crassulacean acid metabolism

(CAM) A form of photosynthesis first described in the family Crassulaceae and since found in many other succulent plants. CAM plants keep their stomata closed during the day to reduce water loss by transpiration. Carbon dioxide can therefore only enter at night when, instead of combining with ribulose bisphosphate (as in conventional *C₃ plants ) it combines with the three-carbon compound phosphoenol pyruvate (PEP) to give the four-carbon oxaloacetic acid. This is then converted to malic acid, which can be stored in the cell vacuoles until daylight, when it is transferred to the cytoplasm. Here it is broken down to release carbon dioxide, which is then fixed in the normal manner. This adaptation allows such plants to flourish in arid habitats but their growth rate is slow. CAM can be induced in certain C₃ plants by water shortage. See also Hatch-Slack pathway.