Auxin word originated from the Greek word Auxin which means “To grow” or increase in growth. Presently auxin term represents the group of plant hormones which promote the growth of stem or coleoptile. Indole Acetic Acid (IAA) and those naturally occurring and artificially synthesized products which have some properties as IAA are termed as Auxins.
Physiological Effects of Auxins on Plant growth are as follows:
1. Apical dominance.
2. Cell elongation.
3. Root initiation.
4. Parthenocarpy.
5. Prevention of lodging.
6. Control of dormancy.
7. Thinning of flowers.
8. Effect on abscission.
9. Eradication of weeds.
10. Shortening of Internodes.
11. Tissue Culture.
1. Apical Dominance:
- It is commonly observed that if the apical bud remains intact and growing, the lateral buds remain suppressed. This is called apical dominance.
- Removal of apical bud causes fast growth of lateral buds, and the plant becomes bush-like.
- It is because of this the hedge is regularly cut and trimmed to remove apical buds so that it becomes dense.
2.Cell Elongation:
- The main function of auxin is to induce elongation of the cells which are formed by division of cells of the apical meristem.
- The high concentration of auxin in apical region promotes cell elongation.
- Because of this, the shoot apex is negatively geotropic and positively phototropic.
3. Root Initiation:
- Auxins promote the initiation of adventitious roots from the nodes or basal region of stem.
- Plant growers use BA and NAA to induce root formation in the stem
- Auxins promote root growth at extremely low concentration (i.e. 10–7 to 10–13 M concentration).
- If the lower end of stem cutting is dipped in auxin and then planted, roots are rapidly developed. Example: Rose, Bougainvillea, Citrus, etc.
4. Parthenocarpy:
- The process of development of ovary into the fruit without fertilization is called parthenocarpy.
- Fruits formed by parthenocarpy are called parthenocarpic or seedless fruits.
- For inducing parthenocarpy, the stamens are removed in the bud stage and auxin is sprayed on the stigma
- Seedless fruits are developed by external application of auxin in tomatoes, apples, cucumbers, orange, lemon, brinjal and grapes etc.
5. Prevention of Lodging:
- In many crop plants such as wheat, the thin and weak stem bends near the base due to strong winds and falls on the ground. This is called lodging.
- If the solution of auxin is sprayed on small young plants, the basal portion of the plants become strong and chances of falling in the strong wind are reduced.
- Hence use of auxin prevents lodging.
6. Control of Dormancy:
- Auxin maintains dormancy in seed and vegetative propagules (Rhizome etc.).
- Auxin prevents germination of seeds and sprouting of buds so that these can be stored for a long duration.
- Potato tubers sprayed with NAA can be stored for about three (3) years.
