Properties and application of ethylene. Physical and chemical properties of ethylene
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Ethylene began to be widely used as a monomer before World War II due to the need to obtain a high-quality insulating material that could replace polyvinyl chloride. After the development of a method for the polymerization of ethylene under high pressure and the study of the dielectric properties of the resulting polyethylene, its production began, first in the UK, and later in other countries.
The main industrial method for producing ethylene is the pyrolysis of liquid petroleum distillates or lower saturated hydrocarbons. The reaction is carried out in tube furnaces at +800-950 °C and a pressure of 0.3 MPa. When straight-run gasoline is used as a raw material, the ethylene yield is approximately 30%. Simultaneously with ethylene, a significant amount of liquid hydrocarbons, including aromatic ones, is also formed. During the pyrolysis of gas oil, the yield of ethylene is approximately 15-25%. The highest yield of ethylene - up to 50% - is achieved when saturated hydrocarbons are used as raw materials: ethane, propane and butane. Their pyrolysis is carried out in the presence of steam.
When released from production, during commodity accounting operations, when checking it for compliance with regulatory and technical documentation, ethylene samples are taken according to the procedure described in GOST 24975.0-89 “Ethylene and propylene. Sampling methods". Ethylene sampling can be carried out both in gaseous and liquefied form in special samplers in accordance with GOST 14921.
Ethylene produced industrially in Russia must comply with the requirements set forth in GOST 25070-2013 “Ethylene. Specifications".
Production structure
Currently, in the structure of ethylene production, 64% falls on large-tonnage pyrolysis plants, ~17% - on small-tonnage gas pyrolysis plants, ~11% is gasoline pyrolysis, and 8% falls on ethane pyrolysis.
Application
Ethylene is the leading product of the main organic synthesis and is used to obtain the following compounds (listed in alphabetical order):
- Dichloroethane / vinyl chloride (3rd place, 12% of the total volume);
- Ethylene oxide (2nd place, 14-15% of the total volume);
- Polyethylene (1st place, up to 60% of the total volume);
Ethylene mixed with oxygen was used in medicine for anesthesia until the mid-1980s in the USSR and the Middle East. Ethylene is a phytohormone in almost all plants, among other things, it is responsible for the fall of needles in conifers.
Electronic and spatial structure of the molecule
Carbon atoms are in the second valence state (sp 2 hybridization). As a result, three hybrid clouds are formed on the plane at an angle of 120°, which form three σ-bonds with carbon and two hydrogen atoms; p-electron, which did not participate in hybridization, forms a π-bond with the p-electron of the neighboring carbon atom in the perpendicular plane. This forms a double bond between carbon atoms. The molecule has a planar structure.
Basic chemical properties
Ethylene is a chemically active substance. Since in the molecule between the carbon atoms there is double bond, then one of them, less strong, is easily torn, and at the place of bond rupture, attachment, oxidation, and polymerization of molecules occur.
- Halogenation:
- Hydrogenation:
- Hydrohalogenation:
- Hydration:
- Oxidation:
- Combustion:
- Polymerization (obtaining polyethylene):
Biological role
Ethylene is the first of the discovered gaseous plant hormones, which has a very wide range of biological effects. Ethylene performs a variety of functions in the life cycle of plants, including control of seedling development, ripening of fruits (in particular, fruits), blooming of buds (flowering process), aging and falling of leaves and flowers. Ethylene is also called the stress hormone, since it is involved in the response of plants to biotic and abiotic stress, and its synthesis in plant organs is enhanced in response to different kind damage. In addition, being a volatile gaseous substance, ethylene provides rapid communication between different plant organs and between plants in a population, which is important. in particular, during the development of stress tolerance.
Among the best known functions of ethylene is the development of the so-called triple response in etiolated (grown in the dark) seedlings upon treatment with this hormone. The triple response includes three reactions: shortening and thickening of the hypocotyl, shortening of the root, and strengthening of the apical hook (a sharp bend in the upper part of the hypocotyl). The response of seedlings to ethylene is extremely important at the first stages of their development, as it contributes to the penetration of seedlings towards the light.
In the commercial harvesting of fruits and fruits, special rooms or chambers are used for ripening fruits, into the atmosphere of which ethylene is injected from special catalytic generators that produce gaseous ethylene from liquid ethanol. Usually, to stimulate the ripening of fruits, the concentration of gaseous ethylene in the chamber atmosphere is from 500 to 2000 ppm for 24-48 hours. At a higher air temperature and a higher concentration of ethylene in the air, fruit ripening is faster. It is important, however, to ensure control of the carbon dioxide content in the atmosphere of the chamber, since high-temperature ripening (at temperatures above 20 degrees Celsius) or ripening at a high concentration of ethylene in the air of the chamber leads to a sharp increase in the release of carbon dioxide by rapidly ripening fruits, sometimes up to 10%. carbon dioxide in the air after 24 hours from the start of ripening, which can lead to carbon dioxide poisoning of both workers who harvest already ripened fruits, and the fruits themselves.
