Understanding:
• Plant shoots respond to the environment by tropisms
Tropisms describe the growth or turning movement of an plant in response to a directional external stimulus
- Phototropism is a growth movement in response to a unidirectional light source
- Geotropism (or gravitropism) is a growth movement in response to gravitational forces
- Other tropisms include hydrotropism (responding to a water gradient) and thigmotropism (responding to a tactile stimulus)
Both phototropism and geotropism are controlled by the distribution of auxin within the plant cells:
- In geotropism, auxin will accumulate on the lower side of the plant in response to the force of gravity
- In phototropism, light receptors (phototropins) trigger the redistribution of auxin to the dark side of the plant
In shoots, high auxin concentrations promote cell elongation, meaning that:
- The dark side of the shoot elongates and shoots grow towards the light (positive phototropism)
- The lower side of the shoot elongates and roots grow away from the ground
In roots, high auxin concentrations inhibit cell elongation, meaning that:
- The dark side of the root becomes shorter and the roots grow away from the light (negative phototropism)
- The lower side of the root becomes shorter and the roots turn downwards into the earth
Tropism in A Typical Plant
