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Guard Cells Elongate: Relationship of Volume and Surface Area during Stomatal Movement

Tobias Meckel ^*, Lars Gall , Stefan Semrau ^*, Ulrike Homann  and Gerhard Thiel 

^* Leiden Institute of Physics, University of Leiden, 2333 CA Leiden, The Netherlands; and Department of Botany, Darmstadt University of Technology, 4287 Darmstadt, Germany

Correspondence: Address reprint requests to Dr. Tobias Meckel, Leiden Institute of Physics, University of Leiden, Niels Bohrweg 2, 2333 CA Leiden, The Netherlands. Tel.: 31-0-71-527-1882; Fax: 31-0-71-527-5936. E-mail: tobias.meckel{at}web.de.

Stomata in the epidermis of photosynthetically active plant organs are formed by pairs of guard cells, which create a pore, to facilitate CO₂ and water exchange with the environment. To control this gas exchange, guard cells actively change their volume and, consequently, surface area to alter the aperture of the stomatal pore. Due to the limited elasticity of the plasma membrane, such changes in surface area require an exocytic addition or endocytic retrieval of membrane during stomatal movement. Using confocal microscopic data, we have reconstructed detailed three-dimensional models of open and closed stomata to precisely quantify the necessary area to be exo- and endocytosed by the guard cells. Images were obtained under a strong emphasis on a precise calibration of the method and by avoiding unphysiological osmotical imbalance, and hence osmocytosis. The data reveal that guard cells of Vicia faba L., whose aperture increases by 111.89 ± 22.39%, increase in volume and surface area by 24.82 ± 6.26% and 14.99 ± 2.62%, respectively. In addition, the precise volume to surface area relationship allows quantitative modeling of the three-dimensional changes. While the major volume change is caused by a slight increase in the cross section of the cells, an elongation of the guard cells achieves the main aperture change.