Stomata open when guard cells swell due to

Options

A. Decreased water potential
B. Increased water potential
C. Endosmosis by efflux of K+ ions
D. Endosmosis by influx of hydrogen ions.

Answer

To understand why stomata open when guard cells swell, we need to analyze the process step by step:

### Step-by-Step Solution:

1. **Understanding Stomata and Guard Cells**:
- Stomata are small openings on the surface of leaves that allow for gas exchange (CO2 in, O2 out) and transpiration (water vapor loss).
- Guard cells are specialized cells that surround each stoma and control its opening and closing.

2. **Role of Water Potential**:
- Water potential is a measure of the potential energy in water, influencing the movement of water in and out of cells.
- When the water potential is high (more water), guard cells become turgid (swollen), causing the stomata to open.

3. **Conditions Leading to Stomatal Opening**:
- On bright sunny days, the rate of transpiration increases due to higher temperatures and light intensity.
- As water vapor exits the leaf through the stomata, guard cells lose water, leading to a decrease in their water potential.

4. **Water Movement**:
- To counteract the decrease in water potential, water from neighboring cells moves into the guard cells to restore turgidity.
- This influx of water causes the guard cells to swell, resulting in the opening of the stomata.

5. **Ionic Movement**:
- The swelling of guard cells is also influenced by the movement of ions, particularly potassium ions (K+).
- When potassium ions are actively transported into the guard cells, it lowers the water potential inside the guard cells, allowing more water to enter by osmosis.

6. **Conclusion**:
- Therefore, stomata open when guard cells swell due to the influx of water, which is driven by the decrease in water potential in the guard cells compared to their surroundings.

### Final Answer:
Stomata open when guard cells swell due to **decreased water potential**.

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