Desert plants close their stomata during the day to prevent transpiration and loss of water. To perform photosynthesis, CO2 is necessary. They take up CO2 during the night through stomata and store it in their photosynthesising cells in the form of an intermediate until the day. And when sunlight is available, they perform photosynthesis to make their food. This is called as CAM (Crassulacean acid metabolism) photosynthesis.
