Some salt is required by all living things, according to the Madsci Network. Like humans and other animals, plants are adapted to a certain amount of salt intake. Too much salt can damage terrestrial or land plants, which are only able to handle the relatively low salt levels they get in their native environments.
The cells in a plant's roots absorb water through the process of osmosis. The water passes from an area with a low concentration of dissolved minerals through the cells to a higher concentration of dissolved minerals. Normally, the minerals dissolved inside the plant cells keep the water flowing continuously in through the roots. When the water in the soil is too salty, however, it throws off the osmosis process. All the dissolved salt in the water can slow down the rate at which the water flows into the roots or actually suck water out of the roots. If the water is salty enough, it can dehydrate and kill the plant.
Salt can damage plant tissues in the long run. In many plants, salt is a poison in high concentrations. If a plant is regularly subjected to salt water, the salt leaches into the plant. This can damage its cell processes, interfere with enzymes used in producing food, increase damage to cells through oxidation and have other negative effects.
Plants sometimes are exposed to salt water due to flooding from nearby bodies of water. When plants are chronically flooded with saltwater, it can damage or destroy the roots, killing the plant. Saltwater flooding also hampers the plant's ability to respire. Soil normally contains small pockets of air. Plant roots need to absorb some oxygen through their roots for this process, and cannot do so in flooded soil.
Some land plants, such as mangroves, have evolved the ability to resist damage from salt. Some of these plants have special glands that secrete excess salt out through the leaves. Others store salt in their leaves, then dispose of it by shedding those leaves every season. Still others have specially adapted roots that prevent salt from entering the plant.