Plant Hormones

This section explains plant hormones, including tropisms and general control, response to gravity, response to light and the commercial uses of plant hormones. 

Plant hormones are chemical substances that regulate plant growth and development. They play a crucial role in controlling the plant's responses to environmental stimuli and ensure that it can adapt to various conditions. These hormones are produced in one part of the plant and act on other parts to influence growth, flowering, fruiting, and other essential functions.

Tropisms and General Control

Tropisms are directional growth responses of plants to external stimuli. These responses are controlled by plant hormones, which ensure that the plant grows in the direction that maximises its chances of survival.

• Positive tropism: Growth towards the stimulus (e.g. growth towards light).
• Negative tropism: Growth away from the stimulus (e.g. roots growing away from light).

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There are three main types of tropisms:

1. Phototropism: The growth of a plant in response to light.
2. Gravitropism (or geotropism): The growth of a plant in response to gravity.
3. Thigmotropism: The growth response to touch (e.g. climbing plants).

Response to Gravity: Gravitropism

Gravitropism (also known as geotropism) is the growth response of plants to gravity. This ensures that roots grow downwards into the soil (positive gravitropism), where they can absorb water and nutrients, and stems grow upwards (negative gravitropism) to capture light for photosynthesis.

Mechanism:

• In roots: Gravity causes cells in the root cap to detect gravity and redistribute the plant hormone auxin. Higher concentrations of auxin accumulate on the lower side of the root, slowing down growth on that side and causing the root to bend downwards.
• In stems: In contrast, gravity causes auxin to accumulate on the lower side of the stem, but auxin promotes cell elongation in the stem. This results in the stem growing upwards against the force of gravity.

Response to Light: Phototropism

Phototropism is the growth of a plant in response to light, ensuring that leaves receive the maximum amount of light for photosynthesis.

Mechanism:

• Auxin plays a central role in phototropism. When a plant is exposed to light from one direction, auxins move to the shaded side of the stem. The higher concentration of auxins on this side causes the cells to elongate, making the plant bend towards the light.
• This response is beneficial as it helps the plant maximise its exposure to light for photosynthesis.

Commercial Uses of Plant Hormones

Plant hormones have various commercial applications in agriculture and horticulture. The three key hormones used in these industries are auxins, gibberellins, and ethene.

Auxins

Role: Auxins are involved in promoting cell elongation, phototropism, and gravitropism. They also regulate the growth of lateral buds, root formation, and flowering.

Commercial Uses: 

• Rooting powders: Auxins are used in rooting powders to encourage root growth in plant cuttings.
• Weed killers: Synthetic auxins, such as 2,4-D, are used in selective weedkillers, as they disrupt the growth of broad-leaved plants without affecting grass crops.

Gibberellins

Role: Gibberellins are involved in stimulating seed germination, promoting stem elongation, and inducing flowering in some plants.

Commercial Uses: 

• Promoting seed germination: Gibberellins are used in agriculture to promote the germination of seeds, especially in crops like barley and rice.
• Fruit production: Gibberellins can be used to promote fruit growth in plants such as grapes, making them larger and improving their shape.
• Increasing yield: They are also used in certain crops to increase the yield, such as in tomatoes, where gibberellins help to elongate stems, preventing overcrowding.

Ethene (Ethylene)

Role: Ethene is a gas that acts as a plant hormone and plays a key role in fruit ripening and the regulation of growth.

Commercial Uses: 

• Ripening of fruits: Ethene is used commercially to control the ripening of fruits like bananas, tomatoes, and apples. This ensures fruits ripen in a controlled way, often while still in storage, before being shipped to market.
• Flower induction: Ethene can also be used to promote the flowering of certain plants, such as pineapples.

Summary of Key Plant Hormones and Their Uses

Understanding how plant hormones function allows us to manipulate plant growth for better agricultural outcomes and ensures that plants can respond appropriately to their environment, increasing yield and efficiency. By understanding these concepts, you'll be better equipped to understand how plants grow, adapt, and are cultivated for commercial purposes.