Carbon dioxide induces increases in guard cell cytosolic free calcium

The hypothesis that increases in cytosolic free calcium ([Ca2+](i)) are a component of the COP signal transduction pathway in stomatal guard cells of Commelina communis has been investigated. This hypothesis was tested using fura-2 fluorescence ratio photometry to measure changes in guard cell [Ca2+](i) in response to challenge with 700 mu l l(-1) CO2. Elevated CO2 induced increases in guard cell [Ca2+](i) which were similar to those previously reported in response to abscisic acid. [Ca2+](i) returned to resting values following removal of the CO2 and further application of CO2 resulted in a second increase in [Ca2+](i). This demonstrated that the CO2-induced increases in [Ca2+](i) were stimulus dependent. Removal of extracellular calcium both prevented the CO2-induced increase in [Ca2+](i) and inhibited the associated reduction in stomatal aperture. These data suggest that Ca2+ acts as a second messenger in the CO2 signal transduction pathway and that an increase in [Ca2+](i) may be a requirement for the stomatal response to CO2.