Auxin has various physiological effects in plants such as :
Cell elongation: Auxin stimulates elongation of cells in the shoot. For example, in phototropic curvature, when unilateral light is unequally distributed in stem tips, the higher concentrations of auxin on the shaded side causes the cells of that side to elongate more rapidly and this results in bending of stem tip towards the unilateral light.
Apical dominance: The apical bud inhibits the growth of axillary buds further down the stems and this phenomenon is called apical dominance. Auxin is produced by the shoot tip and thus, its concentration is highest there. It acts as a growth promoter for the apical bud and inhibits the growth of lateral buds.
Root initiation: Higher concentration of auxin inhibits the elongation of roots but the number of lateral branches in roots is considerably increased.
Prevention of abscission: Auxin prevents abscission of leaves, fruits etc.
Parthenocarpy: Auxin induce the formation of parthenocarpic fruits. Normally fruits do not develop if fertilisation has not occurred but if auxins are applied, then the fruits will develop.