DPD is the abbreviation of diffusion pressure deficit, SF (suction force) is the vacuolar osmotic pressure and TP is the turgor pressure.

The difference between SF and TP determines whether water tends or not to enter the cell. If SF > TP, DPD > 0 and water tends to enter the cell by osmosis. If TP > SF, DPD < 0 and water cannot enter the cell by osmosis.