Stimulus-lnduced Oscillations in Guard Cell Cytosolic Free Calcium

Ca2+ is implicated as a second messenger in the response of stomata to a range of stimuli. However, the mechanism by which stimulus-induced increases in guard cell cytosolic free Ca2+ ([Ca2+],) are transduced into different physiologiCal responses remains to be explained. Oscillations in [Ca2+], may provide one way in which this can occur. We used photometric and imaging techniques to examine this hypothesis in guard cells of Commelina communis. Externa1 Ca2+ ([Ca2+Ie), which causes an increase in [Ca2+],, was used as a closing stimulus. The total increase in [Ca2+], was directly related to the concentration of [Ca2+Ie, both of which correlated closely with the degree of stomatal closure. lncreases were oscillatory in nature, with the pattern of the oscillations dependent on the concentration of [Ca2+]e. At 0.1 mM, [Ca2+Ie induced symmetrical oscillations. In contrast, 1.0 mM [Ca2+]e induced asymmetric oscillations. Oscillations were stimulus dependent and modulated by changing [Ca2+]e. Experiments using Ca2+ channel blockers and Mn2+-quenching studies suggested a role for Ca2+ influx during the oscillatory behavior without excluding the possible involvement of Ca2+ release from intracellular stores. These data suggest a mechanism for encoding the information required to distinguish between a number of different Ca2+-mobilizing stimuli in guard cells, using stimulus-specific patterns of oscillations in [Ca2+Ii.