Uptake and loss of water in guard cells changes their
turgor and modulates stomatal opening and closing (see
Chapters 4 and 18). Because guard cells are located in the
leaf epidermis, they can lose turgor as a result of a direct
loss of water by evaporation to the atmosphere. The
decrease in turgor causes stomatal closure by hydropassive
closure. This closing mechanism is likely to operate in air
of low humidity, when direct water loss from the guard
cells is too rapid to be balanced by water movement into
the guard cells from adjacent epidermal cells.