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

This question checks whether we understand what Net Assimilation Rate (NAR) measures and why it changes as a crop grows.

1. Assertion: NAR is dry matter gain per unit leaf area per unit time, which is exactly a measure of the average photosynthetic efficiency of the leaves in the crop stand, so this statement is true.
2. Reason: In young, small plants, leaf area index is low and self shading between leaves is minimal, so nearly every leaf gets strong, direct sunlight and photosynthesizes efficiently, giving a high NAR. As the canopy fills in with growth, lower leaves fall into shade cast by upper leaves, their photosynthesis slows, and the average NAR across the whole plant drops. So this statement is also true.

Since the drop in self shading in small plants is precisely what keeps every leaf efficient and pushes NAR to its peak, the reason directly explains the assertion.

Let's summarize:

• NAR measures dry matter built per unit leaf area, an index of average leaf efficiency.
• Small plants have little mutual shading, so NAR is highest early in growth.
• Canopy closure later on lowers NAR through increased shading.

Both statements are true and the reason correctly explains the assertion, so the answer is option 1.