Carbon dioxide induces increases in guard cell cytosolic free calcium

The hypothesis that increases in cytosolic free calcium ([Ca 2+ ] i ) are a component of the CO 2 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 [Ca 2+ ] i in response to challenge with 700 µl l −1 CO 2 . Elevated CO 2 induced increases in guard cell [Ca 2+ ] i which were similar to those previously reported in response to abscisic acid. [Ca 2+ ] i returned to resting values following removal of the CO 2 and further application of CO 2 resulted in a second increase in [Ca 2+ ] i . This demonstrated that the CO 2 ‐induced increases in [Ca 2+ ] i were stimulus dependent. Removal of extracellular calcium both prevented the CO 2 ‐induced increase in [Ca 2+ ] i and inhibited the associated reduction in stomatal aperture. These data suggest that Ca 2+ acts as a second messenger in the CO 2 signal transduction pathway and that an increase in [Ca 2+ ] i may be a requirement for the stomatal response to CO 2 .