Ions in Stomatal Opening Mechanism
The ion concentrations are different inside and outside of the cell, allowing a potential.  The cell becomes even more polarized by the pumping out of H+ ions, making the cell even more negative (potential difference). Then this negative potential happens to cause voltage-gated channels to open and the uptake of potassium occurs. This will depolarize the cell (return it to the base potential difference).  It is usually very important for the cell to maintain the negative potential after it recovers. To combat the uptake of potassium, negative charges can be used to balance the increased positive ion influx so the resting potential is stabilized. 

These mechanisms are usually mediated by special proteins that are regulated by actual electrical changes in the membrane.