What is the source of CO2 for photosynthesis during the day in CAM plants?

In CAM (Crassulacean Acid Metabolism) plants, the source of carbon dioxide (CO2) for photosynthesis during the day primarily comes from malic acid. These plants have a unique adaptation that allows them to conserve water in arid environments by opening their stomata at night to take in CO2 and storing it in the form of malic acid. During the day, when the stomata are closed to prevent water loss, the malic acid is converted back to CO2, which is then utilized in the Calvin cycle for photosynthesis. This mechanism enables CAM plants to effectively manage water usage while still carrying out photosynthesis.

The other options do not serve as a direct source of CO2 for photosynthesis in this context. Glucose and starch are products of photosynthesis rather than sources of CO2, while citric acid is part of the Krebs cycle and is not directly involved in the CO2 release needed for daytime photosynthesis in CAM plants. Therefore, malic acid is the correct answer as it plays a critical role in the uniquely adapted photosynthetic process of CAM plants.