Question

Given below are two statements, one is labelled as Assertion (A) and other one labelled as Reason (R).
Assertion (A): C4 plants usually do not reach radiation saturation in direct sunlight.
Reason (R): C4 plants use high radiation levels more efficiently at low levels than under full 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:

C4 plants (maize, sorghum, sugarcane) have evolved to utilize intense solar radiation without saturating, thanks to their carbon-concentrating Kranz anatomy.
This makes them highly productive in high-light, warm tropical environments compared to C3 plants.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
Photosynthetic light-response curves show how the rate of carbon dioxide fixation changes with increasing light intensity (solar radiation).
C3 and C4 plants exhibit fundamentally different light-response behaviors due to their distinct internal leaf anatomy and carbon-fixation enzymes.

Step 2: Detailed Explanation:
Let us analyze the Assertion and Reason statements separately:
- Assertion (A): C3 plants (like wheat and rice) have a relatively low light saturation point, reaching maximum photosynthesis at about \(1/3\) to \(1/2\) of full direct sunlight.
Any light intensity beyond this point does not increase photosynthesis because the Rubisco enzyme becomes saturated with carbon dioxide.
In contrast, C4 plants (like maize, sugarcane, and sorghum) possess Kranz anatomy and use the highly efficient PEP carboxylase enzyme to concentrate carbon dioxide around Rubisco inside bundle sheath cells.
This active carbon-concentrating mechanism prevents photorespiration and allows C4 plants to continuously increase their photosynthetic rate with increasing light intensity, meaning they do not reach light saturation even under full direct sunlight.
Therefore, Assertion (A) is correct.
- Reason (R): C4 plants are adapted to open, high-light environments and use high levels of solar radiation extremely efficiently under full direct sunlight.
The statement that they "use high radiation levels more efficiently at low levels than under full sunlight" is chemically and physically contradictory and incorrect.
At low light levels, C3 and C4 plants have similar efficiencies, but C4 plants are significantly more efficient than C3 plants under high light and high temperature conditions.
Therefore, Reason (R) is incorrect.
Since Assertion (A) is true and Reason (R) is false, option (C) is the correct choice.

Step 2: Final Answer:
Assertion (A) is true but Reason (R) is false.