Top 2 Plant Growth Inhibitors: Abscisic Acid and Ethylene

Let us make an in-depth study of the top two plant growth inhibitors. The top two plant growth inhibitors are: (1) Abscisic Acid and (2) Ethylene.

Some plant hormones result in inhibition, rather than stimulation, of growth and development in plants. Although in the past a number of inhibitory substances have been isolated, their true role as naturally occurring growth regulators remained suspect.

However, one growth inhibitor has been unequivocally established as a category of growth substances equal to auxins, gibberellins and cytokinins. This substance is abscisic acid.

Plant Growth Inhibitors # 1. Abscisic Acid (ABA):

The name abscisic acid (ABA) is derived from the ability of this substance to promote abscission, a discovery made by F.D. Addicott et. al. in California (1963) working on the abscission of cotton bolls. ABA has been found in all higher plant tissues; these include leaves, roots, xylem of tree trunks, xylem sap, phloem sap, pollen, petals, fruits and seeds (Milborrow, 1974). Concentrations, however, vary widely.

The application of ABA to an actively growing twig of a woody plant results in cessation of elongation of the internodes; some of the leaves develop abscission layers and drop off, young developing leaves form scale leaves instead of foliage leaves, and the terminal bud becomes quiescent. Similar response of twigs is normally seen at the onset of the winter season and ABA may be described as a “dormancy-inducing hormone”.

Physiological Roles of ABA:

1. Abscission of leaves and flowers:

Application of ABA causes very fast abscission of leaves and flowers.

2. Senescence:

ABA accelerates the process of ageing by causing break down of proteins and nucleic acids.

3. Cell division and cell elongation:

ABA also delays cell division and cell elongation.

4. Growth promotion:

Interestingly enough, ABA has also been found to promote certain growth processes, such as stimulation of parthenocarpic seed development, rooting of cuttings, elongation of hypocotyls, at very low concentrations.

5. Dormancy:

ABA is known to induce and maintain dormancy in potato tubers and buds, and dormant potato tubers and resting buds contain more ABA than active tubers and buds. Gibberellins and cytokinins are capable of antagonizing the effect of ABA, and break dormancy.

6. ABA as a Stress Hormone:

ABA is generally referred to as “Stress Hormone”, which is a most suitable description of its overall role in the plants. Generally it is formed in response to stress or unfavourable environmental conditions, and it, in turn, changes the plant to withstand that stress. The most striking example of such a response is the rapid synthesis of ABA in response to water stress, that is. a shortage of water.

When a plant is deficient in water, the ABA content of the leaves rises rapidly. This then acts on the guard cells of stomata, “deflating” them so as to close the stomata rapidly, long before such a closure would occur from overall water loss by the plant. Other stresses such as low temperature can also lead to the synthesis of ABA and the closure of stomata.

The change of seasons in temperate zones poses survival problems for plants which are partially overcome by their induction into and release from dormancy. Thus, diminishing day-lengths can induce the formation of ABA that reduces the active vegetative growth of a bud or a developing embryo, and sets in motion the events that lead to winter dormancy. On return of favourable season dormancy is broken by removing the effect of ABA by substances such as cytokinins.

7. Fruit Growth:

Though ripening fruits contain large amount of ABA, yet application of ABA to fruit has little or no effect. However, as an exception, when ABA is applied to ripening grape berries, their ripening is accelerated and their colour changes fast. ABA in the fruit coat does not affect the germination or dormancy of seed. ABA is present in fairly constant amounts throughout the development of seed. However, the aborted fruits contain larger amounts of ABA.

8. Parthenocarpy:

ABA is known to rarely induce parthenocarpic development of fruits in some plants (e.g. in emasculated fruits of Rosa sherardii, the wild rose, as reported by Jackson & Blundell, 1966).

9. Flowering:

ABA does not ordinarily promote growth of flowers in short-day plants. High concentrations of ABA usually inhibit or delay flowering in plants. Both ABA and ethylene appear to act in part through effects on differentially permeable membranes and in part through control of protein synthesis.

Applications of Abscisic Acid:

1. As Antitranspirant:

Application of small quantities of ABA to the leaves reduces the rate of transpiration in a plant by inducing closer of a stomata. This way a lot of water can be conserved by the crop.

