At what time, the stomatal transpiration is completely stopped

Options

A. Morning
B. Night
C. Evening
D. None of the above

Answer

**Step-by-Step Solution:**

1. **Understanding Stomatal Transpiration:**
- Stomatal transpiration refers to the process of water vapor loss from plant leaves through small openings called stomata. This process is primarily regulated by the guard cells surrounding each stoma.

2. **Role of Light:**
- The opening and closing of stomata are influenced by light. During the day, when there is ample light, guard cells take up water and become turgid, causing the stomata to open and allowing transpiration to occur.

3. **Nighttime Conditions:**
- At night, there is no light available for photosynthesis. Consequently, the guard cells do not take up water as they do during the day. Instead, the solute concentration within the guard cells decreases.

4. **Osmotic Changes:**
- The decrease in solute concentration leads to a drop in osmotic potential within the guard cells. As a result, water moves out of the guard cells, causing them to lose turgor pressure.

5. **Closure of Stomata:**
- When the guard cells lose water and become flaccid, the stomatal pores close. This closure effectively halts transpiration, as water vapor can no longer escape from the leaf.

6. **Conclusion:**
- Therefore, stomatal transpiration is completely stopped at night when the stomata are closed due to the flaccidity of guard cells.

**Final Answer:**
- The correct option is "night."

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