Question

A stomata closes when :
(i) it needs carbon dioxide for photosynthesis.
(ii) it does not need carbon dioxide for photosynthesis.
(iii) water flows out of the guard cells.
(iv) water flows into the guard cells.
The correct reason(s) in this process is/are :

• A. (i) only
• B. (i) and (iii)
• C. (ii) and (iii)
• D. (ii) and (iv)

Answer 1

The Correct Option is C

Step 1: Understanding the role of stomata:
Stomata are small openings on the surface of leaves that allow gases (such as oxygen and carbon dioxide) and water vapor to move in and out of the plant.
They are surrounded by guard cells that regulate the opening and closing of the stomatal pore.

Step 2: Factors affecting stomatal closure:
There are certain conditions under which stomata will close, and these include:
- The plant doesn't need carbon dioxide for photosynthesis, such as when it has sufficient carbon dioxide stored or when photosynthesis is not occurring.
- Water loss occurs from the guard cells, causing them to become flaccid and close the pore.

Step 3: Analyzing the given options:
(i) It needs carbon dioxide for photosynthesis:
This would typically lead to the stomata opening to allow carbon dioxide to enter. This is not the reason for stomatal closure.

(ii) It does not need carbon dioxide for photosynthesis:
When the plant has sufficient carbon dioxide or photosynthesis is not actively occurring, the stomata may close to conserve water.

(iii) Water flows out of the guard cells:
When water flows out of the guard cells, they become flaccid, causing the stomata to close. This is a key reason for stomatal closure, especially during water stress.

(iv) Water flows into the guard cells:
When water flows into the guard cells, they become turgid, causing the stomata to open. This is the opposite of what happens during stomatal closure.

Conclusion:
The correct reasons for stomatal closure are when (ii) the plant does not need carbon dioxide for photosynthesis, and (iii) water flows out of the guard cells. Therefore, the correct answer is (ii) and (iii).