Do Stomata Respond to CO2 Concentrations Other than Intercellular?

Most studies on stomatal responses to CO(2) assume that guard cells respond only to intercellular CO(2) concentration and are insensitive to the CO(2) concentrations in the pore and outside the leaf. If stomata are sensitive to the CO(2) concentration at the surface of the leaf or in the stomatal pore, the stomatal response to intercellular CO(2) concentration will be incorrect for a ;normally' operating leaf (where ambient CO(2) concentration is a constant). In this study asymmetric CO(2) concentrations for the two surfaces of amphistomatous leaves were used to vary intercellular and leaf surface CO(2) concentrations independently in Xanthium strumarium L. and Helianthus annuus L. The response of stomata to intercellular CO(2) concentration when the concentration at the leaf surface was held constant was found to be the same as the response when the surface concentration was varied. In addition, stomata did not respond to changes in leaf surface CO(2) concentration when the intercellular concentration for that surface was held constant. It is concluded that stomata respond to intercellular CO(2) concentration and are insensitive to the CO(2) concentration at the surface of the leaf and in the stomatal pore.