The salt content in the environment surrounding a plant cell must be just right for the cell to continue functioning properly. Too much salt, as well as too little, can do irreversible damage to plant cells.
The Direction of Osmosis
In biology, osmosis is the flow of water into and out of a cell. The addition or removal of salt from the environment of a plant can impact this flow of water.
Too Much Salt
Too much salt within a plant will cause a net flow of water out of the plant's cells. They will shrink, causing the plant to look shriveled and dry.
When plant cells are shriveled because of a high salt content in their medium, the cell organelles become pushed together and cannot function properly. Cytoskeletal elements necessary for nutrient transportation within the cell can be destroyed and improper ion concentrations can disrupt crucial enzymes.
Too Little Salt
If a plant is supplied continually with pure water, there will be a net flow of water into the cells that can cause them to rupture. Over-watering is necessary to cause this rupturing effect, because even though plants are usually watered with pure water, the salt contained in the soil dissolves into the water and yields roughly the right balance.
Cells that have been ruptured because of insufficient salt in their medium are destroyed. A plant with too many ruptured cells will die.