During photosynthesis, plants acquire the necessary carbon dioxide and release the produced oxygen back into the atmosphere. Openings in the epidermis of plant leaves is the site of this exchange of gasses. A single opening is called a stoma—derived from the Greek word for “mouth.” Stomata open and close in response to a variety of environmental cues.

Each stoma is flanked by two specialized guard cells that create an opening when these cells take up water. The transport of ions regulates the amount of water in guard cells. When trigger, pumps translocate hydrogen ions out of the guard cell. This hyperpolarization of the membrane causes voltage-gated potassium channels to open and allow solutes, such as potassium ions and sucrose, to enter the guard cells. The increased concentration of solutes drives water into the guard cells, which accumulates in the vacuole. As a result, the guard cells bow and deform into a kidney shape, creating the stoma opening. When solutes leave guard cells, water follows, resulting in guard cell shrinkage, and closure of the opening.

A variety of environmental and internal signals triggers stomata opening. For example, blue light activates light-sensitive receptors on the cell surface that initiate a molecular cascade leading to stomata opening. In addition, when the concentration of carbon dioxide falls within the leaf tissue, stomata opening is induced so cells can access this critical reactant of photosynthesis.

Loss of water vapor is critical for the establishment of transpirational pull: water evaporates on the surface of mesophyll cells and escapes into the atmosphere through open stomata. The water loss creates a transpirational pull that pulls additional water from the soil into the roots and all the way into the leaves.

When sufficient water is not available, as in conditions of drought, stomata close. The hormone abscisic acid (ABA) is important in this process, binding to receptors on guard cell membranes and increasing intracellular solute concentration. ABA is also important in circadian control of stomatal opening, causing more stomata to be open in daylight, and closed in the dark.