2. Other Uses:

It is also used in inducing flowering in short day plants, rooting of stem cuttings in some plants, and for inducing dormancy in buds and seeds.

Plant Growth Inhibitors # 2. Ethylene:

It has been known for many years that over ripe fruits produce something that hastens ripening of adjacent fruits (“one rotten apple will spoil a barrel”). This fruit ripening factor has proved to be the simple unsaturated hydrocarbon, ethylene (C₂H₄). It is synthesized from amino acid methionine. It is component of manufactured coal gas and was identified by the hastening of ripening of fruits exposed to accidental leakages of coal gas.

Ethylene is a natural plant product which is produced by ripe fruits and acts like a plant hormone. Ripe fruits produce ethylene which stimulates ripening of adjacent fruits (and the production of ethylene by them). Fruits such as bananas that are picked green for transport to market are treated with ethylene so that they will be properly ripe when they reach the market.

Premature ripening of fruit in a warehouse can be prevented by ventilation to remove the ethylene and by increasing the content of carbon dioxide in the air, for carbon dioxide counters the effect of ethylene.

Ethylene is a rather different type of hormone from the four previous categories in that it is a gas. It is released from most plant organs in varying concentrations, most obviously from ripening fruits. In trace amounts it interacts with the other plant hormones, especially auxin, to coordinate and regulate a wide variety of growth and developmental processes.

Effects of ethylene are very striking in case of fruit ripening, abscission, breaking of dormancy, flowering, and modification of sex expression. Ethylene as a plant hormone is unique in its structural simplicity and gaseous nature.

The Russian botanist Neljubow (1901) is believed to have been the first to recognize the growth-regulatory properties of ethylene. By 1930 ethylene was recognized to have a wide variety of interesting effects on plants. Gane in 1934 discovered that ethylene was a natural plant product. Soon after, Crocker, Hitchcock & Zimmerman in 1935 reported that ethylene is a fruit-ripening hormone and it also acts as a regulator in vegetative plant organs.

By 1960 ethylene had been clearly identified as an endogenous regulator of fruit-ripening. In-rolling of petals in opened flowers (i.e., sleep disease) is caused by ethylene. Even 1 ppm of ethylene prevents opening of flower bud.

Physiological Roles & Uses of Ethylene:

(i) Ripening of fruit:

Fruit ripening was the first plant response which was clearly shown to be regulated by ethylene. It has now been fully established experimentally that ethylene is a fruit ripening hormone.

(ii) In seedling growth:

Ethylene acts as a regulator of cell shape and seedling behaviour rather than strictly as growth inhibitor.

(iii) In Abscission:

There is sufficient experimental evidence available to indicate that ethylene is involved in the abscission (i.e., separation of organs from the plant) of flower bud, flower and young fruit as well as fruit dehiscence. Plant leaves make ethylene which results in their own abscission.

Application of ethylene to leaves 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 brings about leaf fall.

(iv) Ethylene in normal growth and development:

Ethylene participate in almost all phases of plant growth and behaviour. Ethylene regulates a variety of life processes in plants ranging from release of seed dormancy and early seedling behaviour to leaf abscission and fruit ripening. Ethylene may modify development even when its production rate in the plant is at a very low level.

Commercial Importance of Ethylene:

Ethylene has been commercially exploited in a very big way all over the world for improving the quality or promoting ripening of fruits such as tomatoes, apples, coffee berries and grapes; to facilitate harvesting of cherries, walnuts and cotton by accelerating abscission or fruit dehiscence; increasing rubber production by prolonging latex flow in rubber trees; increasing sugar production in sugarcane; synchronizing flowering in pineapple and accelerating senescence of tobacco leaves. For all such purposes ‘liquid-ethylene’ (ethephon), sold in market under the trade name of Ethrel, which is 2-chloroethyl-phosphonic acid (CICH₂ CH₂ PO₃ H₂), is used.

Conclusion:

Growth and development in any plant is controlled by a group of growth promoters as well as growth inhibitors. The hormones coordinate with each other to bring about growth and differentiation.