Auxin was the first plant hormone to be discovered. They were isolated initially from human urine. The term auxin is given to generally IAA and other natural and synthetic compounds having similar structure and growth regulating properties. Generally, auxins are produced in the growing apices of stem and root where from they migrate to the other parts of the plant. Auxins such as IAA and phenyl acetic acid (PAA) are natural auxins. Synthetic auxins are chemicals synthesised in the laboratory. They are considered as plant growth regulators. eg. Naphthalene acetic acid, 2,4 - Dichlorophenoxy acetic acid.
1. Auxins are well known to promote elongation of stem and coleoptile. It promotes the growth by cell enlargement in stems, particularly by elongation of cells behind the apical meristem.
2. Growth in lateral bud is inhibited when the apical bud of a tall plant remains intact. However, the lateral bud grows rapidly on removal of apical bud.
3. Suppression of growth in lateral bud by apical bud due to auxin produced by apical bud is termed as apical dominance. The reason for this is due to auxin produced in growing tip and it stimulates growth but as it moves downward, suppresses growth in the stems below.
4. Auxin is responsible for initiation and promotion of cell division in cambium, which is responsible for the secondary growth. This property of induction of cell division has been exploited for tissue culture techniques and for the formation of callus.
5. Auxin promotes growth of root only at extremely low concentrations. At higher concentrations, it always inhibits growth of root.
6. When leaves and fruits mature, they shed from the stem. This is called abscission. Auxin prevents abscission.
7. Seedless fruits are produced in tomato and apple, by external application of auxin on flowers. Such seedless fruits are called parthenocarpic fruits.
8. 2,4 - Dichlorophenoxy aceticacid, a synthetic auxin is used to eradicate weeds in the field.