If someone pours salt water on a plant that is supposed to receive fresh water, the effects on the plant are swift and severe, beginning with the draining of existing water out of the plant cell. Then, the cell membrane separates from the cell wall in a process known as plasmolysis. Ultimately, the plant shrivels up and no longer thrives.
Ocean plants are prepared for the influx of salt, as their cell membrane permits the salt in, gathering it in the vacuole of the cell. Then salt concentration can go up even higher than that of the surrounding water, and the cells do not go through plasmolysis. There is just one gene that makes the difference between causing plasmolysis and accepting the salt, and scientists have experimented with fresh water plants by adding that gene to their makeup. After that adjustment, the new plants were tolerant of salt water.
In freshwater plants, though, the membrane holds the salt out, while the water comes into the cell. Unfortunately, this is harmful for the cell wall, which is what causes the plant to start to die. There is no way to reverse plasmolysis once it begins, so knowing which water to use is crucial for the health of plants.