A Volatile Growth Inhibitor
Ethylene, a growth inhibitor, is the simplest known olefin. It is lighter than air under physiological conditions. It is flammable and readily undergoes oxidation to ethylene oxide, and can be hydrolyzed to ethylene glycol. In most plant tissues ethylene can be completely oxidized to CO2. Ethylene is released easily from the tissue and diffuses in gas phase through intercellular spaces and outside the tissue. It is a compound of agriculture importance and stimulates fruit ripening in most of the plants.
History of Discovery
During 19th century, when coal gas was used for street illumination, it was observed that the leaves of the trees growing near the street lamps fall off earlier than other trees. It become apparent that this is due to coal gas that polluted the air. In 1901 a Russian student identified ethylene as the active component of the coal gas.
Later, Dimitry N. Neljubow observed that pea seedlings grown in dark in the laboratory showed reduced stem elongation, increased lateral growth, and abnormal horizontal growth. These conditions were termed triple response. When the plants were grown in fresh air they showed normal growth. The laboratory air was tested for any pollutant and ethylene was found in it.
The role of certain gases to stimulate fruit ripening has been known for many years. H. H Cousins in 1910 reported that oranges should not stored with bananas on ships, because some gas coming out of oranges causes bananas to ripen prematurely. This was first indication that fruits release a gas that stimulates the ripening of fruits. But it was up to R. Gane, who proved that ethylene is synthesized by plants and is responsible for faster ripening. The ethylene was found to promote flowering in mangoes and pineapple.
But, ethylene was not identified as an important plant hormone for 25 years, mainly because many physiologists believed that the effects of the ethylene could be mediated by auxin. They were of the view that ethylene had only an indirect physiological role. However, when gas chromatography was introduced in 1959, ethylene was rediscovered and its physiological significance as plant growth regulator was recognized.
Biosynthesis & Distribution of Ethylene
Ethylene is found in almost all angiosperms, gymnosperms and lower plants including ferns, mosses, liverworts and certain cyanobacteria have also shown ability to produce ethylene. Bacteria and fungi add ethylene to the soil environment. Ethylene is biologically active at very low concentrations. The highest ethylene level is found in senescence tissues or ripening fruits, but all organs of higher plants can synthesize ethylene. Young developing leaves produce more ethylene than mature fully expanded leaves. Ethylene levels increase in wounded tissue.
Ethylene is derived from carbon 3 and 4 amino acid methionine in higher plants.
Methionine is converted into S-adenosyimethionine (SAM) in the presence of ATP.
SAM is converted into 1-aminocyclopropane-1-carboxylic acid (ACC). The enzyme ACC synthase catalyze the conversion of SAM to ACC.
Oxygen promotes the synthesis of ethylene from 1-aminocyclopropane-1-carboxylic acid. An enzyme yet to be isolated, the ethylene forming enzyme (EFE) is thought to catalyze conversion of ACC to ethylene.
Ethylene biosynthesis is stimulated by several factors. Level of ethylene increases as the fruit develops and matures. Stress condition such as drought, flooding, chilling or mechanical wounding increase biosynthesis of ethylene.
Physiological Effects of Ethylene
Ethylene has numerous effects on different plant species and organs. These include are:
Effect on Fruit Ripening
Ethylene accelerate fruit ripening. Addition of ethylene to fruits hasten the fruit ripening. Similarly synthesis of ethylene increases with initiation of ripening. Ethylene biosynthesis inhibitors delay or even prevent ripening. All these observations indicate that ethylene is the main agent controlling ripening. It is found that ripening of apples, bananas and tomatoes is characterized by rise in respiration and ethylene production.
Effect on Abscission
The shedding of leaves, fruits, flowers and other plant organ is termed abscission. Ethylene primarily regulates the abscission process. The auxin suppresses the effect of ethylene and prevents the abscission. Ethylene appears to decrease the activity of auxin both by reducing its synthesis and transport and increasing its destruction. High ethylene levels promote production of specific hydrolytic enzymes that digest cell walls and promote abscission.
