Concept
Ethylene (CH2=CH2) is an unsaturated hydrocarbon gas (alkene) acting as a naturally occurring plant hormone. It is the simplest alkene gas and is the first gas known to act as hormone. It acts at trace levels throughout the life of the plant by stimulating or regulating the ripening of fruit, the opening of flowers, the abscission (or shedding) of leaves and, in aquatic and semi-aquatic species, promoting the 'escape' from submergence by means of rapid elongation of stems or leaves. This escape response is particularly important in rice farming. Commercial fruit-ripening rooms use "catalytic generators" to make ethylene gas from a liquid supply of ethanol. Typically, a gassing level of 500 to 2,000 ppm is used, for 24 to 48 hours. Care must be taken to control carbon dioxide levels in ripening rooms when gassing, as high temperature ripening () has been seen to produce CO2 levels of 10% in 24 hours. Ethylene has along history of use in agriculture. Ancient Egyptians would gash figs in order to stimulate ripening (wounding stimulates ethylene production by plant tissues). The ancient Chinese would burn incense in closed rooms to enhance the ripening of pears. In the 19th century, city dwellers noticed that gas leaks from street lights led to stunting of growth, death of flowers and premature leaf fall. In1874, it was discovered that smoke caused pineapple fields to bloom. Smoke contains ethylene, and once this was realized the smoke was replaced with ethephon or naphthalene acetic acid, which induce ethylene production. The scientific study of ethylene as a factor in plant physiology started in the late 19th century. In 1896, Russian botanist Dimitry Neljubow studied the response pea to illuminating gas to which they showed movement. He discovered ethylene as the active component in the light source that stimulated pea behaviour. He reported his discovery in 1901. Sarah Doubt also showed in 1917 that ethylene from illuminating gas stimulated abscission.
