Plants release oxygen during the day in the presence of natural light through the process of photosynthesis. While at night, the plants uptake oxygen and release carbon dioxide, which is called respiration.

What do plants do during the day?

During the day, plants soak up sunlight during photosynthesis, the process they use to get energy. But when the sun goes down, plants' opportunity to eat disappears and other physiological processes take over, including energy metabolism and growth.

Why do plants close their stomata during the day?

During the day, photosynthesis requires that the leaf mesophyll be exposed to the air to get CO₂. At night, the stomata close to avoid losing water when photosynthesis is not occurring. During the day, stomata close if the leaves experience a lack of water, such as during a drought.