The dormant seed, the miniature of plant body, germinates as soon as it gets ample amount of moisture. Favorable environmental and physiological conditions lead the germinated seed to the state of well defined differentiated plant body. Apart from the nutritional and genetic factors, growth and differentiation of plants depend on some more important hormonal factors, which regulate the distribution and fate of nutrients in different organs (or plants) and the final form of plant body. Growth hormones (the phytohormones) are defined as organic substances which are synthesized in minute quantities in one part of the plant body and transported to another part where they influence specific physiological processes. Often the readers got confused within the two terms 'growth regulator' and 'phytohormones'. The latter as the definition indicates, is implied to those chemical substances which are synthesized by plants and thus, they are naturally occurring. On the other hand, there are several manufactured chemicals which often resemble the hormones in physiological action and even in molecular structure. These cannot be termed hormones and thus, they are called growth regulators. A group of plant hormones including auxins, gibberellins, ethylene, abscisic acid are presently known to regulate growth. Here in this article, we will only discuss the ethylene plant hormone.
In comparison to the other plant hormones, ethylene is a gaseous hormone (volatile gas) present in the atmosphere as a component of smoke and other industrial gases. The consideration of ethylene as a plant growth hormone dates back to 1901, when Neljubow found that ethylene gas alters the tropic responses of roots. Denny (1924) observed that ethylene gas was highly effective in inducing fruit ripening. Subsequent physiological studies led to the discovery of ethylene as a natural product of ripening fruits. It is now well established that ethylene is synthesized in ripening fruits, flowers, leaves, and even roots and acts a natural plant growth hormone.
Physiological Effects of Ethylene:
1) Inhibitory effect on growth
Ethylene prevents elongation of stems and roots in longitudinal direction. However, the inhibition of growth in length is associated with the radial enlargement of the tissues. This results in swelling of plant parts. Some authors have proposed that radial enlargement (swelling) of some roots such as radish may be due to ethylene stimulation of lateral growth.
2) Tropic responses
It is proposed that inhibition of elongation growth due to ethylene is responsible for positive geotropic bending of roots. This response is caused through the differential formation of ethylene on geotropically lower side of roots.
3) Suppression of bud growth
Ethylene inhibits the growth of lateral buds in pea seedlings and thus, causes apical dominance. It is proposed that auxin might be functioning partly through the biosynthesis of ethylene in causing apical dominance.
4) Fruit growth and ripening
The growth of fruit is stimulated by ethylene in some plants. It is one of the most remarkable properties of ethylene to most of the horticulturists. Stimulation of fruit ripening by ethylene is more marked in climacteric fruits. In normal processes, Ethylene is produced in mature but unripe fruits; thereafter its occurrence leads to a chain of reactions that finally leads to ripening.
Ethylene stimulates the formation of a separation layer or abscission zone in leaves, flowers and fruits. Authors have suggested that abscission of plant parts is regulated by a balance between auxin (a retardant) and ethylene (a stimulant). Ethylene stimulates abscission by formation of hydrolases.
Flowering in pineapple, mango and various other plants can be induced by application of ethylene.
7) Root initiation
Ethylene stimulates rooting of cuttings, initiation of lateral roots and growth of root hair. At lower concentration (0.1 ppm) it promotes root growth of Mung whereas at higher concentration (1.0 ppm)
Ethylene is responsible for breaking the dormancy of buds and seeds.
9) Leaf bending (epinasty)
More growth on upper surface than on lower surface of leaf causes epinasty. Such type of bending is controlled by ethylene is many plants. At higher concentration, it reverses the opening movement of petals in many flowers and causes sleep disease.
Ethylene promotes the yellowing and senescence of leaves. It induces flower fading in pollinated orchids.
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