In refrigeration and air conditioning systems, the suction 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.
When a plant cell is placed in a hypotonic solution, water enters into a cell by osmosis and as a result turgor pressure develops in the cell which is in solution . The cell membrane becomes stretched and the osmotic pressure of the cell decreases. As the cell absorbs more and more water its Turgor 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 cells, the DPD can be described as the actual thrust of a 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 the water-absorbing ability of a cell, it is also called suction force or suction pressure. DPD is directly proportional to the height of the plant, tree or organism. DPD is governed by two factors i.e. turgor pressure and osmotic pressure. The formula is DPD = OP - TP. Turgor pressure can also be denoted as wall pressure in some cases. The term diffusion pressure deficit was coined by B.S Meyer in 1938. Originally DPD was described as suction pressure by Renner. It is a reduction in the diffusion pressure of water in solution or cell over its pure state due to the presence of solutes in it and forces opposing diffusion. Diffusion pressure deficit or Diffusion pressure of pure water is maximum and its theoretical value is 1245.69 atm at STP. For pure water DPD and Diffusion pressure are same. DPD of a solution is equal to its osmotic pressure i.e. DPD = OP. The DPD of a cell is influenced by both osmotic pressure and wall 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 the solution. The actual pressure with which a cell absorbs water is called "suction pressure".
