Gibberellins are weakly acidic growth hormones which cause cell elongation of intact plants in general and increased internodal length of genetically dwarfed plants in particular.
Japanese rice farmers observed some abnormally tall and thin seedlings in the rice fields which never bore seeds. Such seedling drowned in water due to their weak stem and eventually, died. This disease was called bakanae or foolish seeding disease.
Formosan pathologist, E. Kurosawa (1926) found that the positive agent of the disease was Gibberella fujikuroi, a fungus of the class ascomycetes which is now called Fusariun. Later, Yabuta and Symiki (1938) isolated the substance responsible for rapid growth from the extracts of the fungus and it was named, gibberellic acid.
Mode of Gibberellin action:
It has been observed that level of DNA and RNA in the tissue of dark plant increased, when treated with gibberellic acid. Gibberellic acid favours synthesis of new m-RNA which results in the synthesis of new enzyme which alters the growth effect.
Bioassay of Gibberellins:
Rice: Seeds of rice are soaked in water for three to four days at room temperature. Germinating seeds with 0.5 mm coleoptile are then transferred to petridishes containing water and gibberellin solution separately. The petridishes are placed in light for three to four days. After four days, the length of hypocotyl is measured. Seedlings treated with gibberellin solution showed marked elongation of hypocotyl.
Dwarf pea: Seeds of dwarf pea are allowed to germinate till the formation of coleoptile. Some of the seedlings were treated with gibberellin solution and the others were kept as control. After five days epicotyl length is measured. Seedlings treated with gibberellin solution showed elongation of epicotyl.
Barley endosperm: Endosperms are detached from embryos, sterilised and allowed to remain in 1ml of test solution for 1-2 days. There is a buildup of reducing sugars. The content of reducing sugar is proportional to gibberrelin concentration. Reducing sugars are not formed in controlled experiment where endosperms are in plain water.
Gibberellins are found in almost all groups of plants such as Algae, fungi, mosses, ferns, gymnosperms and angiosperms. Gibberellins are most abundant in young expanding organs. They are synthesized particularly in growing young shoot tips, young apical leaves, buds, embryos and seeds.
Types of Gibberellins:
Gibberellins have gibbane ring stucture. At present, about 110 naturally occurring gibberellins are known. They are represented as GA1, GA2, GA3 and so on. About 15 types of gibberellins have been isolated from Gibberella fujikuroi alone.
Physiological effects of gibberellins:
Elongation of genetically dwarf plants: One of the most important roles of gibberellins in higher plants is to simulate the elongation of the internodal cells. Dwarf and miniature varieties of plants are often genetic mutants which do not produce gibberellins. They can be made to grow to normal size by application of gibberellins.
When gibberellin is applied to single dwarf mutants, for example, Pisum sativum, Vicia faba, and phaseolus they grow to their normal heights. Thus, reversal of dwarfism in many genetically dwarf plant is obtained by application of gibberellins.
Bolting and flowering in long-day plants: Gibberellins play an important role in controlling a balance between internode growth and leaf development. In rosette-forming plant such as cabbage, internode growth is poor but leaves are large. Therefore, leaves appear to arise in tufts.
The internodes suddenly elongate and the stem becomes normal just before flowering. This is called bolting. Generally, such a rosette plant is a long-day plant or requires cold nights. Under the long-day conditions and low-temperature nights, these plants maintain rosette form.
However, gibberellin treatment causes them to bolt or elongate and flower under non-inducing conditions. Thus, long-day plants can be made to flower in off seasons by gibberellin treatment.
Substitution of cold treatments: Biennial plants flower only when they receive low temperature during winter season. Such plants would, however, flower after treatment with gibberellin even if they do not receive suitable low temperature. Thus, biennial plants can be made to flower in a single year by gibberellin treatment.
Cell division in the cambium: Gibberellin can stimulate cell division of vascular cambium in many deciduous trees.
Parthenocarpy: Gibberellins have been found to be more effective than auxins in inducing parthenocarpy in tomato, apple, pear etc. They are commonly used commercially in increasing the yield and fruit size of seedless grapes.
Breaking of dormancy: Gibberellins can effectively break the dormancy of potato tubers and winter buds of many trees. Gibberellins also act antagonistically to abscisic acid.
Germination of seeds: Light sensitive seeds like lettuce and barley can also be germinated in dark when treated with gibberellins.
Synthesis of hydrolytic enzymes: It induces synthesis of various hydrolytic enzymes such as alpha amylase, ribonuclease and protease.