If a plant is placed in salt water what will most likely happen to the cells of the plant?

If you give plants that are adapted to living on land water that is very salty, it will reverse the osmotic potential in their roots and actually cause water to be removed from the cells of the plant and make them shrivel up, thus leading the plant to experience drought stress even if the soil is moist. A similar process is what causes cucumbers to turn into pickles when they are soaked in brine and vinegar.

There are however, plants that are adapted to salty soil and saltwater. These plants have much more effective water intake systems that can overcome this reversal in osmotic potential provided that the salt concentration in the water isn't too high. Seaweed, for example, has a very high salt tolerance. Plants such as Alkali Sacaton and Russian Thistle have a somewhat lower salt tolerance than seaweed, but a higher one than your typical ornamental plant that you may have at home.