62. Following are some of the statements regarding the effect of CO₂ concentration on photosynthesis in plants.
A.With elevated CO₂ levels, C₃ plants are much more responsive than C₄ plants under well watered conditions.
B. In C₃ plants, increasing intracellular CO₂ partial pressure can stimulate photosynthesis only over a narrow range.
C. In C₄ plants, CO₂ compensation point is nearly zero.
Which one of the following combination of above statements is correct?
(1) A and B                      (2) B and C
(3) A and C                      (4) Only C

Introduction

The effect of atmospheric CO2 concentration on photosynthesis varies between C3 and C4 plants due to their distinct photosynthetic mechanisms. Elevated CO2 often enhances photosynthesis more in C3 plants, while C4 plants maintain a near-zero CO2 compensation point due to their specialized CO2 concentrating mechanism.

Explanation of statements

A. With elevated CO2 levels, C3 plants are much more responsive than C4 plants under well-watered conditions.

• True. C3 plants benefit significantly from elevated CO2 because it suppresses photorespiration and enhances carbon fixation by Rubisco, leading to higher photosynthetic rates. C4 plants are less responsive since their CO2 is already concentrated internally.

B. In C3 plants, increasing intracellular CO2 partial pressure can stimulate photosynthesis only over a narrow range.

• Incorrect or partly misleading. C3 plants show a generally positive response over a broader range of increased CO2 before saturation occurs, depending on environmental conditions.

C. In C4 plants, CO2 compensation point is nearly zero.

• True. Due to the CO2 concentrating mechanism and minimized photorespiration, C4 plants have a CO2 compensation point close to zero, meaning they can fix carbon efficiently even at very low ambient CO2.

Why option (3) A and C is correct

• Both A and C statements are well-established physiological facts about C3 and C4 plants, respectively.

• Statement B is less accurate or too narrow to be fully correct.

This makes option (3) the best choice, reflecting the key differences in CO2 response and compensation points between plant types relevant for exams and plant physiology understanding.