Question

Movement of auxin is
(a) Centripetal
(b) Basipetal
(c) Acropetal
(d) Both (a) and (b)

Answer 1

Auxins are synthesized in shoot tips and move towards base. The unidirectional movement of auxins towards base is also called polar transport. Little or no auxins are produced in roots. From the site of synthesis they move down to the elongation zone in the root or shoot. Their movement is from apex to base.

Complete answer:
The movement of auxin is Basipetal (i.e. from apex to base) and polar. Their short distance transport occurs by cell to cell diffusion, and long- distance transport occurs by phloem transport. Auxins are growth hormones which in low concentration promote cell division and elongation of root and shoot. In high concentration they inhibit growth. Auxins can promote growth of stem, coleoptile sections and decapitated coleoptiles, but in the same concentrations, they are incapable of causing growth in intact plants. Chemically auxins are indole derivatives., synthesized from amino acid tryptophan. The commonest auxins involve indole acetic acid and indole acetonitrile. There are two categories of auxins; natural and synthetic. Former occurs naturally in plants and the latter refers to the chemicals that can evoke physiological responses, similar to those initiated by IAA.
So, the correct answer is ‘Basipetal’.

Additional information:
- Auxins promote the cell elongation and stem growth by promoting synthesis of RNA, proteins and wall materials, increasing osmotic potential and water permeability of cells.
- Auxins initiate and accelerate cell divisions in cambium. This effect is particularly important for secondary growth.b
- Auxins prevent the formation of the abscission layer and thereby prevent the abscission of leaves and fruits.

Note:
- Auxins are well known to induce parthenocarpy in a number of plants. External application of auxin causes the development of seedless fruits in tomatoes, apples, cucumbers etc.
- Auxins generally inhibit flowering. But in pineapple spraying certain auxins initiate uniform flowering in the whole crop.
- Auxins break seed dormancy hence promote seed germination. Auxin promotes synthesis of nucleic acids and proteins and thereby regulates growth and development.