Why cant freshwater plants live in saltwater?

Freshwater plants cannot survive in saltwater because of the difference in osmotic pressure between the plant cells and the surrounding water. Osmosis is the movement of water across a selectively permeable membrane from an area of high water concentration to an area of low water concentration. In freshwater, the water concentration is higher outside the plant cells than inside, so water moves into the cells by osmosis. This influx of water helps to keep the plant cells turgid and allows the plant to transport nutrients from the soil to the leaves.

In saltwater, the water concentration is lower outside the plant cells than inside, so water moves out of the cells by osmosis. This loss of water causes the plant cells to become plasmolyzed, which means that they shrink and pull away from the cell wall. Plasmolysis can damage the cell membrane and disrupt the plant's metabolism, eventually leading to death.

Some plants have evolved adaptations that allow them to tolerate high levels of salinity, but most freshwater plants cannot survive in saltwater.