It's just another term for Water potential, technically speaking

as far as botany is concerned,

difusion pressure deficit  (or in older terminology--->suction pressure , DPD)

is da net  pressure dat causes water 2 enter plant cell

its magnitude z da difference between osmotic pressure (ll) & turgor pressure (TP)

i.e, DPD = ll - TP

see!actully the absorption of water is not depend entirely on OP bt also on TP.The TP produces equal amt of WP due 2 its elasticity.Due 2 which water is thrown out of the cell against OP.

It creats a deficit of TP in entry of water due 2 the OP. Therefore the suction pressure or the total capacity of cell to absorb water or the actul thirst of cell 4 water is designated by the term Diffusion Pressure Deficit which is expressed as:    DPD= OP-TP (bcoz TP=WP).