Ethylene is an unsaturated hydrocarbon. The International Union of Pure and Applied Chemistry (IUPAC) name of ethylene is ethene. It is colourless gas which is higher than air at room temperature and soluble in water. The ancient Egyptians used to make the cut on figs to stimulate the ripening. Later on, it has been proved that ethylene production can be accelerated in plants by wounding the plant tissues. The ancient Chinese used to burn incense, a type of aromatic plant material in closed rooms to enhance the ripening of pears. In 1901 Dimitry Neljubow a Russian scientist reported that street lights gas that caused stunting of growth, twisting of plants and abnormal thickening of stems was ethylene. It was Cousin (1910) who observed that a gas emanated from oranges caused banana ripening stored with oranges during shipping. In 1934 Gane reported that plants synthesise ethylene and demonstrated that ethylene was produced by the apple fruits during ripening in storage. The early work was mainly concerned with the practical application of smoke to induce flowering in pineapple and ripening of banana fruits. In 1935 Crocker suggested that ethylene was the plant hormone responsible for fruit ripening and senescence of vegetative tissues. Ethylene is produced in all parts of higher plants and its production is affected by a variety of developmental factors like seeds germination, fruits ripening, leaves, abscission, flowers senescence and environmental factors like mechanical injury, environmental stress and chemicals like auxins. Ethylene is a natural plant hormone which is widely used for ripening of fruits.
Properties of Ethylene
Ethylene is an unsaturated hydrocarbon with sweet ether-like odour. It is colourless flammable gas lighter than air. Its specific gravity is 0.974 in comparison to 1.000 specific gravity of air. Ethylene readily reacts with mercuric per-chlorate and forms complex compounds. It is released when lithium chloride or HCl is added to the complex and this property of ethylene is exploited in its quantification by the manometer. The potassium permanganate (KMnO4) oxidises ethylene to ethylene glycol and reduces to MnO2. The shelf like of the fruits and cut flowers in storage and transit is enhanced when vermiculite, ceilite or silica gel impregnated with KMnO4 are put with produce because KMnO4 oxidises ethylene. The calcium carbide (CaC2) produces acetylene (CH=CH) which is less active than ethylene.Ethylene is soluble in water and lipids but its solubility is more in lipids compared to water. At 0 degree Celsius and normal temperature 315 ppm ethylene can be solubilized in water. The plant tissues do not evolve the equal amount of ethylene synthesised into the tissues because of ethylene solubility in cellular water, lipids and cover of wax at the surface of plant parts .For example, both apple and banana are climacteric fruits and produce a large amount of ethylene but apple tissues retain up to 2500 ppm ethylene while banana tissues retain not more than 2 ppm.
Ethylene Production site
Ethylene is produced in bacteria, fungus, alga, moss, gymnosperms and higher plants. All parts of higher plants produce ethylene but production is higher in the tissues of meristematic, senescence, ripening and wounding regions. The climacteric fruits produce higher concentration of ethylene during their ripening whereas in non-climacteric fruit ethylene production is quite low.
Ethylene Biosynthesis
Ethylene is biosynthesized from sulfur containing essential amino acid methionine:
The ethylene biosynthesis process involves three steps: In first step ATP (AdenosineTri-Phosphate) activates methionine and S-Adenosyl Methionine (SAM) is synthesized from methionine in the process SAM synthetase enzyme involves. Under second step SAM cleaves into 1-amino cyclopropane-1- carboxylic acid (ACC) and 5-methyl thioadenosine in the presence of catalyzing enzyme ACC synthase that also requires cofactor pyrodoxal phosphate. In the process, ACC also conjugates to form N-malonyl ACC which is storage form of ethylene. The ACC synthesis regulates the ethylene production and the factors which accelerate or slow the ACC synthase activity also affecting the level of ethylene production accordingly. The ACC synthase (ACS) activity is directly proportional to the ethylene production rate. Regulation of ACS activity plays a key role in the rate of ethylene biosynthesis. The malonylation of ACC regulates the level of ACC and thereby production of ethylene. In the third step, ACC is oxidized into ethylene, CO2, and HCN in the presence of ACC oxidase enzyme which is also called ethylene forming enzyme (EFE). Further, HCN converts into formic acid and ammonia. The ascorbate and Fe++ are required for ACC oxidase activity and the ACC oxidation takes place in the presence of light and O2. The ripening and wounding promote ACC oxidase activity while Co++,Cu++ and Zn++ inhibit the activity. The further synthesis of methionine continues from 5 methyl thioadenosine produced during SAM cleaving. The ACC synthesis increases with the high concentration of Indole Acetic Acid (IAA) and cytokinins while ACC synthase is inhibited by abscisic acid. Environmental factors like floods, drought, chilling, physical injury and pathogen attack induce ethylene formation in plants. Under floods, roots deprive to sufficient oxygen that results in the synthesis of ACC which is translocated from roots to leaves and oxidized into leaves resulting ethylene biosynthesis. The ethylene synthesis increases with temperature up to 35 degree Celsius and decreases with a decrease in temperature up to 0-2 degree Celsius. Low concentration of O2 also inhibits ethylene synthesis.
