Question

Hormone antagonist to gibberellin Is
(a) IAA
(b) ABA
(c) Zeatin
(d) Ethylene

Answer 1

Hint: Gibberellins are plant growth hormones that enhance the growth of plant parts, promote fruit growth, cell enlargement, break seed dormancy, bud dormancy, etc., while there are certain hormones that act as growth inhibitors, induce abscission, seed dormancy, bud dormancy, and inhibit flowering in long- day plants(anti gibberellin effects). Abscission is a self- running process, which removes aged and injured organs from the plant body.

Complete step by step answer:
Gibberellin causes the extraordinary elongation of stems and leaf sheaths in intact plants. The elongation of the stem is due to cell division and cell elongation, induced by gibberellic acid. Gibberellin promotes blotting and flowering. Bolting us the rapid growth and premature flowering of plants. One of the most striking effects of gibberellin is the reversal of dwarfism in many genetically dwarf plants. Rosette plants of sugar beet, when treated with GA, undergo marked longitudinal axial growth. Extensive studies have indicated that the dwarf plants have a very low GA level which appears to account for dwarf habit. Abscisic acid (ABA) is a growth inhibitor, partially responsible for abscission. ABA is synthesized in leaves, stems, fruits, and seeds. It induces leaf discussion, seed dormancy, and bud dormancy in a variety of plants. Dormant seeds germinate when the effects of ABA are suppressed by gibberellins. Due to their opposite effect on plant growth, gibberellins, and abscisic acid can be said to be an antagonist to each other.
So, the correct answer is ‘ABA’.

Additional information:
- Growth hormones are organic substances, synthesized in minute quantities in part of the plant body, and transported to another part where they influence specific physiological processes.
- Five major growth hormones are auxins, gibberellins, cytokines, abscisic acid, and ethylene.
- Auxins are growth hormones which in low concentration, promote cell division and elongation of root and shoot.
- Cytokinins are plant hormones that stimulate cell division.
- Ethylene is a growth-inhibiting phytohormone, mainly involved in the ripening of fruits.
 - Zeatin is the first chemically identified natural cytokine. It was isolated from young maize grains(Zea may).

Note: Yabuta and Sumuki successfully isolated gibberellic acid in crystalline form from culture filtered of the fungus Gibberella fujikuroi. Gibberellin promotes fruit growth and induces parthenocarpy. They break seed dormancy, bud dormancy, fruit dormancy, and promote seed germination. ABA inhibits the production of enzymes in the embryo and inhibits vivipary. It causes closure of stomata and thereby controls transportation, water loss, and wilting of the plant during drought.