The leaves have fresh water in them, but the salt in the dressing causes the water from the leaves to enter the dressing. This is because the water is trying to even out the concentration of salt in the dressing and the leaves, hence lots of water is needed in the dressing to reduce the concentration of salt. This means the water from the leaves goes into the dressing, causing the leaves to appear wilted.
Definition of Osmosis: Osmosis is the movement of water or other solvent through a plasma membrane from a region of low solute concentration to a region of high solute concentration. Osmosis is passive transport, meaning it does not require energy to be applied.