4. List five main groups of natural plant growth regulators. Write a note on the discovery, physiological functions and agricultural/horticultural applications
of any one of them.
Five main groups of plant growth regulators are auxin, gibberellins, cytokinins, abscisic acid and ethylene.
Discovery of GIbberellin
Gibberellins were discovered in Japan from rice plants suffering from bakane or foolish seedling disease. Such rice plants were thin, pale green spindle-shaped, longer by 50 % than the healthy plants and were sterile.. The disease was found by Hori and Kurosawa to be caused by Gibberella fujikori. This fungus i is the perfect stage of Fusarium moniliforme. He also reported that active substance from this fungus caused the appearance of symptoms. Later Yabuta separated and named this active substance as gibberellin.
1. Gibberellins help in the growth of stem, leaves and other aerial parts and cause an increase in their size and height
2. Gibberellins can specifically induce internodal growth in some genetically dwarf varieties of plants like pea and maize etc.
3. Gibberellins induce subapical meristem to develop faster. This causes elongation of reduced stem or bolting in case of rosette plants
4. Gibberellins overcome the natural dormancy of buds, tubers, seeds,etc.
5. Gibberellins through a cascade of events solubilize the reserve food in seeds and lead to their germination.
Agricultural/horticultural applications of gibberellins
1. Application of gibberellins increases the number and size of several fruits such as grape, tomato etc. Size and shape can also be enhanced by using GA7 and GA4.
2. Seedless parthenocarpic gruits can be produced by the application of gibberellins.
3. Spraying of sugarcane crop with gibberellin increases the length of the stem and yield of sugarcane.