Gibberellin was discovered by Kurosawn in 1928. He observed that a rice. plant was infected by the fungus Gibberella. It showed excessive stein elongation. The symptom is called “Bakanae– disease. The fungus produces a substance .This substance caused this elongation. This substance was named gibberellin after the name of this fungus.
Gibberellin occurs in algae. fungi. liverworts and higher plants. 62 different types of gibberellins have been discovered. These are abbreviated as GA and are serially numbered from GA, to GAh. The most commonly occurring gibberellin is GA3. It is also called gibberellic acid.
ROLE OF GIBBERELLINS IN PLANT GROWTH
Gibberellins play following physiological roles in plants
1. • Seed Dormancy
The physiological inability of a seed to grow is called seed dormancy. Dormant seeds do not germinate even under favourable conditions of water, air and normal temperature. Gibberellic acid breaks the dormancy in some seeds
- The seeds of lettuce requires stimulus of light. The requirement of light can be replatmli-by-gibberelticatie Therefore, gibberellin treated seeds can germinate in the dark.• The seeds of some temperate species like apple and peach need low temperature (1-7°C) for a few days. Gibberellic acid can replace the need of low temperature for seed germination.
- Seed Geemination
Gibberellin plays an important role in the germination of cereal grains like barley and wheat. The seed imbibes water. It releases gibberellins in the embryo. The gibberellin then moves to the aleurone cells of the endosperm. It stimulates the fresh synthesis of an enzyme called u-amylase. This enzyme hydrolyses starch into sugar. The embryo utilizes the sugar and develops into the seedling. The role of gibberellins in the germination of dicot planlts is unknown.
- Stem Growth
The most important effect of gibberellin is on stern elongation. Application of gibberellin causes rapid stem elongation. It has great effect on two types of plants: genetic dwarfs and rosette plants.
(a) Genetic dwarf:The mutant strains of maize di and d5 remain dwarf.They do not grow beyond 1-2 feet. Such plants are unable to synthesize gibberellin. The ink:modes fail to elongate in the absence of gibberellin. Therefore, the stein remains dwarf. When gibberellic acid is given to these plants, the stem rapidly elongates. The plant acquires the height of normal tall varieties of maize.
- (b) Rosette dwarf: The plants with very short stem and crowed leaves are called rosette plant. The stem is very short in carrot and radish. It is not more than a cm in length. Their leaves are crowded together in vegetative phase. Such a condensed disk like stem is called rosette stem. The rosette habit is due to deficiency of gibberellin.
When gibberellic acid is applied to rosette plants of radish, the stem elongates. It grows to 3 – 4 feet tall. The rapid stem elongation of rosette plant is called bolting. Bolting also occurs in rosette plant naturally at the start of Powering. The plant itself produces large quantity of gibberellin atilt is time.
The deficiency of gibberellin inhibits of cell division at subapical meristem. Therefore, the stein remains very short at the intemodes in the rosette plants. The application of gibberellin promotes cell divisions in the subapical meristem. Therefore, the stem ,elongates rapidly.•
- Rooting of Cuttings.
Gibberellic acid inhibits the formation of adventitious moth on the stem cuttings. Therefore, it is antagonistic to auxin.
- Flowering in Plants ‘
Gibberellic acid causes flowering in two types of plants: rosette long day plants and vernalization requiring biennials.
(a) Rosette long day plants: Rosette plants like Hyoscyamus and Samolus remain vegetative under short days. But they produce flowers in long days. The application of gibberellin makes the plants to flower even under short days.
(b) Vernalization requiring biennials: Similarly carrot plant requires low temperature (vernalization) for flowering. The application of gibberellin overcomes this requirement. Now the plants can flower without exposing to low temperature.
- Sex Expression
The application of gibberellin increases the number of male flowers in many plants like cucumber, ridge gourd and pumpkin. But it decreases the number of female flowers. Thus its affect is reverse to auxin. Female plants of Cannabis produce male flowers when gibberellin is applied. Therefore, GA changes the sex ratio of flowers towards maleness.
- Fruit setting
The start of fruit growth after pollination is called fruit setting. The application of gibberellins cause fruit setting in many plants.