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auxin(awk′sĭn) [Gr. auxe, increase]
auxina type of plant growth-regulating substance involved in the growth of cells and several other functions. The most important auxin is indolacetic acid (IAA; see Fig. 194 ), but many other substances have been classified as auxins, using a BIO-ASSAY method developed by Fritz WENT.
The effects of auxins depend on their concentration in the plant. They are most concentrated at the shoot tip and least concentrated in the root, except for small amounts at the root tip. The major effects of auxin are summarized below:
- Encourages cell growth by elongation, producing a softening of the MIDDLE LAMELLAE of cell walls.
- Stimulates cell division in the PHLOEM of the VASCULAR BUNDLES, so encouraging new growth.
- Promotes positive PHOTOTROPISM in shoots by growth of tissues towards the light source.
- Promotes GEOTROPISM in all parts of the plant (positive in roots, negative in shoots, due to unequal distributions of the hormone).
- Induces APICAL DOMINANCE by suppressing lateral buds.
- Induces lateral root formation.
- Stimulates fruit development, enabling seedless fruits to be produced artificially. (h) Suppresses ABSCISSION in leaves and fruit. (i) Encourages the formation of wound tissues in injured or diseased plants.
Auxins have a number of commercial uses, e.g. to promote the rooting of cuttings, regulate plant height, induce flower formation and control fruit set and fruit drop.