Ethylene has been used to stimulate fruit ripening since Ancient Egypt. The ancient Egyptians intentionally scratched or slightly crushed, beat off dates, figs and other fruits in order to stimulate their ripening (tissue damage stimulates the formation of ethylene by plant tissues). The ancient Chinese burned wooden incense sticks or scented candles indoors to stimulate the ripening of peaches (burning candles or wood releases not only carbon dioxide, but also incompletely oxidized intermediate combustion products, including ethylene). In 1864, it was discovered that natural gas leaking from street lamps caused growth inhibition in the length of nearby plants, their twisting, abnormal thickening of stems and roots, and accelerated fruit ripening. In 1901, Russian scientist Dmitry Nelyubov showed that the active component of natural gas that causes these changes is not its main component, methane, but ethylene present in it in small quantities. Later in 1917, Sarah Dubt proved that ethylene stimulated premature leaf drop. However, it was not until 1934 that Gein discovered that plants themselves synthesize endogenous ethylene. . In 1935, Crocker proposed that ethylene is a plant hormone responsible for the physiological regulation of fruit ripening, as well as senescence of the plant's vegetative tissues, leaf fall, and growth inhibition.
The ethylene biosynthetic cycle begins with the conversion of the amino acid methionine to S-adenosyl methionine (SAMe) by the enzyme methionine adenosyl transferase. Then S-adenosyl-methionine is converted to 1-aminocyclopropane-1-carboxylic acid (ACA, ACC) using the enzyme 1-aminocyclopropane-1-carboxylate synthetase (ACC synthetase). The activity of ACC synthetase limits the rate of the entire cycle; therefore, the regulation of the activity of this enzyme is key in the regulation of ethylene biosynthesis in plants. The last step in ethylene biosynthesis requires oxygen and occurs through the action of the enzyme aminocyclopropane carboxylate oxidase (ACC oxidase), formerly known as the ethylene-forming enzyme. Ethylene biosynthesis in plants is induced by both exogenous and endogenous ethylene (positive Feedback). The activity of ACC synthetase and, accordingly, the formation of ethylene also increases with high levels auxins, especially indoleacetic acid, and cytokinins.
The ethylene signal in plants is perceived by at least five different families of transmembrane receptors, which are protein dimers. Known, in particular, the ethylene receptor ETR 1 in Arabidopsis ( Arabidopsis). The genes encoding ethylene receptors have been cloned in Arabidopsis and then in tomato. Ethylene receptors are encoded by multiple genes in both Arabidopsis and tomato genomes. Mutations in any of the gene family, which consists of five types of ethylene receptors in Arabidopsis and at least six types of receptors in tomato, can lead to plant insensitivity to ethylene and disruption of the processes of maturation, growth and wilting. DNA sequences characteristic of ethylene receptor genes have also been found in many other plant species. Moreover, ethylene-binding protein has even been found in cyanobacteria.
Adverse external factors, such as insufficient oxygen content in the atmosphere, flood, drought, frost, mechanical damage (injury) of the plant, attack by pathogenic microorganisms, fungi or insects, can cause advanced education ethylene in plant tissues. So, for example, during a flood, the roots of a plant suffer from an excess of water and a lack of oxygen (hypoxia), which leads to the biosynthesis of 1-aminocyclopropane-1-carboxylic acid in them. ACC is then transported along pathways in the stems up to the leaves and oxidized to ethylene in the leaves. The resulting ethylene promotes epinastic movements, leading to mechanical shaking of water from the leaves, as well as wilting and falling of leaves, flower petals and fruits, which allows the plant to simultaneously get rid of excess water in the body and reduce the need for oxygen by reducing the total mass of tissues.
Small amounts of endogenous ethylene are also formed in animal cells, including humans, during lipid peroxidation. Some endogenous ethylene is then oxidized to ethylene oxide, which has the ability to alkylate DNA and proteins, including hemoglobin (forming a specific adduct with the N-terminal valine of hemoglobin, N-hydroxyethyl-valine). Endogenous ethylene oxide can also alkylate the guanine bases of DNA, which leads to the formation of the 7-(2-hydroxyethyl)-guanine adduct, and is one of the reasons for the inherent risk of endogenous carcinogenesis in all living beings. Endogenous ethylene oxide is also a mutagen. On the other hand, there is a hypothesis that if it were not for the formation of small amounts of endogenous ethylene and, accordingly, ethylene oxide in the body, the rate of spontaneous mutations and, accordingly, the rate of evolution would be much lower.
Notes
- DevanneyMichael T. Ethylene(English) . SRI Consulting (September 2009). Archived from the original on August 21, 2011.
- Ethylene(English) . WP Report. SRI Consulting (January 2010). Archived from the original on August 21, 2011.
- Gas chromatographic measurement of mass concentrations of hydrocarbons: methane, ethane, ethylene, propane, propylene, butane, alpha-butylene, isopentane in the air of the working area. Methodological instructions. MUK 4.1.1306-03 (Approved by the chief state sanitary doctor of the Russian Federation on March 30, 2003)
- "Growth and development of plants" V. V. Chub (indefinite) (unavailable link). Retrieved January 21, 2007. Archived from the original on January 20, 2007.
- "Delaying Christmas tree needle loss"
- Khomchenko G.P. §16.6. Ethylene and its homologues// Chemistry for applicants to universities. - 2nd ed. - M.: Higher school, 1993. - S. 345. - 447 p. - ISBN 5-06-002965-4.
