73. The table below shows photosynthetic type, temperature and sunlight intensity levels.

Photosynthetic Type   Temperature              Sunlight Intensity
A.   C3 Plant                   i. High                      P.  High
B.   C4 Plant                  ii. Moderate                Q.  Moderate
iii. Low                       R.  Low

Which of the following correctly matches the plant photosynthetic type with the temperature and sunlight conditions in which photosynthetic rate per unit leaf area is maximum for that plant?
(1) A-i-P; B-iii-R      (2) A-iii-P; B-i-Q
(3) A-i-R; B-ii-Q      (4) A-ii-Q; B-i-P

Introduction

C3 and C4 plants differ fundamentally in their physiological preferences for temperature and sunlight, influencing their maximum photosynthetic rates per unit leaf area. Understanding these ecological and biochemical adaptations is crucial in plant biology, agriculture, and climate science.

Explanation of Matching

• C3 Plants (A): These plants, including wheat, rice, and barley, thrive in cooler, moderate temperatures and moderate sunlight. High temperatures and excessive light intensify photorespiration, reducing net photosynthesis. Thus, C3 plants achieve the highest photosynthetic rate under moderate temperature (ii) and moderate sunlight (Q).

• C4 Plants (B): These plants, such as sugarcane and maize, are adapted to hot, sunny environments due to their CO₂ concentrating mechanism and reduced photorespiration. They reach peak photosynthetic rate per leaf area under high temperature (i) and high sunlight (P).

Option Analysis

(1) A-i-P; B-iii-R

• Assigns C3 plants to high temperature/high sunlight, where photorespiration limits photosynthetic rate.

• Assigns C4 plants to low temperature/low sunlight, which does not support their optimal rate.

(2) A-iii-P; B-i-Q

• Assigns C3 plants to low temperature/high sunlight, not optimum; and C4 plants to high temperature/moderate sunlight, also suboptimal.

(3) A-i-R; B-ii-Q

• Assigns C3 plants to high temperature/low sunlight, and C4 to moderate temperature/moderate sunlight—both not matching physiological optima.

(4) A-ii-Q; B-i-P

• Correctly matches C3 plants with moderate temperature/moderate sunlight and C4 plants with high temperature/high sunlight for maximum photosynthetic rates.

Conclusion

Maximum photosynthetic rate per unit leaf area is achieved for C3 plants under moderate temperature and sunlight, and for C4 plants under high temperature and intense sunlight. Option (4) aligns with established plant physiology for optimal growth conditions in these photosynthetic types.