Question

Given below are two statements, one is labelled as Assertion (A) and other one labelled as Reason (R).
Assertion (A) : Light normally increases the transpiration rate in plant.
Reason (R) : Light normally causes opening of stomata and increase in temperature.
In light of the above statements, choose the most appropriate answer from the options given below. :

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

Hint:

Over 90% of water absorbed by plants is lost through stomatal transpiration.
Because stomata are open during the day to allow \(CO_2\) absorption for photosynthesis, transpiration is an unavoidable consequence of carbon assimilation.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
Transpiration is the loss of water in the form of water vapor from the aerial parts of plants, primarily through the stomatal openings on leaf surfaces.
This physiological process is regulated by several environmental factors, including light, temperature, relative humidity, and wind velocity.

Step 2: Detailed Explanation:
Let us analyze both the Assertion and the Reason:
- Assertion (A): During the day, in the presence of sunlight, the rate of transpiration in plants increases significantly compared to nighttime when it remains negligible.
Thus, Assertion (A) is true.
- Reason (R): Light affects transpiration in two main ways:
1. Stomatal Opening: Blue light triggers photoreceptors in guard cells, causing an influx of potassium ions, which leads to osmotic water uptake, turgor increase, and the subsequent opening of stomata.
2. Temperature Increase: Absorbed light raises the temperature of the leaf tissue, which increases the vapor pressure difference between the internal leaf air spaces and the surrounding atmosphere, accelerating water vapor diffusion.
Thus, Reason (R) is true.
- Relationship between (A) and (R):
Since light directly induces stomatal opening and increases leaf temperature (R), more water vapor can escape from the leaf surface, which directly explains the increased rate of transpiration observed under light conditions (A).
Therefore, (R) is the correct physiological explanation of (A).

Step 3: Final Answer:
Both (A) and (R) are correct, and (R) is the correct explanation of (A), corresponding to Option A.