Figure 8.

Simplified model for the regulation of SV channels by cytosolic and luminal Ca²⁺ and Mg²⁺ in fava bean guard cells. A₁, High-affinity Ca²⁺-binding site on the cytosolic side, which is not activated by Mg²⁺. A₂, Low-affinity binding site on the cytosolic side, which can be occupied by either Mg²⁺ or Ca²⁺. B, Vacuolar Ca²⁺-binding site, which is not affected by vacuolar Mg²⁺. For the activation of SV channels, both activation sites A₁ and A₂ need to be occupied (see “Discussion”). The cytosolic and vacuolar membrane sides are labeled.