What would be the expected effect on plants if the atmospheric CO2 concentration was doubled? What would be the expected effect on plants if the atmospheric CO2 concentration was doubled? C4 plants would have faster growth rates; C3 plants would be minimally affected. C3 plants would have faster growth rates; C4 plants would have slower growth rates. All plants would have faster growth rates. C3 plants would have faster growth rates; C4 plants would be minimally affected.

C3 and C4 pathways are the variations of dark reactions of photosynthesis present in green plants. The photosynthetic efficiency of C3 plants is reduced due to the affinity of RuBisCo enzyme for oxygen which in turn leads to the futile pathway of photorespiration. RuBisCo enzyme catalyzes the rate-limiting reaction of the C3 pathway. On the other hand, the C4 plants concentrate CO2 around RuBisCo in their bundle sheath cells of leaves to minimize photorespiration and exhibit higher rates of photosynthesis.

Increased levels of atmospheric CO2 would reduce the photorespiration in C3 plants and would allow them to fix CO2 efficiently due to the increased concentration of CO2 around the enzyme RuBisCo. The increased photosynthetic efficiency would help these plants to exhibit faster growth rates.

However, the photosynthetic rate of C4 plants is not limited by CO2 concentration as they themselves reduce photorespiration by spatial separation of primary carboxylation in mesophyll cell and CO2 fixation in bundle sheath cells. Hence, increased CO2 levels in the atmosphere would not have any impact on their photosynthetic rate and growth.