The term diffusion pressure deficit (DPD) was coined by B.S Meyer in 1938. Originally DPD was described as suction pressure by Renner (1915). It is reduction in the diffusion pressure of water in solution or cell over its pure state due to presence of solutes in it and forces opposing diffusion. Diffusion pressure of water is maximum and its theoretical value is 1236 atm. DPD of a solution is equal to its osmotic pressure i.e. DPD = OP(of solution). DPD of a cell is influence by both osmotic pressure and wall pressure (turgor pressure) which opposes the endosmotic entry of water, i.e. DPD = OP - Wall pressure. DPD is directly proportional to the concentration of the solution. DPD decreases with dilution of solution. The actual pressure with which cell absorbs water is called "suction pressure".
In refrigeration and air conditioning systems, the suction pressure' (also called the low-side pressure) is the intake pressure generated by the system compressor while operating. The suction pressure, along with the suction temperature and the wet bulb temperature of the discharge air are used to determine the correct refrigerant charge in a system. If some solute is dissolved in water its diffusion pressure decreases. The difference between diffusion pressure of pure water and solution is called diffusion pressure deficit (DPD).
When a plant cell is placed in hypotonic solution, water enters into a cell by osmosis and as a result turger pressure develops. The cell membrane get stretched and osmotic pressure of cell decreases. As cell absorbs more and more water its Total Pressure increases and Osmotic Pressure decreases. When a cell is fully turgid, its OP is equal to TP and DPD is zero. Turgid cells cannot absorb any more water. Thus, with reference to plant cell, the DPD can be described as actual thirst of cell for water and can be expressed as DPD=OP-TP.
When DPD is zero, entry of water will stop. Thus it is DPD that tends to equate and represents water absorbing ability of a cell, it is also called suction force (SF) or suction pressure (SP).