Photosynthesis in $C_{4}$ plants is relatively less limited by atmospheric $C O_{2}$ levels because

Four carbons acids are the primary initial

C O_{2}

fixation products.

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The primary fixation of

C O_{2}

is mediated via PEP carboxylase.

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Effective pumping of

C O_{2}

into bundle sheath cells takes place.

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RuBisCO in

C_{4}

plants has higher affinity for

C O_{2}

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Solution

The correct option is A The primary fixation of $C O_{2}$ is mediated via PEP carboxylase.
CO $_{2}$ concentration of the atmosphere is 360 ppm. It is a limiting factor for C $_{3}$ as the available CO $_{2}$ concentration is lower than the optimum for photosynthesis. Increase in its concentration upto 0.05% increases the rate of photosynthesis in most C $_{3}$ plants. When CO $_{2}$ concentration is reduced, there comes a point at which illuminated plant parts stop absorbing CO $_{2}$ from their environment. It is called as CO $_{2}$ compensation point or threshold value. The C $_{4}$ plants have this compensation value low as they are more efficient in CO $_{2}$ fixation through PEP- Carboxylase. The optimum CO $_{2}$ concentration for C $_{4}$ plants is 360 ppm. But the atmospheric conc of CO $_{2}$ is increasing. If the trend of CO $_{2}$ increase continues then that would be harmful for C $_{4}$ plants.

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