Explain phototropism and gravitropism of a shoot as examples of the chemical control of plant growth

Phototropism and gravitropism are two critical examples of how plants chemically control their growth in response to environmental stimuli, specifically light and gravity. Both processes involve the plant hormone auxin, which plays a pivotal role in directing growth patterns in shoots and roots.

Phototropism

Definition: Phototropism is the growth response of a plant toward or away from a light source. In shoots, this response is typically positive, meaning they grow towards the light.

Mechanism of Phototropism

1. Light Perception: Plants contain photoreceptors that detect light direction and intensity, primarily blue light. The most well-studied photoreceptors involved in this process are phytochromes and cryptochromes.
2. Auxin Redistribution: When light is detected, auxin (specifically indole-3-acetic acid, or IAA) redistributes within the plant:
   • Auxin accumulates on the shaded side of the shoot where light is less intense.
   • This uneven distribution causes cells on the shaded side to elongate more than those on the illuminated side.
3. Growth Response: The differential growth results in the bending of the shoot towards the light source. This mechanism allows plants to maximize their exposure to sunlight, enhancing photosynthesis and overall growth.

Gravitropism

Definition: Gravitropism (or geotropism) is the directional growth of plant organs in response to gravity. Shoots exhibit negative gravitropism by growing upwards, while roots show positive gravitropism by growing downwards.

Mechanism of Gravitropism

1. Gravity Perception: Specialized cells called statocytes contain dense organelles known as amyloplasts (or statoliths) 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 at the shoot tip.
2. Auxin Redistribution: When a plant is tilted:
   • In shoots, auxin accumulates on the lower side due to the settling of amyloplasts.
   • This accumulation stimulates cell elongation on the lower side, causing the shoot to bend upwards (negative gravitropism).
   • In roots, auxin also accumulates on the lower side but inhibits cell elongation there, leading to slower growth compared to the upper side. This differential growth causes roots to bend downwards (positive gravitropism).
3. Growth Response: The result is that shoots grow against gravity towards light sources while roots grow towards gravity into the soil, ensuring stability and access to water and nutrients.

Summary of Chemical Control