Question

Why can CAM plants keep their stomata closed during the day time, thus reducing loss of water?

Answer 1

Crassulacean Acid Metabolism (CAM) is observed in plants such as Opuntia, orchids, pineapple, and so on growing in xerophytic conditions. These plants show xerophytic adaptations such as thick cuticle, sunken stomata, reduced leaves. The stomata remain closed during the day and open only at night. This is primarily done to prevent water loss through transpiration. Since the stomata are closed during the day, CO₂ supply for photosynthesis is also cut off.
At night, CO₂ diffuses in and phosphoenol pyruvic acid is carboxylated into oxaloacetic acid which is then converted into malic acid. This malic acid is stored in the vacuole of the cell.
During daytime, malic acid stored in the cell vacuole is decarboxylated to pyruvic acid and CO₂ is evolved. Using the path of Kreb’s cycle, pyruvic acid is converted to phosphoenol pyruvic acid. CO₂ liberated during decarboxylation of malic acid is fixed using the Calvin cycle. Thus, even though diffusion of CO₂ does not occur during daytime, photosynthesis goes on as other metabolic reactions produce CO₂.