Investigate and describe gravitropism and phototropism in shoots and roots

Gravitropism and phototropism are essential growth responses in plants that enable them to orient themselves correctly in relation to gravity and light. These mechanisms ensure that roots grow downward into the soil (positive gravitropism) while shoots grow upward towards sunlight (negative gravitropism). Below is a detailed investigation of these responses as they occur in both shoots and roots.

Gravitropism

Gravitropism, also known as geotropism, is the directional growth of plant organs in response to gravity. This response is crucial for establishing proper orientation, allowing roots to anchor the plant and access water and nutrients, while enabling shoots to reach for light.

Mechanism of Gravitropism

1. Sensing Gravity:
   • Amyloplasts: Specialized organelles called amyloplasts (or statoliths) contain starch granules that settle under the influence of gravity. In roots, these are located in the root cap, while in shoots, they are found in specific cells of the shoot tip.
2. Signal Transduction:
   • When a plant is tilted, the amyloplasts shift to the lower side of the cells, triggering a biochemical response. This contact with the endoplasmic reticulum releases calcium ions, which initiate signaling pathways that result in the redistribution of auxin, a key plant hormone involved in growth regulation.
3. Auxin Distribution:
   • In shoots, auxin accumulates on the lower side, promoting cell elongation there and causing the shoot to bend upwards (negative gravitropism).
   • In roots, auxin also accumulates on the lower side but inhibits cell elongation, leading to slower growth on that side compared to the upper side. This differential growth causes roots to bend downwards (positive gravitropism).

Phototropism

Phototropism is the growth response of plants toward or away from a light source. This response allows plants to maximize their exposure to sunlight for photosynthesis.

Mechanism of Phototropism

1. Light Perception:
   • Plants possess photoreceptors such as phytochromes and cryptochromes that detect light intensity and direction. These receptors are particularly sensitive to blue light, which is most effective in inducing phototropic responses.
2. Auxin Redistribution:
   • Similar to gravitropism, auxin plays a critical role in phototropism. When light shines on one side of a plant:
     • Auxin redistributes towards the shaded side of the shoot.
     • This accumulation promotes cell elongation on the shaded side while inhibiting it on the illuminated side, causing the shoot to bend towards the light source (positive phototropism).
3. Growth Direction:
   • In shoots, this bending allows leaves to orient themselves toward light for optimal photosynthesis.
   • Roots typically exhibit negative phototropism by growing away from light, which helps them remain anchored in soil and seek moisture.

Summary of Responses