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Ethylene: Physiological effects, Movement, inhibitors and source

Physiological effects
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 of apical dominance.
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.
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.
Sex expression and flowering: Usually ethylene inhibits flowering in plant however its application causes uniform flowering in pineapple resulting synchronous maturity for harvesting. In cucumber, pumpkin, ridge guard and melon, ethylene application reduces male flowers and promotes female flowers formation.
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 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.
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.
Fruit ripening: The application of ethylene on climacteric fruits hastens the ripening of fruits like banana, mango, melon etc. This is the reason to call the ethylene as a ripening hormone. Ethylene does not cause ripening of non-climacteric fruits like citrus and grapes.
Fruit degreening: The ethylene degrades the chlorophyll content in citrus, grapes tomato and banana fruits that increase the appearance and marketability of the fruits.

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 abscission layer formation and senescence of older leaves. Ethylene also induces cell wall degrading enzymes and prevents auxin reach to abscission zone. The flowers are more sensitive to ethylene senescence and the packing of cut flowers with KMnO4 reduces the senescence because potassium permanganate oxidizes 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 plant system ethylene can permeate through produce cardboards, shipping boxes, wood and even concrete wall.
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 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 environment pollutant. Silver (Ag+) inhibits an ethylene response by binding to and blocking of the ethylene receptor.

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 market under the trade name ReTain which effects on plant 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.
Alcohols: the alcohols particularly ethanol have anti ethylene action. Wu and his colleagues  reported that 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.
NBD: NBD (2,5-norbornadiene) inhibits the ethylene response by binding to and blocking of the ethylene receptor.
1-MCP: 1-MCP (1-methylcyclopropene) is a cyclic olefine 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.
The ethaphon is artificial source of ethylene. The chemical name of ethaphon (C2H6ClO3P) is 2-Chloroethyl phosphoric acid. It is readily soluble in water, methanol, ethanol and acetone. Ethaphon 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. Ethaphon is used for fruit thinning in apple and loosening the berries. It is frequently used to hasten the ripening of climacteric fruits like banana, mango and tomatoes. 
Ethylene Properties            Ethylene Production Site        Ethylene Biosynthesis                 Biosynthesis Factors                 Ethylene Antagonist 

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