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Plant Hormones - Auxins

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Estimated time: 9 minutes

Definition: Auxin
Definition: Apical dominance
Definition: Parthenocarpy
Definition: Parthenocarpic Fruits
Key Points: Auxins

CISCE: Class 10

Definition: Auxin

Auxin is the first discovered plant growth hormone, coined by F.W. Went in 1928. Derived from the Greek word auxein meaning "to grow," auxins are powerful growth stimulants effective even in low concentrations. They are mainly found in actively growing regions like shoot apex, root apex, and meristems. The main natural auxin is IAA (Indole-3-acetic acid).

CISCE: Class 10

Definition: Apical dominance

Apical dominance is the phenomenon where the apical (terminal) bud of a plant suppresses the growth of lateral buds. This is mainly due to the action of auxins produced in the apical bud.

CISCE: Class 10

Definition: Parthenocarpy

Parthenocarpy is the process of fruit development without fertilization.

CISCE: Class 10

Definition: Parthenocarpic Fruits

Parthenocarpic fruits are those that develop without fertilization. Auxins can induce such fruit formation in plants like apples, tomatoes, and bananas.

Maharashtra State Board: Class 12
CISCE: Class 10

Key Points: Auxins

Introduction — Weak organic acid; first plant hormone discovered. Natural: IAA (from Avena by Went, 1928); precursor: tryptophan. Synthetic: IBA, NAA, 2,4-D.
Synthesis & Transport — Produced at apical meristems; transported polarly.
Cell Effects — Stimulates cell elongation in shoots, cell division in vascular cambium, and xylem differentiation.
Roots & Apical Dominance — Inhibits primary root elongation; stimulates lateral/adventitious roots. Suppresses lateral bud growth (apical dominance).
Abscission & Parthenocarpy — Delays abscission in young parts; promotes in mature parts. Applied to unpollinated pistil → seedless fruits.
Flowering, Lodging & Herbicides — NAA & 2,4-D induce flowering in litchi & pineapple; NAA prevents lodging; 2,4-D used as herbicide; 2,4,5-T used as Agent Orange in the Vietnam War.