58. Atmospheric CO₂  contains the naturally occurring stable carbon isotopes ¹²C and ¹³C in the proportion of 98.9% and 1.1%, respectively. Following are some of the statements regarding CO₂ assimilation:
(A) Both C₃ and C₄ plants assimilates less ¹³CO₂ than ¹²CO₂.
(B) Both C₃ and C₄ platns assimilates less ¹²CO₂ than ¹³CO₂.
(C) C₃ plants assimilates lesser ¹³CO₂ than ¹²CO₂ as compared to C₂ plants.
(D) C₄ plants assimilates lesser ¹³CO₂ than ¹²CO₂ as compared to C₃ plants.
Which one of the following combinations of above statements is true?
(1) A and B                   (2) A and C
(3) C and D                  (4) A and D

Introduction

Carbon isotope discrimination during CO₂ assimilation is a key physiological trait that distinguishes C3 and C4 photosynthetic pathways. Atmospheric CO₂ contains two stable isotopes, 12C (98.9%) and the heavier 13C (1.1%), and plants selectively assimilate these isotopes differently due to enzymatic preferences and CO₂ fixation mechanisms. This isotopic difference helps understand plant physiology and is often tested in biology exams.

Explanation of statements

(A) Both C3 and C4 plants assimilate less 13CO₂ than 12CO₂.

• True. Both C3 and C4 plants discriminate against the heavier 13C isotope during photosynthesis, favoring the lighter 12CO₂ because it is easier for enzymes like Rubisco and PEP carboxylase to fix. However, the extent of discrimination differs between the two.

(B) Both C3 and C4 plants assimilate less 12CO₂ than 13CO₂.

• False. Plants preferentially assimilate the lighter 12CO₂ over the heavier 13CO₂. This statement is the opposite of known isotope discrimination principles.

(C) C3 plants assimilate lesser 13CO₂ than 12CO₂ as compared to C2 plants.

• This statement is unclear or partially incorrect. Generally, C2 plants refer to C3-C4 intermediate plants or photorespiring C3 plants. But the question contrasts C3 and C4 plants, so this is less relevant or ambiguous.

(D) C4 plants assimilate lesser 13CO₂ than 12CO₂ as compared to C3 plants.

• True. The discrimination against 13CO₂ is smaller in C4 plants than in C3 plants due to their CO₂ concentrating mechanism, which reduces the fractionation effect seen in C3 plants during fixation by Rubisco. As a result, C4 plants have less discrimination and a higher 13C/12C ratio in their biomass compared to C3 plants.

Why option (4) A and D is correct

• A is true: Both plant types discriminate against 13CO₂, but to different extents.

• D is true: C4 plants discriminate less against 13CO₂ compared to C3 plants due to their biochemical CO₂ concentrating adaptation.

Options including B and C are incorrect based on the principles of carbon isotope discrimination and photosynthetic physiology.

Hence, option (4) best fits the correct understanding of carbon isotope assimilation differences in C3 and C4 plants.