- V. Sh. Feldblum. Dimerization and disproportionation of olefins. Moscow: Chemistry, 1978
- Lin, Z.; Zhong, S.; Grierson, D. (2009). “Recent advances in ethylene research”. J. Exp. bot. 60 (12): 3311-36. DOI:10.1093/jxb/erp204. PMID.
- Ethylene and Fruit Ripening / J Plant Growth Regul (2007) 26:143-159 doi:10.1007/s00344-007-9002-y
- Lutova L.A. Genetics of plant development / ed. S.G. Inge-Vechtomov. - 2nd ed. - St. Petersburg: N-L, 2010. - S. 432.
- . ne-postharvest.com
A bright representative of unsaturated hydrocarbons is ethene (ethylene). Physical properties: Colorless flammable gas, explosive when mixed with oxygen and air. Significant amounts of ethylene are obtained from petroleum for the subsequent synthesis of valuable organic substances (monohydric and dihydric alcohols, polymers, acetic acid, and other compounds).
ethylene, sp 2 -hybridization
Hydrocarbons similar in structure and properties to ethene are called alkenes. Historically, another term for this group has been fixed - olefins. The general formula C n H 2n reflects the composition of the entire class of substances. Its first representative is ethylene, in the molecule of which carbon atoms form not three, but only two x-bonds with hydrogen. Alkenes are unsaturated or unsaturated compounds, their formula is C 2 H 4 . Only the 2 p- and 1 s-electron cloud of the carbon atom mix in shape and energy, in total three õ-bonds are formed. This state is called sp2 hybridization. The fourth valence of carbon is preserved, a π-bond appears in the molecule. In the structural formula, the feature of the structure is reflected. But the symbols for designating different types of connections in the diagrams are usually used the same - dashes or dots. The structure of ethylene determines its active interaction with substances of different classes. The attachment of water and other particles occurs due to the breaking of a fragile π-bond. The released valences are saturated due to the electrons of oxygen, hydrogen, halogens.
Ethylene: physical properties of matter
Ethene under normal conditions (normal atmospheric pressure and temperature of 18°C) - colorless gas. It has a sweet (ethereal) odor, its inhalation has a narcotic effect on a person. Solidifies at -169.5°C, melts under the same temperature conditions. Ethene boils at -103.8°C. Ignites when heated to 540°C. The gas burns well, the flame is luminous, with a weak soot. Ethylene is soluble in ether and acetone, much less so in water and alcohol. The rounded molar mass of the substance is 28 g/mol. The third and fourth representatives of the ethene homologous series are also gaseous substances. The physical properties of the fifth and following alkenes are different, they are liquids and solids.
Preparation and properties of ethylene
German chemist Johann Becher accidentally used concentrated sulfuric acid in experiments. So for the first time ethene was obtained in laboratory conditions (1680). In the middle of the 19th century, A.M. Butlerov named the compound ethylene. Physical properties and were also described by a famous Russian chemist. Butlerov proposed a structural formula reflecting the structure of matter. Methods for obtaining it in the laboratory:
- Catalytic hydrogenation of acetylene.
- Dehydrohalogenation of chloroethane in reaction with a concentrated alcoholic solution of a strong base (alkali) when heated.
- Cleavage of water from ethyl molecules The reaction takes place in the presence of sulfuric acid. Its equation is: H2C-CH2-OH → H2C=CH2 + H2O
Industrial receiving:
- oil refining - cracking and pyrolysis of hydrocarbon raw materials;
- dehydrogenation of ethane in the presence of a catalyst. H 3 C-CH 3 → H 2 C \u003d CH 2 + H 2
The structure of ethylene explains its typical chemical reactions - the addition of particles by C atoms, which are in a multiple bond:
- Halogenation and hydrohalogenation. The products of these reactions are halogen derivatives.
- Hydrogenation (saturation of ethane.
- Oxidation to dihydric alcohol ethylene glycol. Its formula is: OH-H2C-CH2-OH.
- Polymerization according to the scheme: n(H2C=CH2) → n(-H2C-CH2-).
Applications for ethylene
When fractionated in large volumes The physical properties, structure, chemical nature of the substance make it possible to use it in the production of ethyl alcohol, halogen derivatives, alcohols, oxide, acetic acid and other compounds. Ethene is a monomer of polyethylene and also the parent compound for polystyrene.
Dichloroethane, which is obtained from ethene and chlorine, is a good solvent used in the production of polyvinyl chloride (PVC). Made of low polyurethane high pressure they make film, pipes, dishes, from polystyrene - cases for CDs and other details. PVC is the basis of linoleum, waterproof raincoats. In agriculture, fruits are treated with ethene before harvest to speed up ripening.
The hormonal regulatory system is one of the most important systems in plants and includes phytohormones. Phytohormones are compounds with the help of which the interaction of cells, tissues and organs is carried out and which in small quantities are necessary for the launch and regulation of physiological and morphogenetic programs. Plant hormones are relatively low molecular weight organic matter. They are formed in various fabrics and organs and act in very low concentrations of the order of 10 -13 -10 -5 mol/l.
All phytohormones are divided into stimulants and inhibitors. Inhibitors (from the Latin “Inhibeo” - I stop, restrain) in biology, natural and synthetic substances that inhibit the activity of enzymes (both in the body and in cell-free systems); differ in the nature of the action, specificity, and other properties. Ethylene is a growth inhibitor. A number of compounds have a similar effect on the plant, but are inferior to it in efficiency. Ethylene is the only gaseous plant growth regulator.
