Gibberellins
Gibberellins are synthesized in the apices of young leaves and roots and are transported through xylem. Anti-giberellins are synthetic compounds which interfere with the synthesis of gibberellins in the plant body, hence called growth retardants e.g. maleic hydrazide, phosphon D,
chlorocholine chloride (CCC).
Japanese farmer noticed ‘bakanae’ or ‘foolish seedling disease of rice’. As a result of the disease, certain rice seed lings grew excessively tall and rapidly; and toppled over before forming seeds. Crop losses as high as 40% were reported. In the beginning of twentieth century, it became known that the disease is caused by a fungus Gibberella fujikuroi (perfect state of
Fuasarium moniliforme).
Kurosowa (1926) showed that the sterile filtrates of the fungus are capable of causing the symptoms of bakanae disease in otherwise normal seedlings. Yabuta and Hayashi (1910) isolated the growth inducing principle and called it Gibberellin.
In 1938, Yabuta and Sumiki were able
to isolate the crystalline form of gibberellin from the culture of the fungus. This and other similar compounds isolated were called gibberellins (after the name of the fungus Gibberella) and were numbered GA1, GA2, GA3.
Chemically all gibberellins are terpenes a complex group of plant chemicals related to
lipids. All are weak acids. Gibberellins are found in abundance in young expanding organs being synthesized especially in embryos, young apical leaves, buds, seeds and root tips.
Aftersynthesis, these are translocated up or down the plants from the leaves.
Applications of gibberellins
(i) Internodal elongation
Like auxins, the main effect of gibberellins is on stem elongation mainly by affecting cell elongation. Gibberellins stimulate stem elongation and leaf expansion, but do not affect roots. Thus, gibberellins restore normal size and growth to genetically dwarf varieties of pear and maize. Though stem growth of normal plants is also promoted, but here these do not induce marked elongation. It is believed that certain specific types of dwarfness in plants are due to deficiency of gibberellin.(ii) Bolting
In many plants, leaf development is profuse while internode growth is retarded. This form of growth is called “rosette” e.g., cabbage. Just before the reproductive phase, the internodes elongate enormously causing a marked increase in height. The stem sometimes elongates from 5-6 times the original height of the plant. This is called bolting.Bolting requires either long days or cold nights. So if a cabbage head is kept under warm nights, it returns to its rosette habit. However, application of gibberellins to these plants can induce bolting even under the conditions that would normally favor rosette form. The gibberellins thus appear to have a role in bolting or not bolting of a plant. Some gibberellin-like substances found in greater amounts in bolted plants than in the non-bolted form.
(iii) Germination of seeds
Especially in cereals, is triggered by soaking the seed in water. After imbibition of water, the embryo secretes gibberellin which diffuses to the aleurone layer, stimulating the synthesis of several enzymes, including amylase, proteases, lipases. These enzymes catalyse the breakdown of food reserves in the endosperm and the products liberated diffuse to the embryo, where they are used in growth.(v) Control of flowering
Gibberellins promote flowering in long day plants and inhibit it in short day plants. These also control sex-expression in certain species. In general, the application of gibberellins promotes the production of male flowers in female plants of Callilabis.(vi) Control of fruit growth
Along with gibberellins, the auxins control fruit growth and development. Gibberellins cause parthenocarpy in pome fruits (apple, pear etc.) and are now-a-days used extensively to increase the fruit size and bunch length in grape.(vii) Vernalization
Gibberellins can substitute vernalization.
Comments
Post a Comment