Auxin and gravitropism
Gravitropisms
Growth of plant shoots towards the light. is a response to the stimulus of light, whereas Growth of plant roots towards gravity and plant shoots away from light. (also called geotropism) 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..
- When the stem grows against the force of gravity (upwards), this is known as a negative gravitropism.
- When a root grows in the direction of the force of gravity (downwards), this is known as a positive gravitropism.
Just like phototropism, gravitropism is also caused by an unequal distribution of auxin.
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.
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. This is just like phototropism, in that auxin in the root inhibits growth and auxin in the stem promotes growth.