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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 .
Webb et al. (Fri,) studied this question.