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| Ethylene |
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| It is commonly known that a ripe or injured fruit in a basket hastens the ripening of other fruits. Merchants generally use kerosene lamps and hay to bring about colour developments in plants quickly. It is only recently has it been realized that these effects are due to ethylene. |
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| Ethylene is unique in that it is the only gaseous phytohormone. It is usually present in a minute quantity of about 0.1 ppm and causes marked physiological effects. Some of the inhibitory effects earlier attributed to auxin are now known to be caused by ethylene. High concentrations of auxins promote ethylene formation and hence inhibit growth. |
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| Exposure of plants to ethylene results in a condition known as epinasty, which is characterized by drooping of leaves and flowers. |
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| Inhibitory effect on growth |
| Ethylene inhibits elongation of stem and roots and causes swelling of plant parts. |
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| Flowering |
| Ethylene retards flowering in most plants but also increases flowering in some plants. |
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| Sex modifications |
| Treatment of plants with ethylene increases the number of female flowers and fruits in cucumber. |
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| Tropic responses |
| Ethylene is considered as responsible for positive geotropic bending of roots. |
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| Suppression of bud growth |
| Ethylene inhibits the growth of lateral buds and thus causes apical dominance. |
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| Fruit growth and ripening |
| Ethylene stimulates the growth of fruits in some plants. It is considered responsible for the changes that occur during the ripening of fruits. |
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| Root initiation |
| Ethylene stimulates rooting of cuttings, initiation of lateral roots and growth of root hair. |
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| Senescence |
| Ethylene promotes the yellowing and senescence of leaves. It also induces flower fading in pollinated orchids. |
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| Dormancy |
| Ethylene is responsible for breaking the dormancy of buds and seeds. |
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| Fruit Ripening |
| Exogenous application of ethylene induces climacteric (sudden but brief rise in respiration rate) and the fruits ripen quickly. |
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| Examples: Banana, mango and apple. |
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| Feminising effect |
| Exogenous supply of small quantities of ethylene increases the number of female flowers and hence the fruits. |
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| Example: Cucumber |
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