Other Plant Hormones (Grade 12 NSC Matric Life Sciences): Revision Notes

Other Plant Hormones

Introduction to additional plant hormones

Beyond auxins, there are several other important plant hormones that regulate various growth and development processes. Two key hormones that play crucial roles in plant responses are gibberellins and abscisic acid. These hormones work together with auxins to coordinate plant growth and help plants respond appropriately to environmental conditions.

Gibberellins

Gibberellins are growth-promoting hormones that have several important functions in plant development. These hormones act as natural "wake-up calls" for dormant seeds and play a vital role in flowering and bud development.

Functions of gibberellins:

• Seed germination: They trigger seeds to begin the germination process, breaking seed dormancy when conditions are favourable
• Flower development: They stimulate the formation and development of flowers in many plant species
• Bud sprouting: They encourage dormant buds to begin growing into new shoots and branches

Abscisic acid

Abscisic acid works in many ways as the opposite of gibberellins. While gibberellins promote growth, abscisic acid often inhibits or slows down certain growth processes. This hormone helps plants conserve energy and survive difficult conditions.

Functions of abscisic acid:

• Inhibiting apical bud growth: It prevents the tips of shoots from growing too rapidly, helping the plant allocate resources more efficiently
• Promoting seed dormancy: It keeps seeds in a dormant state until environmental conditions are suitable for successful germination
• Stress response: It helps plants cope with drought and other environmental stresses

Practical applications: Using plant hormones for weed control

The problem with traditional herbicides

Weeds are unwanted plants that compete with crop plants for essential resources like space, nutrients, water, and sunlight. This competition reduces crop productivity and can significantly impact agricultural yields. Traditional weed control methods often involve toxic herbicides, but these chemicals can be harmful to animals, humans, and even beneficial plants.

Plant hormones as eco-friendly alternatives

Plant hormones offer a more environmentally friendly approach to weed control. The key to this method lies in understanding that different plant species respond differently to various hormone concentrations - this is called selective targeting.

How selective weed control works

The effectiveness of hormone-based weed control relies on the differences between weed plants and crop plants:

• Leaf structure differences: Most weeds have broad leaves, while many important cereal crops (like maize and wheat) have narrow leaves
• Species-specific sensitivity: Different plant species react differently to the same hormone concentration
• Targeted application: By choosing the right hormone and concentration, farmers can kill broad-leaved weeds while leaving narrow-leaved crops unharmed

Auxin-based weed control

When auxins are applied to the roots of weeds in high concentrations, they can actually inhibit root growth rather than promote it. This seemingly contradictory effect occurs because:

• High auxin concentrations have an inhibitory rather than stimulatory effect
• The excessive hormone concentration disrupts normal root development
• This ultimately leads to the death of the targeted weed plants

This method allows farmers to eliminate unwanted weeds whilst protecting their valuable crops.

Benefits of hormone-based weed control

Using plant hormones for weed management offers several advantages:

• Environmental safety: Less toxic to animals and humans compared to traditional herbicides
• Selective action: Can target specific types of plants without harming others
• Natural approach: Uses the plant's own regulatory systems against unwanted species
• Reduced chemical pollution: Minimises the introduction of harmful synthetic chemicals into the environment