STIMULUS-INDUCED 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+](i)) are transduced into different physiological responses remains to be explained. Oscillations in [Ca2+]i 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. External Ca2+ ([Ca2+](e)), which causes an increase in [Ca2+](i), was used as a closing stimulus. The total increase in [Ca2+](i) was directly related to the concentration of [Ca2+](e), both of which correlated closely with the degree of stomatal closure. Increases were oscillatory in nature, with the pattern of the oscillations dependent on the concentration of [Ca2+](e). At 0.1 mM, [Ca2+](e) 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+](l).