believed that internal level of auxin controls the process. Thus, auxin level falls with the increasing age of the leaf, fruit maturity.

Auxin is associated with antisenescent activity. In fact, auxins have been shown to cause delay in the degradation of chlorophyll, RNA and proteins. The fall in the amount or hormone may be due to the decrease or inhibition in the synthesis of auxin, or even its transport, oxidation and decarboxylation may be affected.

All these processes are also shown to be affected by ethylene. Ethylene is also known to inhibit conversion of tryptophan to auxin; Auxins may also regulate their synthesis by acting as mobilizers of nutrients. For instance, during fruit development auxins promote the inflow of nutrients. As a result, there is an increased synthesis of auxins in the fruit due to the ready availability of the amino acids.

However, in the mature fruit the nutrients tend to flow out of the fruit resulting in a decline in the level of the auxins within the fruit. Thus depending upon the concentration and point of application, auxin could inhibit or promote abscission.

For instance, young fruits sprayed with NAA are more sensitive to the auxin than the larger ones and therefore^– drop down following NAA application. Application of GA can negate NAA action.

Though the effectiveness of ethylene in stimulating abscission was reported in 1935 its exact method of action has been studied only recently. The tissues distal to the abscission zone are shown to produce relatively large amount of ethylene during tissue senescence and prior to abscission of the organ. Concentration of ethylene when increased in the air can cause abscission and older leaves respond readily.

Application of auxin prevents senescence to the tissues distal to the abscission zone and prevents cells below the zone from responding to ethylene. Ethylene application tends to reduce the auxin content in the maturing fruit.

Thus, fruit senescence is stimulated and abscission is brought about. Ethylene is also known to stimulate cellualse secretion from the cytoplasm to the cell walls in the abscission zone. This hormone is also known to stimulate respiration and produce energy-rich compounds essential for the synthesis of enzymes concerned with abscission.

Another abscission accelerating hormone is shown to be abscisic acid and this hormone regulates ethylene production. Abscisic acid also increases the rate of cellulase and pectinase synthesis, indicating that the mode of action of this hormone is directed towards RNA or protein synthesis.

Gibberellic acid is also found to influence the phenomenon but the effect is believed to be indirect one due to increased vigour. It quickens abscission in explants when applied distally or proximally to the abscission zone. There is also a view that CA acts during abscission by interacting with other plant hormones, especially IAA.

Cytokinins are considered to be the most effective retardants of plant senescence. The application of this hormone retards the loss of chlorophyll, prolongs the activity of peptidases, ribonuclease, etc. and also retards the decline of the auxin level and suppresses increased oxygen uptake during senescence.

Some workers have purified factors which cause senescence and call them as SF. These factors are distinct from IAA or ABA in chemical composition or biochemical action. Thus SF accelerates abscission and immediately stimulates ethylene production.

Supposedly this SF regulates ethylene biosynthesis. In the young tissues SF could be membrane contained. During abscission, or senescence, the membranes are ruptured and SF is released and stimulates ethylene biosynthesis and is followed by abscission.

Abscission clearly involves changes that are unique to the zone where specific organ is separated from the parent plant. It involves precise regulation of cellular senescence. This applies to the timing of the senescence events and to the precise location of the line of cells through which separation will occur.

In recent years ultrastructural and morphogenetic studies are being carried out. It has been observed that cells of the separation layer have more-dense cytoplasm than the rest of the cells. This indicates that there is normally heavy concentration of organelles in the cell and less portion occupied by the vacuoles.

Nucleoli are also enlarged in the separation zone which indicate the high rate of RNA synthesis associated with the increase in the hydrolytic enzymes synthesis which are required to degrade the cell wall. The high accumulation of starch in the separation zone indicates the availability of the substrate for providing energy and also the component for the formation of protective layers (Fig. 27-2).

With the electron microscopic studies, it has also been observed that there is the swelling of middle lamella followed by the development of cavities, sometimes in the vicinity of the plasmodesmata and this leads to the complete disappearance of middle lamella in due course of time. An increase in the amount of rough endoplasmic reticulum (ER) is normally observed before the cell wall degradation starts.

It is thought that hydrolases synthesized in the ER are sequestered in ER cisternae (A) and the vesicles carrying the hydrolytic enzymes are released from ER and differentiate into flattened dictyosomes cisternae (S).

At the maturing phase of dictyosomes vesicles are formed which get fused with the plasmalemma (C) and so the enzymes are released in the paramural region of the cell. Most commonly pectinases diffuse out through the wall (D), loosen and break down the pectinaceous middle lamella and this results into cell separation.