Why do grocery stores spray their produce with water?

The reason they do this is because of the plant's turgor pressure, which is the pressure of the cell's contents against its cell wall, determined by the water content from omosis. If there is more solute in the plant than in its surrounding environment than you have a hypotonic solution where water diffuses into the cell to achieve equilibrium. Thus by routinely spraying the vegetables with water it keeps up their turgor pressure and keeps them nice and crisp for us to eat.