The first stable product in C4 plant is
Oxaloacetic acid
In C4 plants, CO2 fixation occurs in mesophyll cells. Phosphoenol pyruvate is the primary electron acceptor for CO2 which is carboxylated to form oxaloacetic acid (OAA) in the presence of the enzyme PEP carboxylase. OAA is then converted to malic acid or aspartic acid, which is then transported to the bundle sheath cells.