Ethylene

Ethylene stands apart from the other four PGRs by being a gas, produced in minute amounts by almost all plant tissues. Its role as a plant hormone emerged gradually: Denny showed in 1924 that ethylene accelerates lemon ripening; Gane found in 1934 that ripe bananas naturally contain abundant ethylene; and Cocken et al. formally identified it as a natural plant hormone in 1935. Ethylene synthesis peaks during the climacteric ripening of fruits and during tissue senescence, and it is produced in almost every plant organ - roots, leaves, flowers, fruits and seeds. It diffuses easily through the plant via intercellular air spaces rather than needing a vascular transport route, and its precursor is the amino acid methionine, alongside linolenic and fumaric acid. Ethylene levels are measured by gas chromatography, which can detect exact amounts even in tissues like lemon and orange. Physiologically, ethylene stimulates respiration and fruit ripening, promotes radial growth in stem and root while inhibiting linear elongation (shortening internodes), breaks dormancy in buds, seeds and storage organs, triggers formation of the abscission zone (causing premature leaf, flower and fruit shedding), stimulates lateral root and root hair growth (increasing the root's absorptive surface), and causes epinasty. Fruits that show a sharp respiratory rise at ripening (climacteric fruits such as tomato, apple, banana and mango) can be artificially ripened by exposure to about 1 ppm ethylene, often applied as the ethylene-releasing liquid ethephon; non-climacteric fruits such as grapes, watermelon and orange are insensitiv …