The gibberellins are weakly acidic phytohormones which help in longitudinal growth of stem.
Avena test (a) A piece of mica inserted on the shaded side prevented curvature of the coleoptile, (b) but not when it was inserted on the illuminated side, (c) when the tip was removed (d) but was put back with a block of gelatine, (e) normal phototropic curvature occured
In Japan, the farmers noted that certain rice seedlings grow taller, thinner and paler than the normal ones and toppled over. This condition was termed as bakanae disease (bakanae in Japanese means foolish) or foolish seedling disease.
In 1926, a plant pathologist Kurosawa, discovered the causative agent of the disease as a fungus Gibberella fujikoroi.
Yamuta and Sumuki (1938) demonstrated that some substance secreted by this fungus was responsible for the elongation and they later isolated this substance and named it gibberellin A3
or gibberellic acid (GA). Presently more than 100 different gibberellins have been identified, many of them occurring in plants.
Gibberellins are chemically different from auxins, in that they contain gibbane ring system with specific biological properties. There are more than 100 gibberellins reported. Of them GA3 are more important.
Structures of Gibbane Skeleton and Gibberellic Acid (GA3)
Gibberellins cause stem elongation and leaf expansion. It is believed that certain types of dwarfness are due to gibberellin deficiency. But it has no effect on roots.
Gibberellin induces stem elongation in rosette plants. Cabbage is a rosette plant with profuse leaf growth and retarded internodal length. Just prior to flowering, internodes elongate enormously. This is called bolting. Bolting needs either long days or cold nights. When a cabbage head is kept under warm nights, it retains its rosette habit. Bolting can be induced artificially by the application of gibberellins under normal conditions.
Gibberellins promote seed germination in lettuce, cereals.
Gibberellins break dormancy of buds and tubers. But in root tubers it inhibits the development of the root tuber.
Gibberellins cause parthenocarpy in apple and pear.
Gibberellins along with auxin, control the growth and development of fruits.
Gibberellins control flowering in long day plants. Gibberellins promote the production of male flowers, either in place of female flowers in monoecious plants or in genetically female plants such as cucurbits.
Increased yield (larger size) and better shape of grapes is obtained by treating the fruit bunches with GA. It induces parthenocarpy in apples, pears, tomatoes and cucumbers.
Gibberellins delay fruit maturity and senescence in lemons, oranges and cherries. This helps in storing the fruits.
Gibberellins help in the flowering of many long day plants.
Gibberellins treatment helps in breaking dormancy in “seed potatoes” resulting in uniform crop emergence.
Gibberellins have been used to increase synthesis of various hydrolytic enzymes such as a
- amylase, ribonuclease and protease in aleurone cells of barley.
These are chemicals which inhibit the biosynthesis and activity of gibberellins in plants. They usually cause dwarfism in plants.
Examples: Phosphon - D, B - 995 and AMO - 1618