Why does lettuce release water in the presence of salt?

I'm not sure about lettuce cells, but I may be able to give you some information to help with your question. Water move according to an osmotic pressure gradient. When the concentration of water inside of a cell is greater than the concentration outside of the cell, water will leave the cell and the cell will shrivel up. When salt is added to the lettuce the concentration of water is greater inside of the cell than it is outside the cell, so the water leaves the cell until equilibrium is reached. If far too much salt is outside of the cell, equilibrium will not be reached, and all of the water will leave the cell. Perhaps you want to look into lettuce cells' permeability to NaCl, which is what table salt is made of, or the ionized forms: Na and Cl. I doubt the cells are very permeable to these solute, in which case all of the salt would remain outside the cell and water would continue to leave the cells, causing them to shrink..