Growth and Development of Horticultural Crops (1+1)

Physiological effects of Auxin

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.
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.

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 resulted in an early and extensive rooting. This fact is of great practical importance and has been widely utilized to promote root formation in economically useful plants which are propagated by cuttings.
4. Prevention of Abscission

• Natural auxins have controlling influence on the abscission of leaves, fruits etc.
5. Parthenocarpy

• Auxin can induce the formation of parthenocarpic fruits.
  
• In nature also, this phenomenon is common 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.


http://image.wistatutor.com/content/plant-growth-movements/parthenocarpy-illustration.jpeg

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.
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.
8. Vascular Differentiation

• Auxin induces vascular differentiation in plants.
  
• This has 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.