71. Following observations were recorded while studying physiological parameters of sorghum and wheat under similar conditions:
A. Sorghum RUBISCO exhibits relatively higher affinity for CO₂ compared to that of wheat.
B. Light saturation of net photosynthetic flux is relatively lower for sorghum compared to that of wheat.
C. Warburg effect is difficult to record for sorghum and could be said as “not measurable” whereas it could be easily recorded for wheat.
D. Temperature optimum for net photosynthesis is lower for sorghum compared to that of Wheat.
E.  ¹³C/1²C ratio of assimilate is relatively higher sorghum compared to that of wheat.
Which one of the following combination of the above. observation is correct?
(1) Only A, B and C (2) Only B, C and E
(3) Only A, B and D (4) Only A, C and E

Introduction

Comparing physiological features of C3 wheat and C4 sorghum reveals essential knowledge about photosynthesis efficiency, Rubisco characteristics, CO₂ response, and metabolic adaptations in major crop plants.

Statement Analysis

A. Sorghum RUBISCO exhibits relatively higher affinity for CO₂ compared to that of wheat.

• Correct. C4 plants (sorghum) concentrate CO₂ in bundle sheath cells, so their Rubisco is more efficient and less prone to oxygenation compared to C3 plants (wheat).

B. Light saturation of net photosynthetic flux is relatively lower for sorghum compared to that of wheat.

• Incorrect. C4 plants like sorghum tend to tolerate and photosynthesize efficiently at higher light intensities; the light saturation point is generally higher than in C3 plants.

C. Warburg effect is difficult to record for sorghum and could be said as “not measurable” whereas it could be easily recorded for wheat.

• Correct. The Warburg effect—reduction in photosynthetic efficiency due to O₂ inhibition—is prominent in C3 plants (wheat) because of photorespiration but negligible in C4 plants (sorghum) due to their CO₂ concentrating mechanism.

D. Temperature optimum for net photosynthesis is lower for sorghum compared to that of wheat.

• Incorrect. C4 plants have a higher temperature optimum for photosynthesis than C3 plants due to reduced photorespiration and better enzyme function at elevated temperatures.

E. 13C/12C ratio of assimilate is relatively higher in sorghum compared to that of wheat.

• Correct. C4 plants discriminate less against 13CO₂, resulting in a higher 13C/12C ratio, while C3 plants show greater discrimination against 13C.

Why Option (4) Only A, C and E is Correct

• A is correct: Rubisco affinity is higher in sorghum (C4).

• C is correct: Warburg effect is measurable in wheat (C3), negligible in sorghum (C4).

• E is correct: C4 plants assimilate higher 13C/12C ratios than C3 plants.

Statements B and D are incorrect based on physiological principles and crop science evidence.