Diffusion Pressure Deficit

The reduction in the diffusion pressure of water in a solution over its pure state is called diffusion pressure deficit or DPD. It is an older term that was used in place of water potential. Pure water has the maximum diffusion pressure. Diffusion pressure deficit is also called suction pressure. 
                                                                            The term diffusion pressure deficit was given by Meyer (1938). Originally it was described as suction pressure and presently it is replaced by the term water potential. Its value is equal to the osmotic pressure of the solution in a cell minus the wall pressure (= turgor pressure) which opposes the entry of water into it provided the external water is pure.

Interrelationship of OP, TP and DPD:- In a plasmolysed cell the net force with which water enters the cell is equal to its osmotic pressure. As the water enters the cell its turgor pressure increases which acts aganist the osmotic entry of water in a cell.
                                                                                 The net force with which water enters the cell would be equal to the difference of OP and TP.


                                         i.e.            DPD = OP - WP
                                         as               WP = TP
                                     Therfore       DPD = OP - TP

In a flaccid cell
                                             DPD = OP ( as TP = 0 )
In a turgid cell
                                              DPD = 0 ( as OP = TP )