When a plant is watered with salt water, the salt concentration in the soil increases. This causes water to move out of the roots and into the soil in an attempt to equalise the salt concentration on both sides of the root membrane. This process is known as osmosis.

Osmosis is the movement of water through a selectively permeable membrane from an area of high water concentration to an area of low water concentration. The selectively permeable membrane in this case is the root membrane. The water concentration in the soil is higher than the water concentration in the roots, so water moves out of the roots and into the soil.

The loss of water from the roots can cause the plant to wilt and eventually die. This is because the plant cannot transport water and nutrients up from the roots to the leaves.