Question

Given below are two statements, one is labelled as Assertion (A) and other one labelled as Reason (R).
Assertion (A): ABA level is more when plant is under water stress
Reason (R): This is because when the plant is water stressed, the permeability of the chloroplast membranes to ABA increases and allows movement of the hormones to the epidermal cells including the guard cell
In light of the above statements, choose the correct answer from the options given below:

• A. Both (A) and (R) are true and (R) is the correct explanation of (A).
• B. Both (A) and (R) are true but (R) is NOT the correct explanation of (A).
• C. (A) is true but (R) is false.
• D. (A) is false but (R) is true.

Hint:

To remember the mechanism of stomatal closure, think of ABA as the "stress gatekeeper".
It acts as a rapid chemical signal that moves from the mesophyll storage sites to the guard cells within minutes of sensing water stress.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
Plants undergo various physiological and biochemical adaptations when subjected to drought or water stress.
Abscisic acid (ABA) acts as a primary signaling phytohormone coordinating stress responses.
Under stress conditions, plants rapidly accumulate ABA in their foliage to trigger protective mechanisms.
The primary downstream target of this pathway is the regulation of stomatal aperture.

Step 2: Detailed Explanation:
Under water deficit, the concentration of endogenous ABA in leaves can increase significantly.
This high level of ABA is crucial to prevent further transpiration and avoid tissue desiccation.
Thus, Assertion (A) is chemically and biologically true.
At the cellular level, under normal conditions, chloroplasts in mesophyll cells store a high concentration of ABA.
This storage occurs because the stromal pH of healthy chloroplasts is alkaline, which keeps ABA in its anionic, membrane-impermeable form.
When water stress occurs, the stromal pH drops, and the permeability of the chloroplast membrane increases.
This change in chloroplast membrane permeability allows the rapid efflux of ABA from the chloroplasts into the cytosol.
The hormone then moves via the apoplast and transpiration stream to the epidermal tissue.
Once it reaches the guard cells, ABA triggers the opening of anion channels and potassium efflux.
This loss of solutes reduces the turgidity of the guard cells, causing the stomata to close.
Thus, the changes in chloroplast membrane permeability directly explain why ABA levels increase at the target site during stress.
Therefore, Reason (R) is true and represents the correct scientific explanation of Assertion (A).

Step 3: Final Answer:
Both Assertion (A) and Reason (R) are true, and (R) is the correct explanation of (A).