51. What would be effect on photosynthesis in C₃ and C₄ plants on elevating the concentration of CO₂ under light saturated condition?
(1) No effect on both type plants
(2) Increase in C₃ plant and unaffected in C₄ plant
(3) Increase in C₄ plant and low in C₃ plants
(4) Increase in both type plants

Concept: C3 vs C4 response to CO2

C3 plants are generally limited by CO2 at normal atmospheric levels, so increasing CO2 under adequate light boosts their photosynthetic rate until a saturation point. In contrast, C4 plants already concentrate CO2 around RUBISCO via their CO2-concentrating mechanism (PEP carboxylase and Kranz anatomy), so their photosynthesis is not greatly enhanced by further increases in external CO2 under light-saturated conditions.​

Because C3 plants suffer more from photorespiration at normal CO2, elevating CO2 reduces photorespiration and increases net carbon fixation, while C4 plants already minimize photorespiration and gain much less benefit from extra CO2.​

Why option (2) is correct

Option (2): “Increase in C3 plant and unaffected in C4 plant” matches the known physiological response under light-saturated conditions.​

• In C3 plants, higher CO2 increases the carboxylation rate of RUBISCO and suppresses photorespiration, leading to a higher photosynthetic rate and often increased growth and yield.​

• In C4 plants, CO2 is already concentrated near RUBISCO, so raising external CO2 has little additional impact on photosynthesis when light is not limiting, making them nearly unaffected in this specific condition.​

Therefore, under light-saturated conditions with elevated CO2, C3 photosynthesis increases while C4 photosynthesis shows minimal change, making option (2) correct.​

Why option (1) is wrong

Option (1): “No effect on both type plants” is incorrect because it ignores the CO2 limitation of C3 photosynthesis.​

• Experiments and agronomic data show that elevated CO2 enhances photosynthesis and biomass in many C3 crops such as wheat, rice and soybean, proving a clear positive effect.​

• Only C4 plants tend to show small changes under elevated CO2 at saturating light, so saying “no effect” for both types contradicts established plant physiology.​

Why option (3) is wrong

Option (3): “Increase in C4 plant and low in C3 plants” reverses the actual response pattern.​

• C3 plants do not show a decrease in photosynthesis with higher CO2; instead, they generally show increased net photosynthesis because carboxylation is favored over oxygenation.​

• C4 plants may show a slight increase in some cases, but their response is much smaller than that of C3 plants, and the main concept taught in exam biology is that C4 plants are relatively insensitive to elevated CO2.​

Thus, the statement that C3 decreases while C4 increases under elevated CO2 is opposite to the accepted concept and is incorrect.​

Why option (4) is wrong

Option (4): “Increase in both type plants” is partially correct for C3 but overestimates the response of C4 plants under light-saturated conditions.​

• While elevated CO2 clearly stimulates C3 photosynthesis, C4 photosynthesis is already near CO2 saturation at the site of RUBISCO, so any further increase in external CO2 produces little or no significant rise in photosynthetic rate.​

• Because the question specifically asks about light-saturated conditions and exam-level understanding emphasizes the negligible effect on C4 plants, saying both increase is not the best answer.​