List five main groups of natural plant growth regulators. Write a note on discovery, physiological functions and agricultural/horticultural applications of any one of them.

Explanation

### Step-by-Step Solution

**Step 1: Identify the Five Main Groups of Natural Plant Growth Regulators (PGRs)**
The five main groups of natural plant growth regulators are:
1. Auxins
2. Gibberellins
3. Cytokinins
4. Ethylene
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