Plant tropisms
Plants need 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 make sure they grow towards sources of light and water.
Positive tropism is when a plant grows towards the Any change in the environment that can be detected by receptors in an organism..
Responses to stimuli of different parts of the plant
Stem response to light is positive phototropism (grows towards the light).
Root response to gravity is positive geotropism (grows in the direction of the force of gravity).
Auxin
Auxin is a family of plant hormones. They are mostly made in the tips of the growing stems and roots, which are the meristems. They can When particles spread out from a region of higher concentration to a region of lower concentration. to other parts of the stems or roots. Auxin change the rate of elongation in plant cells, controlling their length.
Stems and roots respond differently to high concentrations of auxin:
- cells in stems grow more
- cells in roots grow less
Phototropisms
In a stem, the shaded side contains more auxin and grows longer – causing the stem to bend towards the light.
Question
Can you explain why these three shoots have grown in the way they have?
| 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 |
Auxin has 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.
Gravitropisms
Gravitropism is a growth response in a plant to the force of gravity. If a young plant was placed on its side – as shown in the diagram – these are the growth responses which would occur.
In a root placed horizontally, the bottom side contains more auxin and grows less, causing the root to bend in the direction of the force of gravity.
In a stem placed horizontally, the bottom side contains more auxin and grows more, causing the stem to bend upwards against the force of gravity.
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