Plants absorb water through the process of osmosis. In most plants, saltwater fights with the roots when they absorb water. While the roots try to take water in for the plant, the saltwater pulls the water out at the same time. This causes many plants to eventually shrivel and die.
While all living things need a certain amount of salt to survive, they can only process so much at a time. When the amount of sodium in a plant exceeds the limits the plant can handle, it poisons the plant. Plants divert much of their energy to filtering out the salt and do not grow as much as plants exposed to fresh water.Learn more about Botany