Figure 8.

[Ca²⁺]_i and analysis of the Ca²⁺ flux across the plasma membrane and tonoplast during voltage and [Ca²⁺]_i excursions at the end of the daylight period. Shown are the plasma membrane and tonoplast voltages, and the stomatal aperture (B), the Ca²⁺ influx through the Ca²⁺ channels at the plasma membrane and tonoplast (C), the Ca²⁺ efflux through the plasma membrane and tonplast Ca²⁺ ATPases (D), and the [Ca²⁺]_i (E). Current-voltage curves for these Ca²⁺ transporters at the plasma membrane and tonoplast are shown in E and F, respectively. Total membrane current is indicated in each case by the dotted line; the free-running membrane voltage is defined by the point at which this line crosses the voltage axis. Current-voltage curves are cross-referenced to time points in B to E by letter. As in Figure 7, time point a is characterized by plasma membrane hyperpolarization, which favors Ca²⁺ influx through the Ca²⁺ channels at the plasma membrane; time point c is characterized by plasma membrane depolarization, which favors Ca²⁺ efflux through the plasma membrane Ca²⁺-ATPase. Note the Ca²⁺ influx at the plasma membrane is replaced by Ca²⁺ influx across the tonoplast shortly before plasma membrane depolarization (C); thereafter, the characteristics of the [Ca²⁺]_i rise are determined by Ca²⁺ flux from the tonoplast and, as the tonoplast Ca²⁺ channels inactivate, by [Ca²⁺]_i recovery-mediated Ca²⁺-ATPases at the two membranes (D). Note that positive flux is defined as movement of the ionic species (not the charge) out of the cytosol, either across the plasma membrane or the tonoplast.