How plants control the opening and the closing of the stomata?

The closing and the opening 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 a little water is available the guard cells become flaccid and the ostiole closes. Water goes out and enters the stomata by osmosis.

The Other factors like light carbon dioxide concentration and intensity in the leaves influence the opening and the closing of the stomata and 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 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.