When weather is hot and dry, plants close their stomata to prevent water loss. As a result, carbon dioxide levels in the plant's cells decrease, and oxygen levels rise. This results in photorespiration, a chemical process by which plants use oxygen and release carbon dioxide rather than using carbon dioxide. Photorespiration is not ideal for plants because it releases toxic compounds and fixes far less energy than photosynthesis.
Photorespiration is a problem faced by many species of plants on hot, dry days. When plants photosynthesize normally, two molecules of G3P are produced per reaction. After a series of additional chemical reactions, these G3P molecules are used to produce glucose, which the plant later breaks down for energy. Photorespiration is far less efficient at storing energy than photosynthesis. It produces only one molecule of G3P, along with a toxic phosphoglycolate molecule that the plant must expend energy to convert to a non-toxic substance.
Some plants have evolved mechanisms for avoiding costly photorespiration reactions when carbon dioxide is in short supply. CAM plants, such as orchids and cacti, use a pathway called crassulacean acid metabolism to convert carbon dioxide into an organic acid during the night when it is safe for the stomata to be open. During the day time when the stomata are closed, these organic acids are broken down to release carbon dioxide so that the plant can continue photosynthesis.