Plant Responses
Biology: AQA A Level, topic 6.1.2
Directional Responses in Plants
Plants are not mobile organisms so they cannot move to avoid a harmful environment. Instead they regulate growth in response to direction stimuli. Directional growth responses are called tropisms.
Phototropism
- Phototropism is growth in response to direction of light:
- Shoots are positively phototropic - they grow towards light.
- Roots are negatively phototropic - they grow away from light.
Gravitropism
- Gravitropism is growth in response to direction of gravity:
- Shoots are negatively gravitropic - they grow upwards (away from the gravitational pull).
- Roots are positively gravitropic - they grow downwards (towards the gravitational pull).
Indoleacetic acid (IAA)
IAA is a type of auxin that controls the tropic response in plants.
Distribution of IAA
- IAA can be transported over short distances (by diffusion or active transport) and longer distances (in the phloem) around the plant.
- When a plant detects directional stimuli, IAA is transported to different parts of the plant.
- This creates an uneven distribution of IAA.
- Where the distribution of IAA is uneven, a directional growth response occurs.
Phototropism - shoots
- Shoots are positively phototropic.
- If a shoot is exposed to an uneven light source, IAA is transported to the more shaded part.
- A higher concentration of IAA in the shaded parts cause cells in the shaded area to elongate.
- Cell elongation causes the shoot to bend towards the light.
Phototropism - roots
- Roots are negatively phototropic.
- If a root is exposed to an uneven light source, IAA is transported to the more shaded part.
- A higher concentration of IAA in the shaded parts inhibits cell elongation on the shaded part.
- The root bends away from the light.
Gravitropism - shoots
- Shoots are negatively gravitropic.
- If a shoot is exposed to an uneven gravitational pull, IAA is transported to the underside (closer to the pull of gravity).
- A higher concentration of IAA in the underside cause cells in the underside to elongate.
- Cell elongation causes the shoot to bend away from the gravitational pull.
Gravitropism - roots
- Roots are positively gravitropic.
- If a root is exposed to an uneven gravitational pull, IAA is transported to the underside (closer to the pull of gravity).
- A higher concentration of IAA in the underside inhibits cell elongation on the lower side.
- The root bends towards the gravitational pull.
Test yourself on Plant Responses
Which of the following statements about plant tropisms are true?
Which of the following statements about gravitropism is correct?
Which of the following statements about IAA are true?