63. Following are certain statements regarding C₃, C₄ and CAM plants?
A. The ratio of water loss to CO₂ uptake is higher in CAM plants than it is in either C₃ and C₄ plants.
B. The rate of photosynthesis attains maximum rate at lower intracellular CO2 partial pressure in C₄ plants than in C₃ plants.
C. The compensation point in C₃ plants are always lower than C₄ plants.
D. Plants with C₄ metabolism need less rubisco than C3 plants to achieve a given rate of photosynthesis.
Which one of the following combinations of above statements is correct?
(1) A and B   (2) A and C
(3) C and D   (4)B and D

Introduction

C3, C4, and CAM plants exhibit distinct physiological adaptations to optimize photosynthesis under various environmental conditions. Understanding their differences in water use efficiency, photosynthetic responses to CO2, enzyme requirements, and compensation points is critical for students and researchers studying plant physiology.

Explanation of each statement

A. The ratio of water loss to CO2 uptake is higher in CAM plants than in either C3 and C4 plants.

• False. CAM plants are adapted to arid environments and minimize water loss by opening their stomata at night to fix CO2. Hence, they have a lower ratio of water loss to CO2 uptake (higher water-use efficiency) compared to both C3 and C4 plants.

B. The rate of photosynthesis attains maximum rate at lower intracellular CO2 partial pressure in C4 plants than in C3 plants.

• True. C4 plants possess a CO2-concentrating mechanism enabling them to achieve maximum photosynthesis at lower intracellular CO2 partial pressures relative to C3 plants, which rely directly on atmospheric CO2 concentrations.

C. The compensation point in C3 plants is always lower than in C4 plants.

• False. C3 plants have a higher CO2 compensation point due to higher rates of photorespiration. C4 plants have an efficient mechanism to reduce photorespiration, leading to a lower compensation point (near zero).

D. Plants with C4 metabolism need less Rubisco than C3 plants to achieve a given rate of photosynthesis.

• True. Due to the CO2 concentrating mechanism in C4 plants, they require less Rubisco enzyme to achieve the same photosynthetic output compared to C3 plants, which must compensate for photorespiration losses.

Why option (4) B and D is correct

• Statement B correctly describes the efficient CO2 use in C4 plants.

• Statement D accurately reflects the lower Rubisco requirement in C4 plants due to their CO2 concentration mechanism.

• Statements A and C are incorrect based on physiological facts about water use efficiency and CO2 compensation points.

Thus, the scientifically accurate combination is option (4): B and D.

This knowledge is essential for understanding plant adaptation and responses in botany and plant physiology courses and exams.