How does osmosis affect the turgor pressure of plants?

(diffusion of water across a membrane) causes plant cells to gain or lose water, which increases and decreases turgor pressure, respectively.
Plant cells gain water when the concentration of water is higher outside the cell. This is called hypotonic conditions (hypo=low solute outside). Hypotonic conditions can be achieved by submersing the plant cells in distilled water (no solute).
Plant cells will lose water when the concentration of water is higher inside the cell. This is called hypertonic conditions (hyper=high solute outside). These conditions occur naturally to plants on the side of an icy road that has been salted (solute).
This video discusses the changes that take place in plant cells when placed into pure water or in salt water.

Turgor pressure is the result of these conditions. Increased turgor pressure from hypotonic conditions (low solute concentration outside=high water concentration outside) and decrease in turgor pressure from hypertonic conditions (high solute concentration outside=low water concentration).
This explains why plants on the side of an icy road can wither (water loss) and possibly die if conditions do not improve.