Auxin and phototropism
Greg Foot explains how plants respond to light and gravity, and the role of auxins in controlling and coordinating plant growth
In order to survive, plants require light and water for A chemical process used by plants to make glucose and oxygen from carbon dioxide and water, using light energy. Oxygen is produced as a by-product of photosynthesis. Algae subsumed within plants and some bacteria are also photosynthetic.. They have developed responses called Plant growth response to light, gravity or water. to help ensure they grow towards adequate sources of light and water.
There are two main types of tropisms:
- positive tropisms - the plant grows towards the Any change in the environment that can be detected by receptors in an organism.
- negative tropisms - the plant grows away from the stimulus
Growth of plant shoots towards the light. is the growth of plant shoots towards the light.
Responses to stimuli of different parts of the plant
- In the plant stem, responses to light are known as a positive phototropism, which means the stem grows towards the light.
- In the plant root, responses to light are known as a negative phototropism, which means the root grows away from the light.
Auxins
Plant hormones that control cell elongation. are a family of plant hormones. An example of a specific auxin is IAA, which is known as Indole acetic acid.
They are mostly made in the tips of the growing stems and roots, which are known as apical meristems, and can The movement of molecules from an area of higher concentration to an area of lower concentration. to other parts of the stems or roots.
Unequal distribution of auxin can cause unequal growth rates in plants and shoots. Auxins change the rates of elongation in plant cells and control growth of stems and roots.
Stems and roots respond differently to high concentrations of auxins:
- cells in stems grow more
- cells in roots grow less
Phototropisms
In a stem, the shaded side contains more auxin and grows longer, which causes the stem to grow towards the light. The plant does NOT bend towards the light. It grows because the auxin causes the cells to elongate on the shaded side, so this side grows more. This unequal growth of the two sides, results in the growth of the stem towards the light.
If lit from above, the plant will grow upwards.
| Seedling A | Seedling B | Seedling C | |
| Treatment | The tips have been removed | No light reaches the tips | More light reaches one side of the tips |
| Effect on auxin concentration | No auxin is produced | Equal concentration of auxin on both sides | Greater concentration of auxin on shaded side |
| Result | The stems do not grow longer | The stems grow evenly and longer on both sides | The cells on the darker side of the stems grow longer |
| Treatment | |
|---|---|
| Seedling A | The tips have been removed |
| Seedling B | No light reaches the tips |
| Seedling C | More light reaches one side of the tips |
| Effect on auxin concentration | |
|---|---|
| Seedling A | No auxin is produced |
| Seedling B | Equal concentration of auxin on both sides |
| Seedling C | Greater concentration of auxin on shaded side |
| Result | |
|---|---|
| Seedling A | The stems do not grow longer |
| Seedling B | The stems grow evenly and longer on both sides |
| Seedling C | The cells on the darker side of the stems grow longer |
Auxins have the opposite effect on root cells. In a root, the shaded side contains more auxin and grows less - causing the root to bend away from the light.
Auxin and gravitropism
Gravitropisms
Phototropism is a response to the stimulus of light, whereas gravitropism (also called Growth of plant roots downwards as a result of gravity.) is a response to the stimulus of The force of attraction between all objects. The more mass an object has, the larger the force of gravity it exerts..
Plants responses to gravity:
- when the stem grows against the force of gravity, this is known as a negative gravitropism
- when a root grows in the direction of the force of gravity, this is known as a positive gravitropism
In a root placed horizontally, the bottom side contains more auxin and grows less - causing the root to grow in the direction of the force of gravity.
The opposite happens in a stem. When a stem is placed horizontally, the bottom side contains more auxin and grows more - causing the stem to grow upwards against the force of gravity.