Smoke was used for ripening of bananas in old time. Later it was found that smoke contain ethylene. It was found that this gas is also produced by the plant. It controls a number of physiological processes. Ethylene is produced by bacteria, fungi and higher plants. All the parts of the plant produce ethylene. But ripening fruits and meristematic regions of the plant produce a large amount of ethylene. Ethylene is the only gaseous hormone in plants.
.Ethylene is a colourless unsaturated hydrocarbon gas. It is lighter than air. It has unsaturated bond between the carbon atoms. These bonds are essential for the stimulation of biological activity.
Ethylene is synthesized in every living cell of the plant body. It moves by diffusion in all directions. It regularly escapes out of the plant body in large quantities. Carbon dioxide nullifies the effect of ethylene. Therefore, CO, is called antiethylene. -Salts of heavy metals like silver nitrate also prevent ethylene action by absorbing this gas.
ROLE OF ETHYLENE IN PLANT GROWTH (PHYSIOLOGICAL ROLES)
I. Dormancy of seeds and buds: Application of ethylene breaks dormancy of seeds of several species. It also overcomes dormancy of potato tuber and causes its sprouting.
- Responses in dicot seedlings: Application of ethylene to dicot seedlings shows three distinct responses. These are Inhibition of seedling elongation
(H) Increase in the diameter of stem.
(iii) Distqrbance of normal geotropic ability of roots
- Epinasty: Application of ethylene to dicot seedlings causes drooping down of the leaves. The leaves are normally oriented in horizontal direction: They hang down. It is called epinasty. There is a greater growth of the petiole on the upper side in epinasty. Tomato seedlings are especially sensitive to ethylene.
- Ripening of fruits: It is the most important effect of ethylene. The banana and apple are kept in storage for ripening. The rate of respiration of the fruit increases. It is caied climacteric respiration.Such fruits are tailed. climacteric fruits. The climacteric fruits release a large amount of ethylene in storage. Ethylene starts the climacteric respiration and ripening.
Application of ethylene promotes ripening of climacteric fruits in storage. A chemical called .Ethrel is available in the liquid forms It can be sprayed on the plant. It enters the plant cells and breaks down and release ethylene.