Ethylene is a gaseous hydrocarbon with the structure of H2C=CH2. It is reasonably not necessary for normal vegetative growth of the plant but has a notable effect on the growth of roots and shoots. Ethylene occurs in plants when high intensities of auxins are provided to the plant tissue. It occurs in all plant organs- roots, leaves, stem, bulbs, seeds, fruits, tubers, and so on.
The rate of production of ethylene may vary depending on the structure of the plant as different tissue secrete different amounts of ethylene. It is also determined by the stage of development the plant is going through. Studies have shown that ethylene is generally located in the peripheral tissues of the plants. It also depends on the type we look at, for example in peach and avocado seeds it is noticed that more ethylene concentration is found on the seed coat, but in tomato fruit and mung bean hypocotyls, it takes its origin from the epidermal region.
As it could be seen that ethylene is a basic hydrocarbon derived from ethane molecule. Here both the carbon atom shares a double bond, and the remaining valencies are filled by hydrogen atoms. It is to be noted that the carbon-carbon double bond has a length of about 133.9pm and the carbon-hydrogen bond has a length of 108.7pm and the angle between the carbon and hydrogen from the inner aspect is 121.3⁰
The formula for ethylene is H2C=CH2. Ethylene has high importance in plant physiology and despite that is was difficult to find out how ethylene was produced in the plants and what pathway it followed to be secreted. After the evolution and invention of Gas chromatography, volatile ethylene substance could be isolated and its physiological importance could be studied.
Few scientists in the early 1960s showed that ethylene was actually derived from an amino acid called Methionine in apple tissues. Later in the 1970s Yang and Adam have illustrated that the conversion of Methionine to Ethylene takes place by the production of an intermediate compound called S-adenosylmethionine (SAM) by the apple tissue. They further demonstrated that the accumulation of a compound called 1-aminocyclopropane-1-carboxylic acid (ACC) in apple tissue fed with Methionine had inhibited the production of ethylene in anaerobic conditions but on the introduction of oxygen the labeled ACC was rapidly converted to ethylene.
Ethylene as a Plant Hormone
Ethylene acts as a significant hormone that regulates and mediates complex cycles in plants, regarding their growth and development, and their survival throughout their life cycle.
The main function and interest of the scientist for ethylene is its ability to ripen fruits and achieve senescence. This ability has been the major focus for food biotechnologists as it can be used to fasten the ripening of fruits where ethylene is the main hormone, for example, tomato and banana fruits. Food biotechnologists aim to achieve the ability to control the fruit ripening process by controlling the synthesis of ethylene hormone.
To understand the function of ethylene, we first understand how a plant secretes ethylene in the tissues. Ethylene synthesis is a biochemical pathway consisting of two-steps. It starts with a compound called SAM (S-adenosyl-L-methionine). SAM is changed to ACC with the help of the enzyme ACS (ACC synthase). ACC is turned to ethylene via an enzyme called ACO (ACC oxidase).
It is to be understood that both the enzymes ACS along with ACO are emitted by multiple gene coding family that synchronizes with one another in situations like drought, flood, wound, applying external pressure, and attack from a pathogen.
Ethylene has a wide range of functions in plants. Some of the important functions of ethylene are seed germination, shoot and root growth, root development, abscission of leaves and fruits, the formation of adventitious roots, senescence of leaves and flower and sex determination. For instance, during floods ethylene in plant tissue provokes the generation of air-filled cavities called aerenchyma tissues which helps in oxygenation of plants.
The most important function of ethylene is, however, the ripening of climacteric fruits implying peach, bananas, apples, and tomatoes. For example, if you place a ripened banana in a bag full of unripened avocados, it will accelerate the rate of ripening of the avocados. This is due to the accumulation of ethylene in the bag.
A few fascinating functions of ethylene also incorporate:
The generation of female flowers in a male plant.
Producing root growth to enhance the capability of the root to absorb more water and minerals.
Evoking a phenomenon called epinasty. Epinasty is a complex behavior seen in plants when the roots are flooded. During floods, the top layer of the leaves grows more than the bottom ones. This induces the leaves to drop and rather than being horizontal the leaves become more vertical. This is specially induced by ethylene when it is converted to ACC and transported from the xylem to the tissues of leaves on the upper part.
Ethylene promotes negative geotropism, where it ensures that the growth of the roots is towards the ground. Hence, more area of roots in the soil indicates easy absorption of minerals from the soil.
The sex of a flower can be determined.
Influences the seed germination.
Has a great role in the initiation of root growth and pollination.
The flowering of pineapple flowers can be hastened by ethylene.
It breaks the dormancy of buds, seeds and storage organs of the plants.
It increases the dormancy of lateral buds and improves apical dominance.
Uses of Ethylene
Ethylene is principally used in the agricultural industry due to its varying number of physiological processes. It is used and a plant-growth regulator.
Due to the effect of inducing female flowers in male plants, it is used to increase female flowers so that the production of fruits is increased.
Early sprouting in seeds, rhizomes and tubers are noticed when ethylene is applied to them.
Excess flowers and young fruits such as cotton, walnuts can be thinned out by using ethylene.
Ethylene is also responsible for interfering with auxin transport.
Main hormone in flower induction, flower opening, and fruit ripening.
It inhibits stem elongation.
It is also engaged with a positive feedback mechanism. Whenever a stimulus is taken by the plant, ethylene amplifies the stimulus and produces changes in the plant system.
Apart from using ethylene as a plant-based hormone, it has other uses, such as plastic production in several industrial areas.
It is used for the production of car glasses in the automobile industry due to its durability.
It is also used in the metal industry in metal cutting, high-velocity thermal spraying, and welding.
In medicine, it can also be used as a gas anesthetic for general surgery cases.
In LNG liquefication plants, ethylene is used as a refrigerant.
It is used in the extraction of rubber.
It is the raw material used to produce polyethylene, polystyrene and polyvinyl chloride (PVC). Polythene is a polymerized compound of ethylene which is widely used to manufacture toys, plastic utensils, cable insulations, bags, and boxes.
Ethylene reacts with water in the presence of a catalyst and produces ethanol. Ethanol is the main component of alcoholic beverages and also used in inks, pharmaceuticals, and cosmetics.
It is used as an anti-freezing compound in car radiators, the main component, however, used is ethylene glycol which is a derivative of ethylene.