In this article we will discuss about:- 1. History of Ethylene 2. Functions of Ethylene 3. Uses.
History of Ethylene:
It is a gaseous hormone which stimulates transverse or isodiametric growth but retards the longitudinal one. Businessmen dealing with storing and shipping of fruits had known quite early that a rotten or ripe fruit could trigger early ripening of other fruits present nearby. Cousins (1910) found that ripe oranges produced a volatile substance that hastened ripening of unripe bananas nearby.
With the help of gas chromatography, R. Gane (1934) found that the ripening causing volatile substance was ethylene. Ethylene was recognised as a plant hormone by Crocker (1935). Ethylene is produced in plants from the amino acid methionine. It is formed in almost all plant parts— roots, leaves, flowers, fruits, seeds.
Maximum synthesis occurs during climacteric ripening of fruits and tissues undergoing senescence. Excess of auxin also induces ethylene synthesis. Many effects of excess auxin are actually the effects produced by ethylene.
Functions of Ethylene:
Ethylene inhibits longitudinal growth but stimulates transverse or horizontal growth and swelling of axis.
It decreases the sensitivity to gravity. Roots become Apo-geotropic while stems turn positively geotropic. Leaves and flowers undergo drooping. The phenomenon is called epinasty. Seedlings develop tight epicotyl hook.
It hastens the senescence of leaves and flowers.
Abscission of various parts (leaves, flowers, fruits) is stimulated by ethylene which induces the formation of hydrolases.
5. Apical Dominance:
Ethylene promotes apical dominance and prolongs dormancy of lateral buds.
6. Breaking of Dormancy:
It breaks the dormancy of buds, seeds and storage organs.
7. Abscisic Acid:
It seems that formation of abscisic acid in the leaves under conditions of water stress is mediated through ethylene.
8. Growth of Rice Seedling:
Ethylene promotes rapid elongation of leaf bases and internodes in deep water rice plants. As a result leaves remain above water.
9. Root Initiation:
In low concentration ethylene helps in root initiation, growth of lateral roots and root hairs. This increases the absorption surface of the plant roots.
10. Fruit Ripening:
It aids in ripening of climacteric fruits and dehiscence of dry fruits. Climacteric fruits are fleshy fruits which show a sudden sharp rise of respiration rate at the time of ripening (respiratory climacteric). They are usually transported in green or unripe stage. Ethylene is used to induce artificial ripening of these fruits, e.g., Apple, Mango, Banana, etc.
It stimulates flowering in Pineapple and related plants as well as mango though in other cases the gaseous hormone causes fading of flowers. This helps in synchronizing fruit set.
12. Sex Expression:
Like auxins and cytokinins, ethylene has a feminizing effect on sex expression. The genetically male plants of Cannabis can be induced to produce female flowers in the presence of ethylene. The number of female flowers and hence fruit is enhanced in monoecious plants like Cucumber.
Uses of Ethylene:
Ethylene regulates a number of physiological processes. Therefore, it is widely used PGR in agriculture. The common compound used for obtaining ethylene is ethophen or ethrel which is 2-chloroethyl phosphonic acid. In aqueous solution, ethophen is readily absorbed and transported to various parts. It releases ethylene slowly.
The various commercial uses of ethylene are as follows:
1. Fruit Ripening:
Kerosene lamps and hay were previously used for stimulating colour development and ripening of some fleshy fruits, e.g., Banana, Mango, Apple, and Tomato. The effect is due to ethylene. Ethylene lamps are now specifically used for this purpose.
2. Feminising Effect:
External supply of very small quantity of ethylene increases the number of female flowers and hence fruits in Cucumber.
3. Sprouting of Storage Organs:
Rhizomes, corms, tubers, seeds (e.g., Peanut) and other storage organs can be made to sprout early by exposing them to ethylene.
Excess flowers and young fruits are thinned with the help of ethylene, e.g., Cotton, Cherry, and Walnut. It allows better growth of remaining fruits.