Ethylene gas (C2H4) is rightly referred to as plant hormones, since it is synthesized in plants and regulates their growth at extremely low concentrations, activates fruit ripening, causes aging of leaves and flowers, leaf and fruit abscission, participates in the response of plants to various stress factors and in the regulation of many other important events in the life of a plant (Kulaeva, 1995). Ethylene, more precisely, ethylene producers - compounds, the destruction of which is accompanied by the release of ethylene, are widely used in practice. Agriculture. All this determines the great attention of biochemists, physiologists, geneticists, molecular biologists and practitioners to the study of ethylene.
AT last years Great progress has been made in obtaining and studying mutant plants insensitive to ethylene. These mutants have provided progress in isolating the genes responsible for the perception and transmission of the ethylene signal in plants, and helped to partially decipher the molecular pathways through which the signal passes, causing the activation or suppression of certain physiological programs. This success prompted the author to write an article about ethylene. Its purpose is to consider the regulatory role of ethylene in plants, its practical application, the features of its biosynthesis, as well as the latest data on the mechanism of action of this phytohormone.
The history of the discovery of ethylene
Ethylene was first obtained by the German chemist Johann Becher in 1680 by the action of vitriol oil on alcohol of wine. Initially, it was identified with "combustible air", i.e., with hydrogen. Later, in 1795, the Dutch chemists Deyman, Pots-van-Trusvik, Bond and Lauerenburg similarly obtained ethylene and described it under the name "oil gas", since they discovered the ability of ethylene to add chlorine to form an oily liquid - ethylene chloride ("oil of Dutch chemists").
The study of the properties of ethylene, its derivatives and homologues began in the middle of the 19th century. The beginning of the practical use of these compounds was laid by the classical studies of A.M. Butlerov and his students in the field of unsaturated compounds and especially Butlerov's creation of the theory of chemical structure. In 1860, he obtained ethylene by the action of copper on methylene iodide, establishing the structure of ethylene.
In 1901, Dmitry Nikolaevich Nelyubov grew peas in a laboratory in St. Petersburg, but the seeds produced twisted, shortened seedlings, in which the top was bent with a hook and did not bend. In the greenhouse and in the open air, the seedlings were even, tall, and the top in the light quickly straightened the hook. Nelyubov suggested that the factor causing the physiological effect is in the laboratory air.
At that time, the premises were lit with gas. The same gas burned in the street lamps, and it has long been noticed that in the event of an accident in a gas pipeline, trees standing near the site of a gas leak turn yellow prematurely and shed their leaves.
The lighting gas contained a variety of organic substances. To remove the admixture of gas, Nelyubov passed it through a heated tube with copper oxide. Pea seedlings developed normally in "purified" air. In order to find out exactly which substance causes the response of seedlings, Nelyubov added various components of the lighting gas in turn, and found that the addition of ethylene causes:
1) slow growth in length and thickening of the seedling,
2) "non-bending" apical loop,
3) change in the orientation of the seedling in space.
This physiological response seedlings has been called a triple response to ethylene. Peas were so sensitive to ethylene that they began to use them in bioassays to detect low concentrations of this gas. It was soon discovered that ethylene also causes other effects: leaf fall, fruit ripening, etc. It turned out that plants themselves are capable of synthesizing ethylene; ethylene is a phytohormone.
Physiological role of ethylene
Ethylene properties
Ethylene is a colorless gas with a faint, barely perceptible odor. It is poorly soluble in water (at 0 0 25.6 ml of ethylene dissolves in 100 g of water), burns with a luminous flame, and forms explosive mixtures with air. Thermally less stable than methane. Already at temperatures above 350 0, ethylene partially decomposes into methane and acetylene. At a temperature of about 1200 0, it dissociates mainly into acetyl and hydrogen.
Ethylene does not occur in natural gases (with the exception of volcanic gases). It is formed mainly during the pyrogenetic decomposition of natural compounds containing organic substances.
In very low concentrations, about 0.001-0.1 µl/l, it is able to slow down and change the nature of plant growth, accelerate the ripening of fruits. Ethylene is synthesized in bacteria, fungi, lower and higher plants, and in large quantities. Not all organisms are capable of synthesizing ethylene. Thus, of the studied 228 species of microscopic fungi, only 25% emit ethylene. Organisms control the rate of ethylene synthesis. Thus, its concentration is regulated, in addition, an excess of ethylene can freely diffuse into environment. The rate of ethylene formation is different in different organs and systems. The formation of ethylene increases with aging and abscission of leaves and fruits. It is inhibited by a lack of oxygen (in all agricultural plants except rice) and can be regulated by temperature and light. Affects the synthesis of ethylene and the level of CO 2 . Moreover, in different plants, carbon dioxide can both stimulate and inhibit the formation of ethylene.
