64. The quantum yield of photosynthetic carbon fixation in a C₃ plant and C₄ plant is studied as a function of leaf temperature. Following are some statements based on this study.
A. At lower temperature the quantum yield of C₃ plant is lower than C₄ plant.
B. In C₄ plant quantum yield does not show a temperature dependence.
C. Since the photorespiration is low in C₄ plants temperature dependence. because of CO₂ concentrating mechanism, quantum yield is not affected.
D. At higher temperature the quantum yield of C₃ plant is lower than C₄ plant.
Which one of the following combination of above statements is correct?
(1) A, B and D           (2) B, C and D
(3) A, B and C          (4) A, C and D

Introduction

The quantum yield of photosynthesis refers to the efficiency with which absorbed light is converted into fixed carbon during photosynthesis. It varies with leaf temperature and photosynthetic type, showing distinct patterns in C3 and C4 plants due to differences in photorespiration and CO2 concentration mechanisms.

Explanation of statements

A. At lower temperature the quantum yield of C3 plant is lower than C4 plant.

• True. Several studies show C3 plants have lower quantum yield at lower temperatures compared to C4 plants because photorespiration in C3 plants is temperature sensitive, resulting in reduced efficiency.

B. In C4 plant quantum yield does not show a temperature dependence.

• True. C4 plants have a CO2 concentrating mechanism that eliminates photorespiration, making their quantum yield relatively stable over a range of temperatures.

C. Since the photorespiration is low in C4 plants temperature dependence because of CO2 concentrating mechanism, quantum yield is not affected.

• This statement is somewhat confusing in wording but effectively true; low photorespiration owing to the CO2 concentrating mechanism in C4 plants limits the temperature effect on quantum yield.

D. At higher temperature the quantum yield of C3 plant is lower than C4 plant.

• True. High temperatures increase photorespiration in C3 plants, lowering quantum yield compared to C4 plants, which maintain higher efficiency.

Why option (1) A, B and D is correct

• All statements except C are clearly accurate and supported by plant physiology research.

• Statement C’s wording is unclear but generally reflects the idea behind B, yet is less precise and is a partial restatement.

• Hence, the best choice that includes only clearly true statements from the question is option (1).

The quantum yield temperature dependence distinction between C3 and C4 plants is essential knowledge for understanding plant adaptation, photosynthetic efficiency, and agricultural productivity.