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guard cell
Guard cells are specialized cells in the epidermis of leaves, stems and other organs that are used to control gas exchange.
The guard cells are produced in pairs with a gap between them that forms a stomal pore (Figure 1). The stomatal pores are largest when water is freely available and the guard cells are turgid and closed when water availability is critically low and the guard cells become flaccid. (Figure 1). Photosynthesis depends on the diffusion of Carbon dioxide (CO2) from the air through the stomata into the mesophyll tissues. Oxygen (O2), produced as a byproduct of photosynthesis, exits the plant via the stomata. When the stomata are open water is lost by evaporation and must be replaced via the transpiration stream with water taken up by the roots. Plants must balance the amount of CO2 absorbed from the air with the water loss through the stomatal pores, and this is achieved by both active and passive control of guard cell turgor and stomatal pore size.〔Schroeder JI, Kwak JM, & Allen GJ (2001) Guard cell abscisic acid signalling and engineering drought hardiness in plants. Nature 410:327-330.〕〔Hetherington AM & Woodward FI (2003) The role of stomata in sensing and driving environmental change. Nature 424:901-908.〕〔Shimazaki K, Doi M, Assmann SM, & Kinoshita T (2007) Light regulation of stomatal movement. Annu Rev Plant Biol 58:219-247.〕〔Kwak JM, Mäser P, & Schroeder JI (2008) The clickable guard cell, version II: Interactive model of guard cell signal transduction mechanisms and pathway. The Arabidopsis Book, eds Last R, Chang C, Graham I, Leyser O, McClung R, & Weinig C (American Society of Plant Biologists, Rockville), pp 1-17.〕
== Guard cell function ==
Opening and closure of the stomatal pore (Figure 1) is mediated by changes in the turgor pressure of the two guard cells.The turgor pressure of guard cells is controlled by movements of large quantities of ions and sugars into and out of the guard cells. When guard cells take up these solutes, the water potential (Ψ) inside the cells decreases, causing osmotic water flow into the guard cells. This leads to a turgor pressure increase causing swelling of the guard cells and the stomatal pores open (Figure 2). The ions that are taken up by guard cells are mainly potassium (K+) ions〔Imamura S (1943) Untersuchungen uber den mechanismus der turgorschwankung der spaltoffnungs-schliesszellen. Jap. J. Bot. 12:251-346.〕〔Humble GD & Raschke K (1971) Stomatal opening quantitatively related to potassium transport. Evidence from electron probe analysis. Plant Physiol. 48:447-453.〕〔Schroeder JI, Hedrich R, & Fernandez JM (1984) Potassium-selective single channels in guard cell protoplasts of Vicia faba. Nature 312:361-362.〕 and chloride (Cl−) ions.〔MacRobbie EAC (1981) Ion fluxes in 'isolated' guard cells of Commelina communis L. J. Exp. Bot. 32:535-562.〕 In addition guard cells take up sugars that also contribute to opening of the stomatal pores.
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