1. Light: Intense sunlight increases the temperature of the atmosphere and decreases the relative humidity of the air. It opens the pores. Blue light accelerates pore opening. In some CAM plants (pineapple) the stomata are open at night.
2. Relative humidity: When the relative humidity of the atmosphere is low, the stomata open faster. Again, if the relative humidity of the atmosphere is high, the stomata close.
3. Atmospheric pressure: When the atmospheric pressure is low, the stomata open and when the atmospheric pressure is high, the stomata close.
4. Potassium Ion: Potassium ion helps to open the pores.
5. Water: When water enters the guard cell, the stomata open and when water leaves, the stomata close.
6. Temperature: At high temperature the stomata widen and at low temperature the stomata narrow. Therefore, when the temperature increases, the stomata open.
7. Effect of hormones: Hormones like cytokinin, abscisic acid etc help to open pores.
8. Concentration of CO2 in air: CO2 helps open stomata. But when the concentration of CO2 is high, the stomata close.