C4 plants are adapted to hot, dry climates, when their stomata are partially closed to conserve water. The enzyme PEP carboxylase has a higher affinity for CO2 than rubisco and produces the four-carbon product oxaloacetate in mesophyll cells, then exported to bundle sheath cells. This adaptation maintains a CO2 concentration in the bundle sheath that favors photosynthesis over photorespiration. Review: