These are one of the important groups of plant hormones having multisided roles in plant.
Kogl and Haagen (1931) introduced the term 'auxin' (Gk. auxein=to grow) for those plant hormones responsible for cell enlargement or growth of shoots. F.W. went (1928), a Dutch botanist for the first time undertook the task of isolation, extraction and bioassay of these growth promoting substances.
The most common natural auxin is Indole-3-acetic acid or IAA which is synthesised from an amino acid called tryptophan. It is produced at the shoot tips and translocated to other parts. Its translocation is plar as it occurs from apex to other zone even if experimentally the position of organ is inverted.
Several synthetic chemical substances like Napthal-acetic-acid or NAA. Indole- butyric-acid (IBA), Phenyl-acetic-acid (PAA) and 2.4 dichlorophenoxy acetic acid (2, 4- D) have auxin activities. Substances those limit or inhibit the action of auxins are called anti-auxins. Triiodobenzoic acid (TIBA) is one such compound.
1. Auxin stimulates cell elongation by modifying certain conditions (Devilin, 1969) like, increase in osmotic contents of the cell, increase in permeability of water into cell, decrease in wall pressure, an increase in cell wall synthesis and inducing specific RXA and protein synthesis. The cell elongation results in longitudinal growth of the plant.
2. Auxins promote eambial activity. Jost (1940) suggested that the major function of hormones that migrate from the developing apex of the epicotyl is to activate the differentiation of procambial strands.
3. Auxins initiate rooting on stem cutting. This has enermous application in horticulture. The most commonly employed auxins for rooting are IAA, NAA and 2, 4-D etc.
4. Auxins inhibit the growth of lateral buds and accelerate the growth of apical bud- a phenomenon called apical dominance.
5. Auxins inhibit or delay abscission of leaves. The leaf fall is retarded by the basi petal movement of auxins.
6. Auxins induce flowering and fruiting. Auxins play significant role in fruiting by promoting parthenocarpic development of fruits and also by preventing premature dropping of fruits.