The following points highlight the eight physiological effects of auxin in plants. The physiological effects are: 1. Cell Elongation 2. Apical Dominance 3. Root Initiation 4. Prevention of Abscission 5. Parthenocarpy 6. Respiration 7. Callus Formation and 8. Vascular Differentiation.
Physiological Effect # 1. Cell Elongation:
The primary physiological effect of auxin in plants is to stimulate the elongation of cells in shoot. A very common example of this can be observed in phototropic curvatures where the unilateral light unequally distributes the auxin in the stem tip (i.e., more auxin on shaded side than on illuminated side).
The higher concentration of auxin on the shaded side causes the cells on that side to elongate more rapidly resulting in bending of the stem tip towards the unilateral light. Many theories have been proposed to explain the mechanism of cell elongation due to auxin.
Accordingly, the auxin causes cell elongation probably:
(i) By increasing the osmotic solutes of the cells,
(ii) By reducing the wall pressure,
(iii) By increasing the permeability of cells to water,
(iv) By an increase in the wall synthesis and
(v) By inducing the synthesis of specific DNA dependent new m-RNA and specific enzymic proteins. The latter bringing about an increase in cell plasticity and extension resulting ultimately in cell enlargement.
Physiological Effect # 2. Apical Dominance:
It has been a common observation in many vascular plants especially the tall and sparsely branched ones that if the terminal bud is intact and growing, the growth of the lateral buds just below it remained suppressed. Removal of the apical bud results in the rapid growth of the lateral buds. This phenomenon in which the apical bud dominates over the lateral buds and does not allow the latter to grow is called as apical dominance.
Skoog and Thimann (1934) first pointed out that the apical dominance might be under the control of auxin produced at the terminal bud and which is transported downward through the stem to the lateral buds and hinders their growth. They removed the apical bud of broad bean plant and replaced it with agar block. This resulted in rapid growth of lateral buds. But, when they replaced the apical bud with agar block containing auxin, the lateral buds remained suppressed and did not grow.
In recent years, experiment with transgenic plants by plant physiologists and plant molecular biologists have made it quite clear that by removing the apical bud (or decapitating) the shoot), the concentration of auxin in lateral buds situated below was not decreased but there was in-fact a manifold increase in auxin conc., a few hours after decapitation. For instance, Gocal et al (1991) have observed fivefold increase in auxin conc. in the axillary buds of Phaseolus vulgaris 4 hours after decapitation.
It is now generally held that inhibitory effect of auxin from shoot apex on lateral buds is not direct but is indirect possibly through the involvement of other growth hormones such as cytokinins and abscisic acid (ABA).
(In many plant species it has been observed that application of cytokinins to lateral buds stimulates bud growth. The auxin in shoot apex makes it a sink for cytokinins which are synthesized in roots. By decapitating the shoot, the supply of cytokinins is diverted to lateral buds, thereby relieving the buds from apical dominance.
Application of auxin to cut apical stump retards accumulation of cytokinins in lateral buds resulting in inhibition of bud growth. The ratio of auxin to cytokinins appears to be one of the key factors involved in apical dominance. Higher auxin to cytokinins ratio suppresses the growth of lateral buds.
ABA, which is well known growth inhibitor has been observed in dormant lateral buds in intact plants. When the shoot is intact, the auxin in the shoot apex helps to maintain high conc. of ABA in lateral buds so that their growth is suppressed. By removing the shoot apex, the conc. of ABA in lateral buds decreases so that lateral buds grow rapidly).
Physiological Effect # 3. Root Initiation:
In contrast to the stem, the higher concentration of auxin inhibits the elongation of root but the number of lateral branch roots is considerably increased i.e., the higher conc. of auxin initiates more lateral branch roots. Application of IAA in lanolin paste to the cut end of a young stem results in an early and extensive rooting. This fact is of great practical importance and has been widely utilised to promote root formation in economically useful plants which are propagated by cuttings.
Physiological Effect # 4. Prevention of Abscission:
Natural auxins have controlling influence on the abscission of leaves, fruits etc.
Physiological Effect # 5. Parthenocarpy:
Auxin can induce the formation of parthenocarpic fruits. In nature also, this phenomenon is not uncommon and in such cases the concentration of auxins in the ovaries has been found to be higher than in the ovaries of plants which produce fruits only after fertilization. In the latter cases, the concentration of the auxin in ovaries increases after pollination and fertilization.
Physiological Effect # 6. Respiration:
It has been established that the auxin stimulates respiration and there is a correlation between auxin induced growth and an increased respiration rate. According to French and Beevers (1953), the auxin may increase the rate of respiration indirectly through increased supply of ADP (Adenosine diphosphate) by rapidly utilizing the ATP in the expanding cells.
Physiological Effect # 7. Callus Formation:
Besides cell elongation, the auxin may also be active in cell division. In fact, in many tissue cultures where the callus growth is quite normal, the continued growth of such callus takes place only after the addition of auxin.
Physiological Effect # 8. Vascular Differentiation:
Auxin induces vascular differentiation in plants. This has also been confirmed in tissue culture experiments and from studies with transgenic plants. Cytokinins are also known to participate in differentiation of vascular tissues and it is believed that vascular differentiation in plants is probably under the control of both auxin and cytokinins.