Question

Which of the following hormones is NOT auxin?
1. IBA
2. 2, 4, D
3. IAA
4. ABA

Answer 1

Correct Answer - Option 4 : ABA

Concept:

• Plant growth hormones or phytohormones are simple molecules with complex chemical composition, that regulates the growth and physiology of plants.
• Based on their functions on the living plant body they can be classified into two groups:

1. Plant growth promoters - Promotes activities like cell division, cell enlargement, flowering, fruiting, etc. Eg. Auxins, Gibberellins & cytokinins.
2. Plant growth inhibitors - They are involved in various growth-inhibiting activities like dormancy, and abscission. Eg. Abscisic acid.

• Gaseous PGR like ethylene can be put into both groups, however, it is largely an inhibitor of growth activities.

Explanation:

• Auxin:
• Promote flowering in plants like pineapple.
• Help to initiate rooting in stem cuttings.
• Prevent the dropping of fruits and leaves too early.
• Promote natural detachment (abscission) of older leaves and fruits.
• Control xylem differentiation and help in cell division.
• Eg.  IAA (Indole acetic acid) is a type of naturally occurring auxin that prevents premature abscission of plant organs.
• 2, 4, D (2, 4-dichlorophenoxyacetic)  is synthetic auxin used as a herbicide to control the growth of weeds.
• ABA (Abscisic acid):
• It is also known as the stress hormone because it protects the plants from extreme environmental conditions like water stress.
• ABA promotes aging and abscission (falling) of leaves and fruits.
• It inhibits the process of cell division and cell elongation.

Gibberellins	Increase the axis length in plants such as grape stalks. Delay senescence (i.e. aging) in fruits. As a result, their market period is extended. Help fruits like apples to elongate and improve their shape.
Cytokinins	Help in the formation of new leaves and chloroplasts. Promote lateral shoot growth and adventitious shoot formation. Help overcome apical dominance. Promote nutrient mobilization which in turn helps delay leaf senescence.