5.2 - Phytohormone

Phytohormones are plant hormones that stimulate and coordinate the responses in plants. Due to the fact that plants do not have nervous system like humans and animals to control their responses, plants are very sensitive towards phytohormones stimulation even at very low concentration. Phytohormones are synthesized in specific organs and carried to the target locations through the phloem.

The Functions of Phytohormones

Auxsin

• Involved in phototropism and geotropism
• Promotes the development of apical dominance in shoots and roots
• Promotes the germination of adventitious roots at the end of stem cuttings
• Promotes cell division at cambium during secondary growth
• Inhibits the abscission of fruits and young leaves
• Inhibits the growth of lateral buds

Gibberellin

• Encourages the division and elongation of stem cell
• Promotes the development of leaves, flowers and fruits
• Promotes the development and germination of seeds
• Causes the production of flowers in dwarf plants
• Inhibits the development of roots

Cytokinin

• Promotes the division and elongation of root cells and stem cells in the presence of auxin
• Promoted the germination of seeds
• Inhibits the development of apical dominance
• Slows down the process of aging
• Promotes the growth of lateral buds

Abscisic acid

• Inhibits the growth of plants
• Promotes the abscission of fruits, leaves and flowers that have matured
• Promotes seeds dormancy
• Induces stomata closing during drought
• Inhibits the growth of buds and germination of seeds

Ethylene

• Promotes the ripening of fruits
• Induces plants aging
• Promotes abscission of leaves and fruits

Effects of Auxin on Growth Response

Phytohormone auxin control the growth responses of plants.

Figure 1 - The above experiments investigate the effect of auxin on the growth of coleoptile at the shoot tip. The results of experiment I show that the coleoptile grows back upwards. Experiment II also shows that the coleoptile grows upwards. Experiment III shows that coleoptile bends towards the part that were not covered. These experiments show that auxin promotes the elongation of cells at the tip of the coleoptile. Auxin distribution also affects the direction the growth of the shoot. Even distribution allows the growth upwards. The part of the shoot that receives more auxin will elongate towards the site with less auxin concentration.

Figure 2 - The external stimuli such as light and gravitational pull can affect the distribution of auxin in plants. Auxin gives a different effect at different parts of the plants such as the shoots and roots. The graph above shows that high concentration of auxin promotes the elongation of the shoots but inhibits the elongation of the roots.

The Roles of Auxins in Plant Responses

The direction of growth of the shoot is determined by the direction of the light stimulus. If light comes from all directions, auxin at the tip of the shoot will be evenly distributed. As a result, the plant will grow upwards. If the light comes from one direction only, the concentration of auxin will be higher at the side that has no light. Therefore, the shaded side will elongate more than the exposed side. Consequently, the shoot will bend towards the light. This is an example of a positive phototropism.

Figure 3 - The effect of auxin on the tip of coleoptile towards phototropism response.

1. Plant as control. As a result, the coleoptile bends towards the direction of the light.
2. The tip of the coleoptile is cut and hence no growth occurs.
3. The tip of the coleoptile is covered with an opaque cover. After 2 to 3 days, the coleoptile grows back upwards.
4. The tip of the coleoptile is covered with a transparent cover. The coleoptile grows by bending towards the light.
5. The coleoptile tip is covered with an opaque object. The coleoptile still grows by bending towards the direction of the light.
6. The coleoptile tip is separated by an agar block. The coleoptile also grows by bending towards the light.
7. The coleoptile tip is separated by mica. As a result, no growth occurs. Auxin will give a different effect in the geotropism response. The auxin will accumulate at the bottom side of the shoots and roots due to gravity. The roots show positive geotropism by growing towards the gravity (downwards) whereas the shoots show negative geotropism by growing against the gravity (upwards).

Figure 4 - The effect of auxin at the tips of the shoot and root on geotropism response. At the tip of the shoot, the higher concentration of auxin at the bottom side will cause elongation faster so the shoot bends upwards. At the tip of the root, the higher concentration of auxin at the bottom side inhibits cell elongation and causes the root to bend downwards.