Here's how it works:
1. Sensing Gravity:
- Specialized cells in the root tip and shoot tip called statocytes contain amyloplasts. These amyloplasts are dense starch granules that act like tiny "weights" within the cells.
- When the plant is oriented horizontally, the amyloplasts settle to the bottom of the statocytes due to gravity.
2. Hormonal Signaling:
- The settling of amyloplasts triggers the release of plant hormones, particularly auxin.
- Auxin is transported to the lower side of the shoot, where it accumulates.
3. Differential Growth:
- Auxin promotes cell elongation on the lower side of the shoot. This causes the shoot to bend upwards, growing against gravity.
4. Positive Phototropism:
- Simultaneously, the shoot will also bend towards a light source (positive phototropism). This ensures that the leaves receive the maximum amount of sunlight for photosynthesis.
In summary:
- Gravity causes amyloplasts in statocytes to settle, triggering auxin production and transport.
- Auxin accumulates on the lower side of the shoot, promoting cell elongation.
- This differential growth causes the shoot to bend upwards, growing against gravity.
This process ensures that the shoot grows towards sunlight, maximizing its access to energy for photosynthesis.
