When fusicoccin activates proton pumps, they start the change in membrane potential which then leads guard cells to take up potassium ions. Water then flows into the guard cells, opening the stomates. This would then allow the continual transpirational loss of water, leading to severe wilting.
Work Step by Step
The key is to review the text to see the role of the proton pumps in starting stomatal opening. It is like a tool needed to open a fire hydrant. Once the tool is used, the hydrant releases a tremendous amount of water.