Question

Given below are two statements, one labelled as Assertion (A) and other one labelled as Reason (R).

Assertion (A): The Net Assimilation Rate (NAR) is a measure of the average photosynthetic efficiency of leaves in a crop community.

Reason (R): It is highest when the plants are small and most of the leaves are exposed to sunlight.

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:

Think about how much shading occurs between leaves when a crop is still small.

Answer 1

The Correct Option is A

Step 1: Understanding the Question:
The Assertion defines Net Assimilation Rate (NAR) as a measure of average photosynthetic efficiency of leaves in a crop stand, and the Reason states that NAR is highest when plants are small with most leaves exposed to sunlight. We need to judge both and see if the Reason explains the Assertion.

Step 2: Key Formula or Approach:
NAR is defined as the rate of increase in dry matter per unit leaf area per unit time, so it captures how efficiently, on average, each unit of leaf surface in the crop is building dry matter through photosynthesis after subtracting respiration losses.
\[ NAR = \frac{1}{LA} \times \frac{dW}{dt} \]
where \(LA\) is leaf area and \(W\) is plant dry weight.

Step 3: Detailed Explanation:
Because NAR is dry matter gained per unit leaf area, it directly reflects how well each unit of leaf is photosynthesizing on average, so the Assertion's definition is correct.
Early in crop growth, the plant is small, the leaf area index is low, and almost every leaf sits in full sunlight with little shading from neighbouring leaves. Under this good light exposure, each leaf photosynthesizes near its full capacity, so the average efficiency per unit leaf area, that is NAR, is at its highest. As the crop grows bigger and the canopy closes, lower leaves start shading each other, so many leaves get less light and photosynthesize less efficiently, pulling the average NAR down. This confirms the Reason is true and it is exactly why NAR peaks when the plant is small and well lit.

Step 4: Final Answer:
Both (A) and (R) are true, and (R) correctly explains why NAR behaves the way the Assertion describes, so the answer is option 1.