Stomata open when guard cells:

Options

A. sense an increase in `CO_2` in the air spaces of the leaf
B. flop open due to a decrease in turgor pressure
C. become more turgid due to an influx of K, followed by the osmotic flow of water
D. accumulate water by active transport

Answer

**Step-by-Step Solution:**

1. **Understanding Guard Cells**:
- Guard cells are specialized cells that surround the stomata (tiny openings on the leaf surface). They play a crucial role in regulating the opening and closing of these stomata.

2. **Turgidity of Guard Cells**:
- The stomata open when the guard cells become turgid, meaning they are swollen with water. This turgidity is essential for the stomata to open.

3. **Mechanism of Opening**:
- When guard cells take up potassium ions (K+), water follows by osmosis, causing the guard cells to swell. This swelling leads to the opening of the stomata.

4. **Active K+ Theory**:
- The process of K+ influx (the movement of potassium ions into the guard cells) is crucial. This theory, proposed by scientist Levitt, explains that the influx of K+ ions causes the guard cells to become turgid and thus opens the stomata.

5. **Closing of Stomata**:
- Conversely, when K+ ions exit the guard cells (efflux), the guard cells lose water, become flaccid, and the stomata close.

6. **Summary**:
- In summary, stomata open when guard cells are turgid, which occurs due to the influx of K+ ions. This process is essential for gas exchange in plants.

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