Question

The opening and closing of stomota is controlled by a hormone called :

• A. Gibberellins
• B. Auxin
• C. Ethylene
• D. Abscisic Acid

Hint:

Function keywords for plant hormones: Auxin = Apical dominance/elongation. Gibberellins = Stem elongation/seed germination. Ethylene = Fruit ripening. Abscisic Acid (ABA) = Stress tolerance/stomatal closure.

Answer 1

The Correct Option is D

Step 1: Concept:
This question requires identifying the specific plant hormone (phytohormone) responsible for regulating stomatal aperture, particularly during environmental stress.

Step 2: Step-by-step Explanation:

• Stomata are pores on the leaf surface that facilitate gas exchange but also allow water loss via transpiration.

• Abscisic Acid (ABA) is commonly known as the "stress hormone" in plants.

• During conditions of water scarcity or drought stress, ABA is synthesized in the roots and transported to the leaves.

• ABA binds to receptors in the guard cells surrounding the stomata, causing an efflux of potassium ions ($K^+$) and water out of the guard cells. The guard cells become flaccid, causing the stomata to strictly close, thereby conserving water.

• While other hormones (like auxins and cytokinins) play minor roles in stomatal opening, ABA is the primary master controller for rapid stomatal closure.

Step 3: Final Answer:
Abscisic Acid controls stomatal closing. This matches option (D).