Sorghum is a C4 plant (adapted to dry tropical regions). in the C4 pathway, the first stable product of CO2 fixation is a 4-carbon organic acid identified as oxaloacetic acid (OAA) , . This occurs in the mesophyll cells where phosphoenolpyruvate (PEP) is the primary CO2 acceptor . In contrast, the first stable product in C3 plants is 3-phosphoglyceric acid (PGA) , .