As shown in the experiments of D.N. Nelyubov, ethylene inhibits the growth of the stem in length and causes its thickening. Subsequently, scientists found that this occurs due to a change in the direction of growth of stem cells, which corresponds to a change in the orientation of elements of the cytoskeleton. Ethylene inhibits root growth, accelerates aging, which can be clearly seen on the leaves and flowers of plants. Ethylene also accelerates the ripening of fruits, causes leaf and fruit abscission. It induces the formation of a special separating layer of cells in the petiole, along which the leaf is torn off from the plant, and instead of the wound, an ethylene-induced protective layer of cells with corky walls remains at the place of rupture. This phytohormone affects the sex of flowers, causing the formation of female flowers in plants that are characterized by separate female and male flowers, such as cucumber, pumpkin and squash.
The formation of roots on the stem and the formation of a special tissue in the stem - aerenchyma, through which oxygen enters the roots, are induced by ethylene. This saves plants in conditions of oxygen starvation of the roots, in which they fall when the soil is flooded. In addition, ethylene causes other changes in plants. For example, epinasty, which changes the angle of the leaf in relation to the stem (leaves fall).
Ethylene is also involved in plant responses to various damaging effects - mechanical, chemical and biological. It is involved in the response of plants to the attack of pathogens. Ethylene enables plant defense systems against pathogens. At the same time, it induces the synthesis of a large number of enzymes, for example, enzymes that destroy cell wall fungi (chitinases, specific glucanases), as well as enzymes involved in the synthesis of phytoalexins - compounds that are toxic to the pathogen.
When plants are injured, ethylene is synthesized and released. There is evidence that when the leaves of woody plants are eaten by animals, the eaten plant releases ethylene and, under its influence, substances can be synthesized in the leaves of neighboring plants that make the leaves tasteless to animals.
Biosynthesis of ethylene
The key compound for ethylene biosynthesis in plants is the amino acid methionine. When methionine interacts with the macroergic compound ATP, an intermediate product S-adenosylmethionine arises, which is then converted into 1-aminocyclopropane-1-carboxylic acid (ACC), the direct precursor of ethylene in plants. Then, ACC decomposes in the presence of oxygen to form ethylene, ammonia, formic acid, and CO2. Each step is catalyzed by a specific enzyme. The key enzyme at the level of which ethylene biosynthesis is regulated is ACC synthase. ACC synthase is not constantly synthesized in cells, but is induced by inducers - substances that cause its synthesis. Such enzymes are called inducible. The synthesis of ACC synthase is induced by high concentrations of auxin, molecules - chemical signals of fungal infection, and ethylene itself. The synthesis of ACC synthase continues as long as the inducer is present. Then the synthesis stops, and the formed enzyme molecules are rapidly destroyed, since their half-life is 20-30 minutes. This emphasizes how tightly the plant controls ethylene synthesis at the level of formation and destruction of the key enzyme of ACC synthase biosynthesis.
It is significant that in the plant genome there is a large family of ACC synthase genes that differ in their regulation: some are switched on at different stages of normal plant development, others - when injured, others - under the action of a pathogen, etc. This provides a multifactorial system for the regulation of ethylene synthesis in plants. The genes for ACC synthase and ACC oxidase attract a lot of attention from genetic engineers, since modification of plants by these genes makes it possible to regulate ethylene synthesis and, consequently, regulate the rate of fruit ripening. Along the way, American genetic engineers have obtained transgenic tomato plants with a month-long fruit shelf life.
The next step in ethylene biosynthesis is the oxidation of ACC. It is oxygen-dependent and does not proceed under conditions of oxygen starvation (anaerobiosis). This situation occurs in the roots when the soil is flooded. Without oxygen, root respiration, ATP synthesis and related processes are suppressed. The supply of shoots with water, mineral nutrients, hormones (cytokinins) and other waste products of the root is disrupted. All this threatens the death of plants. This is where the ethylene protection system kicks in. Under conditions of anaerobiosis, the conversion of ACC into ethylene in the roots stops. ACC enters as part of the sap - a solution that comes from the roots to the shoots, to the above-ground organs, where there is no lack of O2, and turns into ethylene there. Ethylene induces epinasty in shoots - a change in the angle of inclination of the petiole to the stem, as a result of which the leaves go down, away from the direct action of sunlight. At the same time, the leaves heat up less and evaporate less water. Ethylene induces the formation of roots on the stems, which do not perform an absorbing function, but carry out specific synthetic processes necessary for the normal functioning of the shoot, including restoring the supply of cytokinins to aboveground organs. In addition, ethylene induces the formation of aerenchyma in the stem - a tissue through which O2 enters the roots from the stems and ensures their normal vital activity. This example illustrates well how ethylene provides plant adaptation to the conditions of oxygen deficiency in the root zone that occurs when the soil is flooded.
During the normal course of plant life, ethylene is actively synthesized in ripening fruits and aging leaves. This is understandable: it induces fruit ripening, senescence and leaf fall. However, a high level of ethylene synthesis is also characteristic of meristematic tissues - zones of cell division. This is still difficult to explain. The synthesis of ethylene in plants is caused by high concentrations of auxin, which occurs at the level of induction of ACC synthase genes. The synthesized ethylene suppresses the reactions caused by auxin. For example, in a certain range of concentrations, auxin activates root growth. Their excess induces the synthesis of ethylene, which inhibits root growth. Thus, ethylene is included in the control of the action of auxin by the plant according to the feedback principle. Ethylene plays the same role in plant responses to high concentrations of cytokinins.