Factors affecting ethylene biosynthesis
Oxygen effect on ethylene biosynthesis
The biosynthesis of ethylene includes oxidation of aminocyclopropane carboxylic acid (ACC) that requires oxygen; therefore the anaerobic conditions inhibit ethylene biosynthesis in plant tissues although methionine a precursor of ethylene is available in the tissues. The wax coating or storage of climacteric fruits into low oxygen atmosphere delays the ripening of the fruits because the low availability of oxygen prevents the conversion of ACC to ethylene which accelerates fruit ripening and senescence.
Temperature effect on ethylene biosynthesis
The optimum range of temperature requires for ethylene biosynthesis, but the range varies with plant species. The 30 degree Celsius temperature is optimum in apple for ethylene production. Normally temperature beyond the optimum range increases in the ethylene biosynthesis. The ethylene production increases many folds in the plant which is first exposed to chilling temperature thereafter higher temperature in comparison to the plant exposed constantly to a higher temperature.
Stress effect on ethylene biosynthesis
The biotic and abiotic stresses increase ethylene production in plants. The ethylene production increases in the plants under the stress caused by insects and parts damage, bacterial fungal and viral diseases, drought, water logging, higher temperature and physical injury to the plant tissues because under stress the synthesis of enzyme ACC synthase is increased leading more synthesis of ACC that subsequently oxidized to ethylene. The pineapple plants flower early and coleus leaves abscise faster when these plants are placed horizontally than in vertical position because of more ethylene production in a horizontal position.
Light effect on ethylene biosynthesis
It is reported that ethylene biosynthesis is increased in dark in the seedlings of a pea, similarly light inhibited the ethylene production in wheat leaves. Light decreases the ACC with a corresponding increase in the MACC (Malonyl ACC) which is biologically inactive. The CO2 increases the ethylene biosynthesis at low concentration whereas it inhibits ethylene action at very higher concentration. In the dark CO2 accumulates in the tissues and promotes ethylene biosynthesis.
Plant Growth Regulators effect on ethylene biosynthesis
The higher biosynthesis of ethylene in those sites of plants where auxin concentration remains higher indicates the effect of auxin on ethylene production. Auxin increases the ethylene biosynthesis by enhancing the conversion rate of S-adenosylmethionine (SAM) to ACC. Removal of the site of auxin biosynthesis like apical bud or application of TIBA (2,3,5-Tri iodobenzoic acid ) an antagonist of auxin reduces the ethylene production. The synthetic auxins NAA and IBA are effective for a longer period to produce ethylene than natural auxin IAA. Similar to auxin; Gibberellins and Cytokinins are also reported to increase the ethylene biosynthesis. The ethrel or ethaphon when applied releases ethylene without the involvement of enzymes. The ethrel molecule contains CH2=CH2 group and structurally resembles methionine a precursor of ethylene.
Movement of Ethylene
Ethylene moves into the plants by diffusion through space between the cells. Normally longitudinal diffusion is about 100 times faster than diffusion in the radial direction. It is soluble in the water and lipids and the dissolve ethylene movement is passive and systematic. The ripening fruits produce relatively more ethylene which diffuses out to air through cut and of pedicel, stomata and lenticels openings of the fruit surface. Out of the plant system ethylene can permeate through produce cardboards, shipping boxes, wood and even concrete walls.
Ethephon as a Source of Ethylene
The ethephon is an artificial source of ethylene. The chemical name of ethephon (C2H6ClO3P) is 2-Chloroethyl phosphoric acid. It is readily soluble in water, methanol, ethanol and acetone. Ethephon is available in the market with trade names of Ethereal, CEPA and CEPHA. It is stable in the aqueous solution below pH 4 but decomposes into ethylene and phosphate as well as chlorine ions: It penetrates, moves and decomposes to ethylene in plant tissues and decomposition rate increases with pH. Ethephon is used in the ornamental industry to delay flowering, selectively abort flowers, abscise leaves, reduce stem elongation and increase stem strength. Ethephon is used to control height and increase stem strength in Narcissus and Hyacinthus. Ethephon is used for fruit thinning in apples and loosening the berries. It is frequently used to hasten the ripening of climacteric fruits like banana, mango and tomatoes.
Ethylene Inhibitors
Ethylene Inhibitors-Silver and STS
Beyer first reported that silver ions inhibited the ethylene action. He further reported that senescence of carnations and orchids could be retarded by silver ions. The feeding of silver (Ag+) through the stem to inhibit the action of ethylene was unsuccessful because of immobility of the silver ion. Veen and Van de Geijn found that silver thiosulfate (STS) was mobile in a cut stem and provided sufficient silver to the petal for preventing the effect of ethylene. The findings became the tool for horticulturists to extend the vase life of cut flowers with the use of STS. Later on, STS was commercially used in pulse treatment of cut flowers to extend the vase life but now its use is banned because it contains silver, an environmental pollutant. Silver (Ag+) inhibits an ethylene response by binding to and blocking of the ethylene receptor.
