Explanation

In $C_{4}$-plants the initial fixation of carbon dioxide occurs in mesophyll cell. The primary acceptor of $CO_{2}$ is phosphoenol pyruvate or PEP. It combines with carbon dioxide in presence of PEP carboxylase or PEPcase to form oxaloacetic acid or oxaloacetate $PEP + CO_{2} + H_{2}O\overset{\ PEP\ carboxylase\ \ }{\rightarrow}$ Oxaloacetic acid + $H_{3}PO_{4}$