In Hatch and Slack pathway, the primary ${\text{CO}}_2$ acceptor is:

In ${\text{C}}_4$ plants (Hatch-Slack pathway), the primary ${\text{CO}}_2$ acceptor is a 3-carbon molecule called phosphoenol pyruvate (PEP), present in the mesophyll cells. The reaction is catalyzed by PEP carboxylase. Oxaloacetic acid (OAA) is the first stable product.