Step 1: Understanding the Question:
The question asks to identify the specific phytohormone that promotes the growth of the main central stem of a plant while inhibiting the growth of side branches (axillary buds). This physiological phenomenon is known as apical dominance.
Step 2: Key Formula or Approach:
The approach involves identifying the functions of major plant growth regulators:
- Auxins: Cell elongation and apical dominance.
- Cytokinins: Cell division and overcoming apical dominance.
- Gibberellins: Stem elongation and bolting.
- Ethylene: Fruit ripening and senescence.
Step 3: Detailed Explanation:
1. Source: Auxins, such as Indole-3-acetic acid (IAA), are synthesized in the shoot apical meristems (the very tips of the plant).
2. Mechanism: From the apex, auxin is transported downward (basipetally) through the stem.
3. Action: High concentrations of auxin at the tip inhibit the growth of lateral or axillary buds. This ensures the plant grows vertically toward light.
4. Validation: If the apical bud is removed (a process called decapitation), the auxin source is eliminated. This allows the lateral buds to sprout, making the plant "bushy."
Step 4: Final Answer:
The hormone primarily responsible for maintaining apical dominance is Auxin.