ABSTRACT
Plants transpire water into the atmosphere through stomata that are tiny openings embedded in green plant surface tissues. The width of each pore is altered by changes in guard cell osmotic pressure, which is determined by solute influx in response to a protonmotive force created by plasma membrane HþATPases. Alterations of environmental state variables such as soil water, air vapor pressure deficit, light, nutrition availability, air carbon dioxide concentration and others, directly or indirectly, activate signal transduction pathways that transmit information throughout the plant. Stomatal responses to the environment are a result of various metabolic reactions that are triggered by this information. Stomatal closing or opening is thus highly co-ordinated, causing difficulties in finding adequate mathematical expressions for simulating stomatal behavior. As water resources are getting increasingly scarce, there is yet a growing need for finding expressions that can be practically applied in quantifying stomatal control of transpiration. The following gives an overview over practical methodologies that are frequently applied in quantifying stomatal responses to the principal controlling environmental factors: air vapor pressure deficit, water, carbon dioxide, and light.
