The special substances produced by the plants which influence the growth and plant responses are called plant hormones. Plant hormones are auxins, gibberellins, cytokinins, abscisic acid and ethylene.
The natural auxin is indole acetic acid (IAA). A large number of synthetic auxins are present. These synthetic auxins are more effective than natural auxins. The important synthetic auxins are:
- Indole butyric acid (IBA)
- Naphthalene acetic acid (NAA)
- 2,4-dichlorophenoxyacetic acid (2,4-0)
- Para chlorophenoxy acetic acid
ROLE OF AUXINS IN PLANT GROWTH (PHYSIOLOGICAL ROLE)
Auxins play following roles in plant growth:
- Coleoptile and Stem Growth
Auxin was discovered in coleoptile. It causes growth of the coleoptile of wheat, maize and other cereals. The tip of coleoptile produces auxin. Therefore, coleoptile shows rapid growth. When the tip is removed, the growth of the coleoptile stump stops. Auxin causes growth of the coleoptile by cell elongation. It also causes growth in stem by cell elongation.
- Cell Elongation
Auxin causes loosening of cell wall. The rigid cell wall becomes soft and its plasticity increases. The loosening of cell walk decreases the wall pressure (WP) or turgor pressure (TP). So cell absorbs more water. It increases the size of vacuole. As a result the cell stretches and elongates.
- Secondary Growth
It is found that the tip of sunflower seedlings produces auxin. This auxin causes secondary growth. If this tip is removed, secondary growth also stops. Auxin stimulates the formation of secondary vascular cambium. Therefore, auxin controls secondary growth or thickness of the stem.
- Rooting of Cuttings
Stern cuttings of some plants like rose term adventitious roots at their basal end in the moist soil. The leaves and buds on the cutting promote the rooting. These leaves and buds produce auxin. The application of auxin promotes rooting of stem cuttings. Synthetic auxins like IDA and NAA are more effective.
Thus auxins are widely used to induce rooting in vegetative propagation of fruit and ornamental plants. However, auxins fail to cause root formation in a few species like guava, mango. Eucalyptus and others. ,
- Apical Dominance
The phenomenon in which shoot apex inhibits the growth of lateral buds is called apical dominance. The terminal bud produces auxins. It inhibits the growth of lateral bud. Therefore, the main stem continues to grow. If the tip is cut off. the influence of the apex on the lateral buds is lost. Now lateral buds emerge and form branches.
Auxin alone is not responsible for apical dominance. Other hormones like cytokinin and gibberellin are also involved in apical dominance.
- Sex Expression
Auxins change the sex ratio in some plants. The flowers of some plants like cucumber and ridge gourd arc unisexual. But both male and female flowers are borne on the same plant (Monoecious). These plants produce a large number of Male flowers. But it. produces only a few female flowers. Application of auxin increases the number of female flowers . and decreases the number of male flowers.
In dioecious species like Cannabis, genetically male plants produce exclusively male flowers. it starts producing female flowers’ with the application of auxin. Here the effect of auxin is indirect. Auxin produces another hormone ethylene. Ethylene changes the sex ratio of flowers and develops more female flowers.
The phenomenon in which ovary changes into fruit without fertilization is called parthenocarpy. The production of auxin increases in ovary after pollination. The endosperm produces auxin in the ovule. This auxin changes ovary into fruit. The flower falls or withers without pollination. The application of auxin develops the fruit without pollination and fertilization in some plants like tomato and brinjal. It is parthenocarpy. Parthenocarpic fruits are seedless. Some fruits like banana and orange are naturally parthenocarpic. Their ovary has high concentration of auxin even before pollination. This auxin develops their ovary into the fruit.
The falling of leaves, flowers and fruits is called abscission. The green lamina of young leaves produce auxin. This auxin prevents the abscission of leaves. The old leaves cannot produce sufficient auxin. So they fall. If auxin is applied on such leaves. abscission is prevented. Similarly, application of auxin prevents abscission of flowers and fruits. The NAA and 2, 4-D are used commercially to prevent premature fall of immature citrus and apple fruits.
The cell of the abscission layer secretes hydrolytic enzymes like cellulase and pectinase. These enzymes dissolve the cell wall. The organ detaches at this point and falls. Auxin inhibits the action of these enzymes and prevents abscission.
The bending of plant organs like coleoptile, stem and root toward or away from stimulus due to unequal growth is called tropism. It may be phototropism if the stimulus is light. Or it may be geotropism if the stimulus is gravity. Auxin also controls phototropism and geotropism.
• Phototropism: A stem is exposed to light from one side. One half of the stern receives the light. The other half is shaJed. Auxin produced at the tip. It is transported dommard. The lighted half of the stem gets about 35% of auxin. But the,sbaded side receives about 65 % of auxin. As a result, the shaded half ‘of.-thestem grows more than the lighted half. It bends the stem towards light.
- Geotropism: A coleoptilel is placed horizontally. The lower half of the coleoptile possesses more auXin than the upper half. Therefore, lower side grows more thin the upper side. It bends the coleoptile upward.