The plant hormone ___________ leads to closing of stomata.

Explanation

**Step-by-Step Solution:**

1. **Understanding Plant Hormones**: Plant hormones are chemical substances that regulate various physiological processes in plants, including growth, development, and responses to environmental stimuli.

2. **Identifying the Function of Stomata**: Stomata are small openings on the surfaces of leaves that allow for gas exchange (CO2 in, O2 out) and transpiration (water vapor loss).

3. **Recognizing the Role of Abscisic Acid (ABA)**: Abscisic acid is a plant hormone that plays a crucial role in regulating stomatal closure, especially during times of water stress.

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