Explain the role of auxin in plant responses. (i) phototropism. (ii) geotropism

Phytohormone

5.2

Phytohormone

Definiton of Phytohormone

Phytohormones or plant hormones are chemical substances that stimulate and coordinate responses in plants, at low concentrations.

Types of Phytohormone	Function
Auxin	Stimulates shoot growth and growth. Stimulates root growth at low concentration. Stimulates fruit development after fertilisation. Inhibit growth of lateral buds. Inhibits root growth at high concentration. Delay senecence of fruits and leaves.
Gibberellin	Stimulates cell elongation at stem. Stimulates seed germination. Stimulates development of certain fruits.
Cytokinin	Stimulates cell division in the presence of auxin. Stimulates growth of lateral buds. Delays senescence of leaves.
Abscisic acid	Stimulates closing of stomata during water stress. Inhibits seed germination.
Ethylene	Stimulates ripening in fruits. Stimulates senescence of fruits and leaves.

Effects of Auxin on Growth Response

Plants respond to stimulus in the form of growth.
This growth response is controlled by the auxin.
Auxin distribution influences the direction of the shoot growth.
Uniform distribution of auxin causes the coleptile to grow upwards.
The area which receives more auxin will grow faster and cause coleptiles to curve to the side that receives less or none of the auxin.
Tropism is closely related to auxin distribution.
External stimuli such as light and gravity influences the auxin distribution in plants.
Auxin produces different effects to the cells in the shoots and the cells in the roots.
High auxin concentrations stimulates cell elongation in the shoots but also inhibits cell elongation in the roots.

The Roles of Auxins in Plant Responses

Phototropism and geotropism are two main types of tropisms which can be shown by shoot and root:
- Phototropism is the growth of plant parts which can respond towards light.
- Geotropism is the growth of plant parts which can respond towards gravity.
The Role of Auxin in Phototropism:
- The response direction of shoot tip depends on the direction of the light stimulus.
- The distribution of auxins in the shoots is uniform if the shoots are exposed to the light from all directions.
- This causes the shoots to grow upwards.
- When the shoots are exposed to the light from only one direction, the auxin will move away from the light.
- The auxin concentration is higher on the shaded side.
- The concentration of auxin becomes uneven on the shoots. The cells in the shaded side elongate more than the cells in the bright side.
The Role of Auxin in Geotropism:
- Auxin also controls plant response to gravity.
- Seeds germinate horizontally in the soil.
- Due to the pull of gravity, auxin pools at the bottom side of the tips and roots.
The table below shows the role of auxin in shoot tips and root tips on plant growth response based on phototropism and geotropism.

Tropism	Stimulus	Response
Positive phototropism	Unilateral light	Shoot bend towards the light.
Negative phototropism	Unilateral light	Root bends away from the light.
Positive geotropism	Gravity	Roots grow downward to the gravitational pull.
Negative geotropism	Gravity	Shoots grow upward away from the gravitational pull.