# Auxins & Apical Dominance ## What are auxins? - **Auxins** are a type of plant hormone that play a crucial role in controlling plant growth - The most common naturally occurring auxin is **indole-3-acetic acid (IAA)** - Auxins are produced at the **growing tip** at the **apex** (the top) of a plant stem - These hormones control two main processes: - They cause the stem to grow **upwards** - They **stop lateral (side) buds from growing**

Apical dominance is the phenomenon where the main central stem of a plant grows more strongly than lateral (side) shoots

## The mechanism of apical dominance - The auxins produced at the apex of the plant stem prevent lateral buds from developing - This inhibition of lateral bud growth is known as **apical dominance** - The main reason for this response is that upward growth is most beneficial for plants - Growing upwards towards light ensures the plant has access to more energy for photosynthesis - Most of the time, sideways growth is not as useful to plants - Apical dominance ensures that growth is **preferentially upwards** ## What happens when the apex is removed? - If the growing tip at the apex of a plant is removed (e.g. when a gardener prunes a plant or an animal grazes on it), lateral buds grow from the top of the plant - This is because the **source of auxins has been removed** and there is no longer apical dominance - With time, however, the lateral shoots that grow from these lateral buds do curl up towards the light - This means the plant continues to grow in an upwards direction ## Experimental evidence for the role of auxins - The role of auxins in apical dominance can be easily shown experimentally - These experiments demonstrate the control that auxin concentration has over lateral bud growth ### Experiment 1: Removing the apical bud 1. The apical bud of a test plant is removed (decapitated) 2. **Result:** This allows the lateral buds to grow 3. **Explanation:** The source of auxin has been removed, so there is no inhibition of lateral bud growth ### Experiment 2: Replacing the apical bud with auxin 1. A second (genetically identical) test plant is decapitated 2. This time the cut tip is immediately replaced with an **agar block containing auxin** 3. **Result:** This restores the inhibition of lateral bud growth and no lateral buds grow 4. **Explanation:** The auxin from the agar block mimics the effect of the natural apical bud

The agar block experiment proves that it is the chemical signal (auxin) from the apex, not the physical presence of the tip itself, that causes apical dominance

## Practical applications Understanding apical dominance has important practical applications: - **Gardening:** When you pinch out the growing tips of plants like tomatoes or basil, you're removing the source of auxin - This encourages bushier growth with more side branches - The plant becomes fuller rather than tall and spindly - **Agriculture:** Farmers can manipulate plant growth patterns by controlling auxin levels - This can increase crop yields by promoting more fruiting sites - **Horticulture:** Understanding apical dominance helps with: - Shaping ornamental plants - Creating hedges with dense foliage - Managing the growth patterns of fruit trees ## Summary of auxin effects on apical dominance | Condition | Auxin present? | Effect on lateral buds | Growth pattern | |-----------|---------------|----------------------|----------------| | Normal plant with intact apex | Yes - produced at apex | Inhibited | Upward growth dominant | | Apex removed | No | Not inhibited - lateral buds grow | Bushier, sideways growth | | Apex removed but replaced with auxin | Yes - supplied artificially | Inhibited | Upward growth restored |

The concentration of auxin is critical - different concentrations can have different effects on plant growth and development

## Key points to remember - Auxins are produced at the apex of the plant stem - They promote upward (vertical) growth - They inhibit lateral (sideways) bud growth - this is apical dominance - Removing the apex removes the source of auxin - This allows lateral buds to grow - Replacing the apex with an auxin source restores apical dominance - This demonstrates that auxins, not the physical tip, control the response