The answer is In a cactus, carbon is fixed only during the night.
There are three types of carbon fixation in plants - C3, C4, and CAM fixation. C3 and C4 plants open their stomata during the day while CAM plants open stomata during the night. Mostly, plants fix carbon during the day. However, plants in arid locations, such as cactuses, use CAM carbon fixation because of adaptation to water deficiency. Their stomata are closed during the day and water loss is reduced. In the night when is colder, stomata open and carbon dioxide enters through stomata.
