Figure 5.

Membrane voltage and abscisic acid (ABA) interact to facilitate [Ca²⁺]_i increases. Data from Vicia guard cells bathed in 5 mM Ca²⁺-Mes, pH 6.1, and 10 mM KCl before and after adding 20 μM ABA to the bath. (A) ABA displaces the threshold for [Ca²⁺]_i rise to more positive voltages. Fura2 fluorescence ratios recorded while driving membrane voltage in 1-min ramps from −50 to −200 mV. Data from one Vicia guard cell before (lower trace) and 4 min after (upper trace) adding ABA. [Ca²⁺]_i signals (points) smoothed (solid lines) by empirical, least-squares fitting of data points to a sigmoid function (27). Thresholds (arrows) taken as time points at which the fitted curves first exceeded 10 nM above the means determined from the 10-s period preceding the voltage ramps (fine horizontal lines). Scale: horizontal, 5 s; vertical, 40 nM. (B) [Ca²⁺]_i increases are augmented and prolonged by ABA, but are potentiated only on membrane hyperpolarization. Data from one Vicia guard cell before and during ABA exposure (diagonal-filled bar). Fura2 fluorescence ratios recorded while driving membrane voltage in 20-s steps from −50 to −180 mV at times indicated above trace (⊔). Time scale (bar), 1 min.