Explanation

Mesophyll cell. After the fixing of $CO_{2}$ to Oxaloacetic Acid (OAA) in $C_{4}$ cycle, the oxaloacetic acid changes into the malic aspartic acid $OAA + NADPH\overset{\ Malic\ acid\ dehydrogenase\ }{\rightarrow}$ Malic acid + $NADP^{+}$ $OAA + NH_{3} + NADPH\overset{\ Transaminase\ }{\rightarrow}$ Aspartic acid + $NADP^{+} + H_{2}O$ Both of these reactions occur in mesophyll cell