A variant of C₃ photosynthesis in which CO₂ is first concentrated at the site of photosynthesis by carboxylation of phosphoenolpyruvate to oxaloacetate, which is then transported as such or as an interchangeable 4-carbon dicarboxylic acid to the site of photosynthesis. This is an advantage to the plant because the key enzyme, ribulose bisphosphate carboxylase, which in photosynthesis forms 2 mol of 3-phosphoglycerate, has a competing activity, O₂ photorespiration, which produces both 3-phosphoglycerate and phosphoglycolate.