Ethylene as a mechanical stress hormone
The release of ethylene is closely related to the mechanical effect on plant cells. Let us take the example of the response of a pea seedling observed by Nelyubov. Until the germ reaches the surface, the delicate cells of the apical meristem must be protected from damage. Therefore, there is a bend and the formation of an apical loop. It is not the meristem that grows through the soil, but a stronger underlying area.
When a mechanical obstacle (stone) appears on the path of the seedling, the seedling releases more ethylene, growth in length stops and thickening begins. The seedling seeks to overcome the obstacle by increasing the pressure. If this is successful, the ethylene concentration drops and the growth in length is restored. But if the obstacle is too large, then the production of ethylene is further enhanced. The seedling deviates from the vertical and goes around the pebble.
AT air environment the ethylene concentration falls, the seedlings unbend the apical meristem, and leaf development begins.
Ethylene and touch
Until 1991, plant physiologists had rather sketchy ideas about exactly how plants feel touch. By subtracting c-DNA libraries, it was found that spraying Arabidopsis thaliana plants with water causes the synthesis of new messenger RNAs - after 10-15 minutes their level rose hundreds of times.
Spraying is a complex factor: the humidity of the air changes, a shadow is created from water vapor, and, finally, the leaves are subjected to mechanical stress. Each of the factors was studied separately. It turned out that humidity does not play any role, but if the plant is rubbed with a glass rod, it will feel it and in 10-15 minutes it will respond with the expression of new mRNAs. The discovered genes were designated as TCH1, TCH2, TCH3, TCH4, TCH5 (from English touch).
If, without touching the plant, you suddenly cover it with a black cap, then the level of TCH matrices in it also increases. The creation of sufficiently powerful sound effects did not lead to the desired result: TCH messenger RNAs did not appear in the cells.
What are the genes responsible for, the products of which appear in cells when touched? They turned out to be very similar to the known calcium-binding proteins - calmodulins. These proteins, together with Ca 2+, activate the work of the cytoskeleton and promote the transition from sol to gel of many structures in the plant cell. Plants that were often disturbed with a glass rod noticeably lag behind in growth from those that were not touched, however, they turn out to be mechanically stronger and hardened.
The protein product of the TCH 4 gene turned out to be xyloglucan endotransglycosylase. The synthesis of this protein can also be induced by brassinosteroids. The same effects can be produced by adding ethylene. At the same time, the synthesis of Ca-binding TCH proteins also occurs.
Ethylene and wound healing
Many plants form lactifers that contain latex (natural rubber). However, the rubber does not "freeze" inside the milkers (just as the blood does not coagulate in the vessels). But as soon as the plant is damaged, latex appears on the surface, which quickly hardens and clogs the place of damage. Latex glues the spores of fungi and bacteria, freezes in the mouth apparatus of insects or sticks them to a drop of rubber that has come out.
For a long time, nothing would have been known about what causes latex to harden quickly when a plant is damaged, if it were not for the demands of agriculture. On hevea plantations, the hardening of latex is a harmful process: you have to re-notch tree trunks, substitute vessels for collecting rubber in new places, which creates a lot of unnecessary work.
It turned out that the latex hardens under the action of ethylene. Important role at the same time, the minor protein of latex, hevein, plays. Latex curing can be controlled to some extent by treating plants with ethylene synthesis inhibitors. The most well-known inhibitor is silver ions, but there are also cheaper ones. Thus, in rubber plants, ethylene promotes the healing of mechanical damage.
In addition, under the action of ethylene, a special tissue, the wound periderm, is activated. A cork cambium is formed, which forms a layer of suberinized cork that separates healthy (live) tissue from diseased (dead) tissue. The cork is highly hydrophobic, which makes it possible to effectively stop the spread of fungi and bacteria that have entered the wound, and protects healthy tissue from excessive evaporation.
The size and place of formation of the wound periderm differ in different plants. So the lungwort forms a wound periderm a few millimeters from the damaged zone (for example, by fungi). The leaf area surrounded by the wounded periderm falls out.
In the bean, the wounded periderm at the base of the leaf blade is activated, and the plant sacrifices the damaged part of the compound leaf for the safety of the whole plant.
It would seem that wound periderm can be useful only when attacked by bacteria and fungi. However, when attacked by insects and ticks, it plays an important role. Under the action of ethylene, local "leaf fall" occurs - the damaged leaf falls to the ground along with the pest. Pests have less chances to get to the crown again. Protective "leaf fall" is observed, for example, in roses when attacked by a spider mite.
Leaf fall regulation in temperate latitudes
Ethylene regulates the phenomenon of leaf fall. This reaction impressed plant physiologists so much that ethylene is sometimes considered the hormone of plant aging. The phenomenon of leaf fall is not just aging. So, in the tropics, individual leaves live 3-4 years (often more). The shortening of leaf life is associated with a protective reaction to mechanical stress.
When the leaves fall, many open wounds form at the attachment points. In order for the leaf to separate without harm to the whole plant, a separating layer is formed at its base. Its work is almost identical to that of the wound periderm. The place of future damage is closed with a cork, the overlying tissue loosens and becomes fragile, the sheet falls off. To loosen the cell wall, pectinases are secreted into it. During the splitting of pectin, physiologically active substances are released - oligosaccharins, which stimulate further softening of the cell walls.
