Investigate gravitropism and phototropism in shoots and roots

SouravNovember 3, 2024

Answer

Gravitropism and phototropism are two essential growth responses in plants that allow them to orient themselves appropriately in relation to gravity and light. Here’s an overview of both processes, focusing on their mechanisms and significance in shoots and roots.

Gravitropism

Definition: Gravitropism, also known as geotropism, is the directional growth of plant organs in response to gravity.

Mechanisms:

1. Positive and Negative Gravitropism:
   • Roots exhibit positive gravitropism, growing downward toward gravity, which helps anchor the plant and access water and nutrients from the soil.
   • Shoots display negative gravitropism, growing upward away from gravity, allowing them to reach for sunlight for photosynthesis.
2. Sensing Gravity:
   • Plants sense gravity using specialized cells called statocytes, which contain dense organelles known as amyloplasts (or statoliths). These organelles settle at the bottom of the cells in response to gravity.
3. Auxin Distribution:
   • The settling of amyloplasts triggers a signaling cascade that leads to the asymmetric distribution of the plant hormone auxin. In roots, higher concentrations of auxin on the lower side inhibit cell elongation, causing the root to bend downward. In shoots, higher auxin concentrations on the lower side stimulate cell elongation, resulting in upward growth.
4. Curvature Response:
   • The differential growth caused by auxin redistribution leads to a curvature response that allows roots to grow downward and shoots to grow upward, maintaining proper orientation relative to gravitational forces.

Phototropism

Definition: Phototropism is the growth response of plants toward or away from light.

Mechanisms:

1. Positive and Negative Phototropism:
   • Shoots typically exhibit positive phototropism, bending towards light sources to maximize photosynthesis.
   • Roots, on the other hand, can show negative phototropism by growing away from light, which helps them remain anchored in the soil and seek moisture.
2. Light Perception:
   • Plants perceive light through photoreceptors such as phototropins, which are sensitive primarily to blue light. These receptors are concentrated in the shoot tips.
3. Auxin Redistribution:
   • Similar to gravitropism, when light shines on one side of a plant, auxin redistributes towards the shaded side. This causes cells on the shaded side of the shoot to elongate more than those on the illuminated side, resulting in bending towards the light source.
4. Growth Optimization:
   • The ability to bend toward light allows plants to optimize their leaf area for photosynthesis, enhancing energy production and overall growth