How does water potential affect plant cells?
When a solution inside a plant cell is separated from pure water by a selectively permeable membrane, water will move by osmosis from the surrounding area where water potential is higher (ψ = 0) into the cell where water potential is lower due to the presence of solutes (ψ is negative). [We will assume, for purposes of explanation, that solutes are not diffusing.] The movement of water into the cell causes the cell to swell, and the cell membrane pushes against the cell wall to produce in increase in pressure. This increased pressure is called turgor. It is what makes plant stems stand upright. Wilting occurs when water flows out, either as soil becomes drier than the plant tissue or as the soil becomes salty. What causes soil to become salty (i.e., have a greater concentration of solute than plants)? Positive turgor pressure inside the cell continues to build up until the water potential of the cell equals the water potential of the pure water outside the cell (ψcell = ψoutside = 0).
