65. Which one of the following graphs best represents the net CO₂ fixation of typical C₃ and C₄ plants under increasing CO₂ concentration and saturating light?

Introduction

Understanding how C3 and C4 plant photosynthesis responds to increasing CO₂ concentrations under high light is a classic topic in plant physiology and exam questions. The response of net CO₂ fixation for each type is distinct and determined by their enzymatic and anatomical adaptation.

Detailed explanation of the question

Under saturating light:

• C3 plants: Net CO₂ fixation rises sharply with increasing CO₂, as higher CO₂ reduces photorespiration. The curve shows a steep initial rise that saturates at higher CO₂ concentrations.

• C4 plants: CO₂ fixation is high even at low atmospheric CO₂ due to their CO₂ concentrating mechanism (Kranz anatomy + PEP carboxylase). The curve is flatter, less responsive to rising CO₂ after a low threshold.

Analysis of each graph option

Option 1 [Correct]

• C3 curve: Lower at low CO₂, rises steeply, then saturates at high CO₂.

• C4 curve: Higher initially, nearly flat, indicating little further increase in response to elevated CO₂.

• This matches textbook physiology: C3 plants are highly CO₂-responsive, C4 plants less so under saturating light.​

Option 2

• Both C3 and C4 curves show similar saturating patterns and starting points, which misrepresents C4 behavior—the C4 curve should start higher at low CO₂ and be less responsive to increases.

Option 3

• C4 curve is below the C3 curve at low CO₂, which is incorrect; C4 plants fix CO₂ efficiently even at low concentrations.

Option 4

• Both curves start high but C4 is always above C3, even as CO₂ increases. It misrepresents the pattern: C3 should rise and eventually approach C4 at high CO₂, not remain below throughout.

Conclusion

Only Option (1) correctly depicts net CO₂ fixation:

• C3 plant fixation is low at low CO₂, increases sharply with more CO₂, reaches saturation.

• C4 plant fixation is high and relatively constant, showing limited increase beyond low CO₂ concentrations.