How do plants control the opening and the closing of the stomata?
The opening and the closing of the stomata depend upon the necessity of the plant to lose water and heat through transpiration (exit of water vapor means elimination of heat). When the plant has excessive water the guard cells become turgid and the ostiole opens. When little water is available the guard cells become flaccid and the ostiole closes. Water enters and goes out the stomata by osmosis. Other factors like light intensity and carbon dioxide concentration in the leaves influence the opening and the closing of the stomata. When luminosity is high the photosynthesis rate increases and the stomata open to absorb more carbon dioxide from the environment and release heat; when luminosity is low the stomata tend to close. When the carbon dioxide concentration in the photosynthetic parenchyma is low the stomata open to absorb more of the gas and make photosynthesis possible; when such concentration is high the stomata tend to close.
