Commercial Uses of Ethylene in Plants

The below mentioned article provides a study note on the commercial uses of ethylene in plants.

Ethylene is one of the most widely used plant growth hormones in agriculture. But, due to its gaseous nature and high diffusion rate, ethylene cannot be administered to plants without confining them in closed chambers and it is very difficult to be applied in gaseous form in the field.

However, this problem has been overcome due to availability of some synthetic chemical compounds which when sprayed on plants in aqueous solution are readily absorbed and trans located within them and breakdown to release ethylene.

One such most commonly used chemi­cal compound is ethephon (2-chIoro ethylphosphonic acid) which is known by various trade names such as ethrel. Ethephon is stable at low pH, but slowly breaks down at pH 4 or more. Since, the pH of plant cells is less acidic (about 6); ethephon breaks down in plant cells and releases ethylene which exerts its hormonal effect.

Conversion of ethephon into ethylene is non-enzymatic and is a simple base catalysed reaction in which phosphoric acid and chloride ions are the byproducts:

Breakdown of ethephon into ethylene is a very slow process and may continue for several days in plant cells.

(a) Aqueous solution of ethephon is sprayed on plants in desired concentrations to has­ten fruit ripening, in tomato and apple and de-greening of citrus fruits. It is also effectively used in synchronizing flowering and fruits set in pineapple and hastening abscission of flow­ers and fruits.

Other commercial uses of ethephon (ethylene) are:

(i) To induce fruit thinning (fruit drop) in cotton, cherry and walnut.

(ii) To inhibit terminal bud growth in some plants so their flowering stems are made more compact.

(iii) To promote formation (expression) of female flowers in cucumber, avoid self pollina­tion and increase yield.

(b) Sometimes, promoters of ethylene biosynthesis such as auxins and ACC are also used in agriculture practice which trigger natural biosynthesis of ethylene in plants.

(c) Contrary to the above, reverse measures are often employed on commercial scale to reduce rate of ripening, preventing over ripening to enhance post-harvest preservation of fruits and to increase longevity of cut carnations and other flowers by inhibiting or reducing the natural biosynthesis of ethylene in plant tissues or removing ethylene from storage chambers.

This can be accomplished in various ways:

(i) By Controlling Storage Atmosphere:

Low O₂ conc. and low temp, inhibit biosynthesis of ethylene. Low atmospheric pressure is used to remove ethylene and O₂ from the storage chambers that reduces rate of fruit ripening. CO₂ at higher concentrations (5-10%) acts as antagonist of ethylene action and helps in pre­venting over-ripening. Potassium permanganate (KMnO₄) is a very effective absorbent of eth­ylene and is used in apple storage chambers to delay ripening and extending shelf life of the fruits.

(ii) By Using Inhibitors of Ethylene Biosynthesis:

AVG, a potent inhibitor of ethylene biosynthesis can be used to retard fruit ripening and flower fading.

(iii) By Using Antagonists of Ethylene Action:

Besides higher conc. of CO₂, silver ions especially as silver thiosulphate are potent and much more effective inhibitors of ethylene action and are extensively used in delaying senes­cence of cut carnations and other flowers. 1-Methyl cyclopropene (MCP), a synthetic volatile olefmic compound is emerging as yet another antagonist of ethylene action for use in many post-harvest agricultural practices.

(iv) Through Biotechnology:

By making expression of an antisense version of ACC synthase and ACC oxidase in to­mato, the biosynthesis of ethylene can be blocked and fruit-ripening completely inhibited. Fruit ripening in such genetically modified or transgenic tomatoes can be restored by externally applied ethylene only when needed. Ethylene biosynthesis can be blocked in many other plants such as Petunia also through biotechnology or genetic engineering to increase longevity of cut flowers to several weeks.

(vi) Morphactins:

In 1960s (1964-65 onwards) a new group of synthetic growth regulators called as morphactins (meaning morphologically active substances) had come into prominence and aroused great interest among plant physiologists because of their polyvalent action (i.e., wide range of action) on the natural regulation mechanism of plants. Their action is usually inhibitory to de­velopment and growth.

The morphactins which are synthetic derivatives of fluorene-9-carboxyIic acid are absorbed via seeds, leaves or roots and are distributed in the plant not strictly polarly (as IAA) but basipetally and acropetally.

The morphactins are peculiar in that:

(i) They are non-toxic over a wide range of concentrations and

(ii) They effect the organs which develop after their application (i.e., the new growth).

Characteristic Physiological and Morphological Effects:

(i) General inhibition of internodes elongation.

(ii) Reduction in laminar area.

(iii) Reduction of apical dominance of the main shoots (promotion of branching).

(iv) Strong inhibition of lateral root formations (i.e., reinforcement of apical dominance of the tap roots).

(v) Abolition of phototropism of shoots and geotropism of roots.