Stomata open and close in response to -

Responses

water being pumped in and out of guard cells
water being pumped in and out of guard cells, EndFragment,

crystal formation in the plant due to excess glucose.

StartFragment, crystal formation in the plant due to excess glucose.,

the lack of available sunlight during the night
the lack of available sunlight during the night, EndFragment,

stimulation by auxins producing rapid growth
stimulation by auxins producing rapid growth, EndFragment,

high levels of carbon dioxide in the atmosphere

high levels of carbon dioxide in the atmosphere,

changes in humidity levels
changes in humidity levels,

temperature fluctuations.