Plant hormones
Hormones promote growth within plants. Plant hormones are unequally distributed throughout the stems and roots, which results in parts of the plant growing in a particular direction.
Auxins 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 photosynthesis. They have developed responses called tropisms 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 stimulus
- negative tropisms – the plant grows away from the stimulus
Phototropism is a response to the stimulus of 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
Auxins are a family of plant hormones. They are mostly made in the tips of the growing stems and roots, which are known as apical meristems, and can diffuse to other parts of the stems or roots.
Auxins control the growth of plants by promoting cell division and causing elongation in plant cells (the cells get longer).
Stems and roots respond differently to high concentrations of auxins:
- cells in stems grow more
- cells in roots grow less
Phototropism
This is caused by an unequal distribution of auxin.
In a stem, the shaded side contains more auxin and grows longer, which causes the stem to grow towards the light. It is vital to note that the plant does NOT bend towards the light.
| 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 the 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 |
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.