guard cell A specialized type of plant epidermal cell (see EPIDERMIS), 2 of which surround each stoma. Changes in their turgidity cause stomatal opening and closing. This procees is not fully understood but includes:
a. the effects of the overall water potential of the plant on the stomata;

b. a feedback mechanism, whereby when water is plentiful abscisic acid is formed and stomata open and when water is scarce abscisic acid is not released and the stomata close;

c. low carbon dioxide levels promote stomatal opening;

d. except in CAM plants, stomata normally open in light and close in darkness;

e. generally, an increase in temperature tends to increase stomatal opening.

guard cell Either of a pair of cells that control opening and closing of a leaf pore (stoma). Each is a sausage- or kidney-shaped cell whose wall varies in rigidity. The wall bordering the pore is thickened and rigid, whereas the outside wall is thin and extensible. As the paired cells absorb water they swell, and the thin-walled region bends outwards, pulling the nonextensible thicker wall with it and opening the pore. Loss of water has the reverse effect, resulting in shrinking of the guard cells and closure of the pore. Movement of water in and out of the cell is controlled by the transport of potassium ions (K⁺) into and out of the cell. At sunrise, K⁺ ions are pumped into the guard cells, raising their internal ionic concentration and causing water to follow by osmosis. Hence the guard cells swell and the pore opens. In order to close the pore, K⁺ ions are pumped out of the guard cells, causing them to lose water osmotically and shrink.