Photosynthesis in desert plants has evolved adaptations that conserve water. In harsh, dry heat, every drop of water must be used to survive. Because stomata must open to allow for the uptake of CO₂, water escapes from the leaf during active photosynthesis. Desert plants have evolved processes to conserve water and deal with harsh conditions. A more efficient use of CO₂ allows plants to adapt to living with less water. 

Some plants such as cacti can prepare materials for photosynthesis during the night by a temporary carbon fixation and storage process, because opening the stomata at this time conserves water due to cooler temperatures. In addition, cacti have evolved the ability to carry out low levels of photosynthesis without opening stomata at all, a mechanism for surviving extremely dry periods.

Cactus

The harsh conditions of the desert have led plants like these cacti to evolve variations of the light-independent reactions of photosynthesis. These variations increase the efficiency of water usage, helping to conserve water and energy.

CAM Photosynthesis

Xerophytes, such as cacti and most succulents, also use phosphoenolpyruvate (PEP) carboxylase to capture carbon dioxide in a process called crassulacean acid metabolism (CAM). In contrast to C₄ metabolism, which physically separates the CO₂ fixation to PEP from the Calvin cycle, CAM temporally separates these two processes. 

CAM plants have a different leaf anatomy from C₃ plants, and fix the CO₂ at night, when their stomata are open. CAM plants store the CO₂ mostly in the form of malic acid via carboxylation of phosphoenolpyruvate to oxaloacetate, which is then reduced to malate. Decarboxylation of malate during the day releases CO₂ inside the leaves, thus allowing carbon fixation to 3-phosphoglycerate by RuBisCO. Sixteen thousand species of plants use CAM.

Cross section of agave, a CAM plant

Cross section of a CAM (crassulacean acid metabolism) plant, specifically of an agave leaf. Vascular bundles shown. Drawing based on microscopic images courtesy of Cambridge University Plant Sciences Department.

C₄ Carbon Fixation

The C4 pathway bears resemblance to CAM; both act to concentrate CO₂ around RuBisCO, thereby increasing its efficiency. CAM concentrates it temporally, providing CO₂ during the day and not at night, when respiration is the dominant reaction. 

C₄ plants, in contrast, concentrate CO₂ spatially, with a RuBisCO reaction centre in a "bundle sheath cell" that is inundated with CO₂. Due to the inactivity required by the CAM mechanism, C₄ carbon fixation has a greater efficiency in terms of PGA synthesis.

Cross section of maize, a C₄ plant

Cross section of a C4 plant, specifically of a maize leaf. Drawing based on microscopic images courtesy of Cambridge University Plant Sciences Department.

C₄ plants can produce more sugar than C₃ plants in conditions of high light and temperature. Many important crop plants are C₄ plants, including maize, sorghum, sugarcane, and millet. Plants that do not use PEP-carboxylase in carbon fixation are called C3 plants because the primary carboxylation reaction, catalyzed by RuBisCO, produces the three-carbon 3-phosphoglyceric acids directly in the Calvin-Benson cycle. Over 90% of plants use C₃ carbon fixation, compared to 3% that use C₄ carbon fixation; however, the evolution of C₄ in over 60 plant lineages makes it a striking example of convergent evolution.