What causes the opening and closing of guard cells of stomata during transpiration?

Explanation

(I ) stoma are open in the day and closed at night .
(ii ) The opening and closing of the stomata is due to the change in turgidity of the guard cells .
(iii ) when water enters into the guard cells , they become turgid and the stoma open .
(iv ) when the guard cells lose water ,it becomes flaccid and the stoma closes.