Final Answer:
CAM plants keep their stomata open only at night.A.
Step-by-step explanation:
A. CAM (Crassulacean Acid Metabolism) plants keep their stomata open only at night. This unique photosynthetic adaptation is crucial for water conservation in arid environments. Unlike C3 and C4 plants that open their stomata during the day, CAM plants, such as succulents and certain orchids, open their stomata at night to minimize water loss through transpiration.
In the first phase of CAM photosynthesis, called the nighttime phase, the stomata open, allowing the plant to uptake carbon dioxide. During this time, the carbon dioxide is converted into malic acid and stored in vacuoles. The stomata remain closed during the day to prevent water loss through transpiration while still allowing photosynthesis to occur. In the second phase, the daytime phase, the stored malic acid is decarboxylated, releasing carbon dioxide for photosynthesis. This strategy of nocturnal stomatal opening helps CAM plants thrive in environments where water is scarce.
The other options—C3 and C4 plants—follow different photosynthetic pathways. C3 plants typically open their stomata during the day, while C4 plants have a modified pathway for carbon fixation that enhances efficiency in more temperate climates. The option "No plants keep their stomata open only at night" is incorrect, as CAM plants specifically exhibit this behavior. Lastly, the option "All plants do" is also inaccurate, as various plants have different strategies for stomatal regulation based on their environmental adaptations.