How is a plant experiencing a water shortage connected to the way a guard cell works?
Water in the cells control the swelling (which opens/closes the stoma)
The guard cell has a less stiff cell wall on the outside (away from the stoma). This means that the cell will bend to the outside in times of abundance of water. When the water gets scarce the water drains out of the cell. This means that the cell will revert to its old form, closing the gap.