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Photosynthesis in $C_4$ plants is relatively less limited by atmospheric $CO_2$ levels because
A
The primary fixation of $CO_2$ is mediated via PEP carboxylase
B
Effective pumping of $CO_2$ into bundle sheath cells
C
Four carbon acids are the primary initial $CO_2$ fixation products
D
Rubisco in $C_4$ plants has higher affinity for $CO_2$
Detailed Solution
In $C_4$ plants, $CO_2$ is first fixed in the mesophyll cells by PEP carboxylase into a 4-carbon acid.
This acid moves into the bundle sheath cells and is decarboxylated, releasing $CO_2$ there.
This works as a $CO_2$ pump: it raises the concentration of $CO_2$ in the bundle sheath cells, where RuBisCO and the Calvin cycle are located, to a level much higher than in the atmosphere.
Because of this effective pumping, RuBisCO works at near-saturating $CO_2$ even when the atmospheric $CO_2$ is low, and photorespiration is almost eliminated.
So photosynthesis is less limited by atmospheric $CO_2$ levels; the $CO_2$ compensation point of $C_4$ plants is very low.
RuBisCO itself is the same enzyme as in $C_3$ plants, and the fixation by PEP carboxylase or the formation of 4-carbon acids is only a part of this mechanism.
So the reason is the effective pumping of $CO_2$ into the bundle sheath cells.
This acid moves into the bundle sheath cells and is decarboxylated, releasing $CO_2$ there.
This works as a $CO_2$ pump: it raises the concentration of $CO_2$ in the bundle sheath cells, where RuBisCO and the Calvin cycle are located, to a level much higher than in the atmosphere.
Because of this effective pumping, RuBisCO works at near-saturating $CO_2$ even when the atmospheric $CO_2$ is low, and photorespiration is almost eliminated.
So photosynthesis is less limited by atmospheric $CO_2$ levels; the $CO_2$ compensation point of $C_4$ plants is very low.
RuBisCO itself is the same enzyme as in $C_3$ plants, and the fixation by PEP carboxylase or the formation of 4-carbon acids is only a part of this mechanism.
So the reason is the effective pumping of $CO_2$ into the bundle sheath cells.