Effect on Epinasty
The downward curvature of the leaves that occurs when the upper side of the petiole grows faster than the lower side termed epinasty. Epinasty is induced by ethylene and high concentration of auxin, but the action of auxin is indirect as it induces synthesis of ethylene and primary effector is the ethylene. Waterlogged or anaerobic conditions around roots in tomatoes result in enhanced production of ethylene in the shoot causes epinasty.
Effect on Seedling Growth
Ethylene changes the growth pattern of seedling by reducing the rate of elongation and increasing lateral expansion, leading to swelling of region below thee hook. These responses to ethylene are common to growing shoots of most dicots and to coleoptiles of members of Poaceae such as wheat or oat. Ethylene inhibits the elongation and lateral expansion of cells by altering the arrangement of cellulose microfibrils of cell wall from transverse to longitudinal orientation by ethylene.
Ethylene production in germinating seedling induce horizontal growth and it helps seedlings to find suitable place to come out of the soil.
The terminal shoot-apex of dark grown seedlings usually becomes hook-shaped. This shape facilitate displacement of the seedling through the ground and protects the tender apical meristem. The hook closure and opening is induced by ethylene due to asymmetric growth. The outer side grows at faster rate than the inner one. When the hook is exposed, it opens because the rate of growth of its inner side increases.
Effect on Seed & Bud Dormancy
The ethylene, when applied to dormant seeds such as those of cereals, break dormancy and initiates germination. In peanut, ethylene production and germination are closely related . Ethylene also increase the rate of seed germination in several species. Bid dormancy may also be broken by ethylene. Ethylene is used to promote sprouting in potato tubers and other bulbs.
Growth Promotion – Stem Elongation
Ethylene promotes stem elongation in several monocots such as rice, in contrast to its common role in inhibition of elongation in most seedlings. In rice seedling waterlogged conditions result in anaerobic environment that inhibit ethylene synthesis in roots. The available ethylene diffuses very slowly in waterlogged soil. So the submerged rice plants are exposed to high ethylene concentrations that increase stem elongation.
Induction of Roots
Ethylene is capable of inducing root formation in leaves, stems, flower stems and even other roots, when present in high concentrations.
Ethylene inhibits flowering in most species, but induces flowering in pineapples and is used commercially for fruit set in this species. Similarly, flowering in mango is also initiated by ethylene. On plants that have separate male and female flowers, ethylene may change the sex of developing flowers, e.g., formation of female flowers is promoted by ethylene in cucumber.
Flower & Leaf Senescence
Onset of leaf and flower senescence is hastened by ethylene and can be delayed by carbon dioxide. The ethylene synthesis is associated with chlorophyll loss and color fading (etiolation).
Applications of Ethylene in Agriculture
Since ethylene regulates many physiological processes in plant development, therefore it is widely used plant hormone in agriculture.
Ethylene has a high diffusion rate, because of its high diffusion rate ethylene cannot be used in its proper form, i.e., gas form in the field, therefore, ethylene releasing compounds such as ethephon (commercial name being ethylene) is sprayed in aqueous solution. Ethephon hastens fruit ripening of apples and tomatoes flowering and fruit set in pineapples and acceleration abscission of flowers and fruits.
Ethylene (ethephon) promoted fruit thinning or fruit drop in cotton, cherries and walnuts.
Ethylene is also used to promote female flowering in cucumber, to prevent self-pollination and increase yield, and to inhibit terminal growth of some plants in order to promote lateral growth and compact flowering.
Storage facilities can be developed by inhibiting ethylene production by creating a atmosphere containing low oxygen concentration and low temperature. Similarly, a high carbon dioxide concentration prevents ethylene action as ripening promoter. Low pressure is also used to remove ethylene from storage chambers thus reducing the rate of ripening and over-ripening.