Leaves that are preparing for leaf fall transfer nitrogen compounds and carbohydrates to other parts of the plant. Chlorophyll is destroyed and the leaf turns yellow. Harmful substances accumulate in the tissues, which will be removed from the plant by leaf fall.
Thus, the phenomena of leaf fall and damage protection are closely related. In the case of leaf fall in temperate latitudes, we see an anticipatory physiological response. In winter, the leaves are damaged by frost, snow falls on them, causing an increase in the mechanical load on the branches. The plant, as it were, "foresees" future mechanical stress and is freed from leaves in advance. Therefore, it is not surprising that all processes associated with the loss of leaves in areas with cold and snowy winters are under the control of ethylene (Prokhorov, 1978).
Formation and ripening of fruits
The beginning of the life of the fetus lies still in the flower, more precisely in the ovary. Pollen grains fall on the surface of the stigma, they begin to germinate and mechanically press on the conductive tissue of the column in order to reach the ovules hidden in the depths of the pistil. Naturally, during the germination of pollen, the column tissues begin to release ethylene.
Different parts of the flower respond differently to the ethylene signal. So, all the organs that attracted pollinating insects either die off or change color. In a matter of hours after pollination, morning glory petals lose their turgor and fade. In the tepals of a lily, a separating layer is activated at the base, and they fall off (compare with the phenomenon of leaf fall). In lungwort, the pH (acidity) of the vacuolar juice changes and the flowers turn from pink to blue. In the calla (Calla palustris), ethylene causes the veil to change color from white to green. In the future, the plant uses the spathe as an additional source of photoassimilates for developing fruits. Note that in some cases, ethylene causes the destruction of chlorophyll, yellowing and leaf fall, while in others, it enhances photosynthesis.
The stamens wither under the action of ethylene, and the ovaries begin to grow actively, attracting new nutrients.
Ethylene is especially important last step ripening of juicy fruits. Almost all the considered effects "play" here. The fetus stops growing (as well as the seedling, which stumbled upon an obstacle), the cells of the fetus begin to secrete pectinase into the apoplast - the fruits become soft. In addition, physiologically active fragments of pectin - oligosaccharins are formed. In the legs of the fruits, the separation layer is activated and a wound periderm is formed (as during leaf fall), the pH changes - the fruits become less acidic, and their color also changes from green to more yellow or red (like the petals of some plants).
Note that damaged fruits ripen and fall off earlier than others. Mechanical stress is caused by birds, insect larvae or phytopathogenic fungi. As in the case of leaves, the plant tends to discard poor-quality fruit so that the rest of the fruits are as healthy as possible.
Fruit ripening under the influence of ethylene is the same preemptive physiological reaction as leaf fall. The succulent fruits are spread by birds and mammals, which damage the fruits when eaten, and the plant produces ethylene in advance.
The ability to accelerate the ripening of fruits was discovered in ethylene long ago, back in the 20s, and since then it has been widely used. During transportation, it is important that the fruits remain firm and green. To do this, they are transported in a ventilated container, protecting the fruits from mechanical damage that causes ethylene synthesis. In addition, ethylene biosynthesis slows down at low temperatures and at high concentrations of carbon dioxide in the air. In principle, inhibitors of ethylene biosynthesis could also be used, if not for their toxicity to humans. The only place where inhibitors are used is in the storage of cut flowers. In Holland, flowers are placed not in ordinary water, but in a special solution, which, in addition to mineral salts, photosynthesis products and antiseptics, contains ethylene synthesis inhibitors. With the help of such additives, merchants manage to keep bouquets fresh for many days.
To prevent the formation of ethylene in fruits, mutants with impaired ethylene biosynthesis are obtained. Tomato varieties based on such mutants have already been obtained. These tomatoes can be stored for a very long time and transported over long distances. Shortly before selling, they are treated with ethylene, and the fruits ripen quickly. However, this technology significantly reduces the taste of the fruit.
There is a saying that one rotten apple spoils a whole barrel. It really is. A rotten apple serves as a source of ethylene, which causes tissue softening in other apples. Moreover, each fruit begins to produce its own ethylene as it ripens, and a "chain reaction" of ethylene production begins in the barrel.
About a thousand years ago, is told in one oriental legend, an old gardener lived at the court of the Khan. The fruits and flowers that he grew in the garden of his sovereign were famous far beyond the borders of the country. There were many strange plants in the garden. And among them is a small pear tree, which the Khan received as a gift from the Indian Maharaja.
One day, the khan said to the old man: - This autumn, the fruits of the pear tree should decorate my table. Otherwise, don't blow your head off.
The gardener's heart sank. Pear fruits ripen only in very hot summers. And this year it was windy and cold. The old man did not leave the tree day and night: he warmed, fed. But a fierce hurricane swept over the garden, knocked the still unripe pears from the tree.
Now only a miracle could save the gardener. He gathered the fruits, brought them to his cramped hut. Then he took a censer with hot coals, put fragrant incense on top and began to pray to the gods to help him.
The censer was "smoked" for three days in a row. For three days a sweetish smoke of incense flowed in the hut. And a miracle happened: the pears became amber-yellow, ripened.
Centuries passed, and someone decided to check: can this happen?
