Which of the following physiological effects is caused in plants by gibberellic acid?

Options

A. shortening of genetically tall plants
B. Elongation of genetically dwarf plants
C. Rooting in stem cuttings
D. Yellowing of young leaves

Explanation

To determine which physiological effects are caused in plants by gibberellic acid, we can follow these steps:

### Step-by-Step Solution:

1. **Understand Gibberellic Acid (Gibberellin)**: Gibberellic acid is a plant hormone that plays a crucial role in various growth processes, particularly in stem elongation.

2. **Identify Key Functions**: One of the primary functions of gibberellic acid is to promote stem elongation. It does this by stimulating the growth of internodes, which are the segments of the stem between the leaves.

3. **Bolting in Rosette Plants**: Gibberellic acid induces bolting in rosette plants. Bolting is the rapid elongation of the stem that occurs before flowering. This is essential for the plant to reach a height that allows it to reproduce effectively.

4. **Effect on Dwarf Plants**: Dwarf plants typically have low levels of gibberellic acid, which restricts their growth. When gibberellic acid is applied externally to these plants, it stimulates growth and results in increased height, allowing them to reach a more normal stature.

5. **Evaluate the Options**:
- **Shortening of genetically tall plants**: Incorrect, as gibberellic acid promotes elongation, not shortening.
- **Elongation of genetically dwarf plants**: Correct, as gibberellic acid stimulates growth in dwarf plants.
- **Rooting in stem cuttings**: Incorrect, as this process is primarily induced by auxin, not gibberellic acid.
- **Yellowing of young leaves**: Incorrect, as this is associated with senescence and involves ethylene, not gibberellic acid.

6. **Conclusion**: The physiological effect caused by gibberellic acid in plants is the elongation of genetically dwarf plants.

### Final Answer:
The correct option is **elongation of genetically dwarf plants**.

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