Integration of Cellular and Physiological Functions of Guard Cells

plantʹs aerial surfaces are covered by a water-impermeable barrier that limits water loss to stomata, which also permit the uptake of carbon dioxide. Water loss through stomata is required for the ascent of nutrient-laden sap, and evaporation of water from the leaf can be an important cooling mechanism. In most terrestrial environments, however, water is a limiting resource, and its loss must be regulated lest the plant desiccate. Balanced against the essential demand for regulated water loss is the less urgent requirement to acquire carbon dioxide. Regulation of water loss and carbon dioxide uptake is achieved through turgor fluctuations of the pair of guard cells that surround each stoma. When a guard-cell pair accumulates solutes, the resultant turgor and volume changes cause the guard cells to bow outward because of cell-wall architecture, enlarging the pore between them. This simple explanation belies the underlying complexity of guard-cell turgor regulation and whole-plant responses, aspects of which have been the topics of many focused reviews. This review is general and provides an overview of cellular mechanisms that are involved in turgor regulation. It emphasizes regulatory substances that can be altered externally and therefore includes a description of integrative environmental and whole-plant physiological signals that coordinate guard-cell responses.