Breathing is the process of gas exchange. When plants breathe, they take in carbon dioxide (CO2) and give off water vapour and oxygen. Plants breathe during the day because they need CO2 for photosynthesis, which requires the sun. A few desert plants evolved mechanisms to either take in CO2 rapidly so the stomata open a short time to reduce water loss, or to open their stomata to exchange gasses at night.
With enough water, gas exchange pores on the leaves, called stomata, open. Stomata close and breathing stops at sunset when the light reaction of photosynthesis stops. The stomata open to breathe in CO2 gas from the air as a source of carbon atoms for photosynthesis. When photosynthesis stops, the higher concentration of CO2 molecules inside the plant signals the stomata to close. Plants also stop breathing to prevent water vapour loss in dry conditions.
Plants depend on stomata to breathe out water vapour to create pressure that draws water up into the roots from the soil, like using a straw to drink lemonade. If the sun dries the soil, the stomata must close so the plant does not lose more water than can be replaced. Closed stomata mean the plant stops breathing and no more water can be pulled up or CO2 taken in, so photosynthesis will stop.
Byproduct - Oxygen
Plants photosynthesise to capture energy from the sun and store it in sugar molecules for later use. Sugar is composed of carbon, oxygen and hydrogen. The carbon and oxygen atoms come from CO2 gas. The hydrogen comes from water (H2O) that has been split into hydrogen and oxygen atoms. The oxygen gas left over from splitting water during photosynthesis must be breathed out through the stomata so it does not poison the cells.
C4 plants growing in hot, dry sunny habitats use an enzyme that allows them to take in CO2 quickly so that the rate of photosynthesis is increased. The stomata of C4 plants do not have to open as long. CAM plants have another adaptation. They breathe in CO2 gas at night and convert it to an acid that can be used for photosynthesis during the day. CAM plants are able to stop breathing during droughts.