Succulent plants such as cactus and pineapple, which are native to very hot, very dry environments, have another variation on photosynthetic CO 2 fixation, which reduces loss of water vapor through the pores (stomata) by which CO 2 and O 2 must enter leaf tissue.

Instead of separating the initial trapping of CO 2 and its fixation by rubisco across space (as do the C 4 plants), they separate these two events over time. At night, when the air is cooler and moister, the stomata open to allow entry of CO 2 , which is then fixed into oxaloacetate by PEP carboxylase. The oxaloacetate is reduced to malate and stored in the vacuoles, to protect cytosolic and plastid enzymes from the low pH produced by malic acid dissociation.

During the day the stomata close, preventing the water loss that would result from high daytime temperatures, and the CO 2 trapped overnight in malate is released as CO 2 by the NADP-linked malic enzyme. This CO 2 is now assimilated by the action of rubisco and the Calvin cycle enzymes.

Because this method of CO 2 fixation was first discovered in stonecrops, perennial flowering plants of the family Crassulaceae, it is called crassulacean acid metabolism, and the plants are called CAM plants.