Low CO2 makes stomata open even at night. Photosynthesis uses up CO2. In drought water stress, stomata will stay closed in day. Circadian rhythm aka internal biological clock correlates opening and closing with what should be light and dark. Cell turgid, walls bend outward and open pore.
Water leaves cell by osmosis.
Cells collapse and pores close. Thru open stomata, with lots of surface area from loose mesophyll cells. Cooling Water brings minerals as it moves thru plant. Water absorbed at night, stomata close Permanent wilting cannot recover. Drought soil doesn't have enough water.