Plants that lose turgidity (lose water from their cells) undergo plasmolysis. When this happens, cells are no longer rigid and the plant loses structure, giving the "wilty" appearance. Wilting might not always be caused by a lack of water, but might be an expression of plants under salt (salinity) stress or can by caused by ethylene (a plant hormone).

"Plants wilt, not only when they lose water through evaporation, but also when they are surrounded by an aqueous solution of common salt, potassium chloride, magnesium chloride, sugar or other substance, if the solution is of higher osmotic pressure, whereas they do not wilt if the osmotic pressure is lower."