Ethylene Inhibitors-AVG and AOA
AVG (aminoethoxyvinyl glycine) , a bacterial toxin, was used to extend the life of carnations and other ethylene sensitive flowers. A chemical AOA (amino oxyacetic acid) inhibitor of pyridoxal phosphate-requiring enzymes was used in extending the life of the carnation. These inhibitors are useful to inhibit the endogenous biosynthesis of ethylene during marketing when endogenous ethylene production induces due to exposure of plants to stresses. AVG and AOA inhibit the ethylene biosynthesis but do not improve the display life of plants and flowers exposed to external ethylene. AVG is marketed under the trade name ReTain which affects plants similar to the Alar. ReTain delayed maturity of fruits, maintained firmness and superficial scald in storage. AVG blocks ethylene synthesis and is applied pre harvest. The fruits do not produce much ethylene after treatment, so there is no ethylene response.
Ethylene Inhibitors-Alcohols
The alcohols particularly ethanol have anti ethylene action. Wu and his colleagues reported that a vase solution containing 4 percent ethanol extended the longevity of carnation flowers. The ethanol was acting by reducing the binding of ethylene to its binding site.
Ethylene Inhibitors-NBD (2,5-norbornadiene)
NBD (2,5-norbornadiene) inhibits the ethylene response by binding to and blocking the ethylene receptor.
Ethylene Inhibitors-1-MCP
1-MCP (1-methylcyclopropene) is a cyclic olefin that has potential to inhibit ethylene action. 1-MCP inhibits ethylene action by binding irreversibly to the ethylene binding site. It inhibits the action of both exogenously and endogenously produced ethylene. 1-MCP treatment is as effective as STS treatment and this compound may be the substitute of STS in ornamental horticulture. The 1-MCP is a patented chemical and available in the market under the trade name ‘Ethyl Bloc’. 1-MCP blocks ethylene by binding to its receptor and is applied postharvest. The fruits may produce some ethylene even after getting 1-MCP treatment but there is no response to the ethylene.
Antagonists of Ethylene
The presence of CO2 reduces the response of plants to ethylene. Because of this reason, fruits stored with CO2 remain fresh for a longer period whereas removal of CO2 from fruit stores by absorbing CO2 in potassium hydroxide enhances ethylene activity and thereby increases ripening of stored fruits. Silver nitrate (Ag NO3) also prevents ethylene action.
Physiological effects of Ethylene
Physiological effects of Ethylene on seed and bud dormancy
Ethylene stimulates the action of hydrolytic enzymes and thereby overcomes seed dormancy and promotes germination. It is reported to overcome the seed dormancy in seeds of apple and lettuce and promotes the sprouting of buds in potato tubers, rhizomes, corms, and bulbs. It resembles auxin in showing apical dominance.
Physiological effects of Ethylene on root formation
The ethylene treatment promotes callus formation and adventitious roots initiation on the stem cuttings. Its application also promotes the formation of secondary roots and root hairs.
Physiological effects of Ethylene on shoot and root growth
Ethylene inhibits longitudinal growth but promotes lateral growth that causes bulging of stem and root in dicotyledons whereas stem of monocotyledons generally does not show any growth effect except in paddy where it promotes the growth of plants.
Physiological effects of Ethylene on sex expression and flowering
Usually ethylene inhibits flowering in plants however its application causes uniform flowering in pineapple resulting in synchronous maturity for harvesting. In cucumber, pumpkin, ridge gourd and melon, ethylene application reduces male flowers and promotes female flowers formation.
Physiological effects of Ethylene on epinasty
The bending of leaves is termed epinasty. Ethylene causes swelling of the cells on the upper part of the petiole of leaves which results in drooping (bending) of leaves. The epinasty of leaves under stress conditions like high temperature, drought and under auxin application also takes place because of ethylene formation under these conditions. The epinastic effect is reversible after removal of ethylene and the monocotyledons do not show epinastic bending due to ethylene.
Physiological effects of Ethylene on plumule bending
The plumules of emerging seedlings in the dark and from a hard stress soil form hook-like structure due to bending because of ethylene formation in plumules under dark and hard soil conditions. Once plumule comes out from the soil and exposed to light its growth becomes symmetrical due to a reduction in ethylene synthesis into the plumuls.
Physiological effects of Ethylene on fruit ripening
The application of ethylene in climacteric fruits hastens the ripening of fruits like banana, mango, melon etc. This is the reason to call ethylene a ripening hormone. Ethylene does not cause ripening of non-climacteric fruits like citrus and grapes.
Physiological effects of Ethylene on fruit degreening
The ethylene degrades the chlorophyll content in citrus, grapes, tomato and banana fruits that increase the appearance and marketability of the fruits.
Physiological effects of Ethylene on abscission and senescence
Ethylene promotes abscission and senescence of plant parts such as leaves, stem, flowers, and fruits. The older leaves produce more ethylene than younger leaves resulting in abscission layer formation and senescence of older leaves. Ethylene also induces cell wall degrading enzymes and prevents auxin reach to the abscission zone. The flowers are more sensitive to ethylene senescence and the packing of cut flowers with KMnO4 reduces the senescence because potassium permanganate oxidises ethylene.
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