The fragrant smoke of incense really had a magical effect on unripe fruits. But many more years passed before they figured out why this was happening.
It turned out that the “culprit” of the miracle was a colorless gas with a sweetish odor, which was found in the smoke of incense: ethylene. By this time, they learned to get it from oil and natural gas. And then turn into polyethylene. "The king of plastics" - this is how chemists called the material.
Lightweight and durable water pipes, furniture covers, unbreakable dishes, perfume bottles are made from polyethylene. What about plastic wrap? Perhaps the best packaging material you can imagine.
If you wrap bread in foil, it will remain fresh even after a week. And you can turn the film into a bag that looks like a huge sausage. It will replace the bulky barge. The tug will easily drag such “sausages” with cargoes, such as oil. It is possible to build greenhouses and greenhouses from the film. You can make a shelter for the grain. You can't enumerate everything that the material, born of a gas with a sweet smell, goes to.
And that's why ethylene gas so miraculously affects the fruits, it turned out relatively recently.
It turned out that a colorless gas is formed in the pulp of the fruit. It is abundant in mature fruits and vegetables. The green ones are few. To fumigate them with ethylene means to saturate them with the substance necessary for maturation.
The old gardener brought the fruits of one tree to ripeness. These days, they do this with many tons of fruits and vegetables. The Khan's servant laid out the fruits in his hut. Now they are placed in a special ethylene chamber. Sometimes stacked directly on the shelves. Sometimes they are brought into boxes with holes.
The gardener fumigated the fruit with incense smoke. Pure ethylene is blown into the chamber once a day. Lemons, apples, pears, tomatoes ripen two or even five times faster, absorbing the gas with a sweet smell.
Among vegetable growers who are professionally engaged in the cultivation and supply of crops, it is customary to collect fruits that have not passed the ripening stage. This approach allows you to keep vegetables and fruits longer and transport them over long distances without any problems. Since green bananas or, for example, tomatoes are unlikely to be in serious demand among the average consumer, and natural ripening can take a long time, gases are used to speed up the process. ethylene and acetylene. At first glance, this approach may be bewildering, but having delved into the physiology of the process, it becomes clear why modern vegetable growers actively use this technology.
Gas ripening hormone for vegetables and fruits
The influence of specific gases on the rate of crop maturation was first noticed by the Russian botanist Dmitry Nelyubov, who at the beginning of the 20th century. determined a certain dependence of the “ripeness” of lemons on the atmosphere in the room. It turned out that in warehouses with an old heating system, which was not very tight and let steam into the atmosphere, lemons ripened much faster. Through a simple analysis, it was found that this effect was achieved due to ethylene and acetylene, which were part of the steam emanating from the pipes.
At first, such a discovery was neglected by entrepreneurs, only rare innovators tried to saturate their storage facilities with ethylene gas to improve productivity. Only in the middle of the 20th century "gas hormone" for vegetables and fruits was adopted by fairly large enterprises.
To implement the technology, cylinders are usually used, the valve system of which allows you to accurately adjust the gas output and achieve the required concentration in the room. It is very important that at the same time ordinary air, which contains oxygen, the main oxidizing agent for agricultural products, is displaced from the storage. By the way, the technology of replacing oxygen with another substance is actively used to increase the shelf life of not only fruits, but also other food products - meat, fish, cheeses, etc. For this purpose, nitrogen and carbon dioxide are used, as detailed.
Why is ethylene gas called "banana" gas?
So, the ethylene environment allows you to speed up the process of ripening fruits and vegetables. But why is this happening? The fact is that in the process of maturation, many cultures emit a special substance, which is ethylene, which, when it enters the environment, affects not only the source of emission itself, but also its neighbors.
so apples help with ripening
Each type of fruit produces different amount maturation hormone. The most different in this respect are:
- apples;
- pears;
- apricots;
- bananas.
The latter get into our country, overcoming a considerable distance, so they are not transported in a ripe form. In order for the banana peel to acquire its natural bright yellow color, many entrepreneurs place them in a special chamber that is filled with ethylene. The cycle of such processing averages 24 hours, after which bananas receive a kind of impetus to accelerated ripening. Interestingly, without such a procedure, the favorite fruit of many children and adults will be in a semi-ripe state for a very long time. Therefore, "banana" gas in this case is simply necessary.
sent for maturation
Ways to create the required gas concentration in the fruit storage chamber
It has already been noted above that gas cylinders are usually used to provide the required concentration of ethylene / acetylene in a room for storing vegetables and fruits. In order to save money, some vegetable growers sometimes resort to a different method. In the room with fruits, a piece of calcium carbide is placed, on which water drips at a frequency of 2-3 drops / hour. As a result chemical reaction acetylene is released, gradually filling the internal atmosphere.
Such a "grandfather" method, although attractive with its simplicity, is more typical for private households, since it does not allow achieving the exact concentration of gas in the room. Therefore, on average and large enterprises where it is important to calculate the required amount of “gas hormone” for each crop, balloon plants are often used.
The correct formation of the gaseous environment during storage and production of food products plays a huge role, allowing you to improve appearance product, its taste and increase shelf life. Read more about the methods of packaging and storage of products in a series of articles on food gas mixtures, and you can order these products by selecting the required gas and, if desired, getting advice on its proper operation.