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: Stomata Function
Stomata are pores on leaf surfaces that facilitate gas exchange (oxygen, carbon dioxide) and water vapor transpiration.
Guard cells surround these pores and control their aperture.

Step 2: Stomatal Closure Triggers
Stomata close under specific conditions, including:
- Insufficient demand for carbon dioxide in photosynthesis, either due to adequate internal reserves or cessation of photosynthetic activity.
- Dehydration of guard cells, leading to flaccidity and pore closure.

Step 3: Option Evaluation
(i) Requirement for carbon dioxide in photosynthesis:
This condition typically promotes stomatal opening for CO2 uptake, not closure.

(ii) Lack of requirement for carbon dioxide in photosynthesis:
When CO2 levels are sufficient or photosynthesis is inactive, stomata may close to minimize water loss.

(iii) Efflux of water from guard cells:
Water loss from guard cells causes them to become flaccid, resulting in stomatal closure, particularly under water deficit conditions.

(iv) Influx of water into guard cells:
Water uptake by guard cells leads to turgor and stomatal opening, the opposite of closure.

Conclusion:
Stomatal closure is induced by (ii) the plant's lack of need for carbon dioxide for photosynthesis and (iii) the outward movement of water from guard cells. The correct answer is therefore (ii) and (iii).