Diffusion Pressure Deficit (DPD) or Suction Pressure (SP)

Diffusion Pressure Deficit, a term coined by Meyer in 1938, measures how much lower a solution's diffusion pressure is compared with that of its pure solvent at the same temperature and atmospheric pressure; since adding any solute to a pure solvent always lowers its diffusion pressure, DPD rises the more concentrated a solution becomes. Because a higher DPD makes a solution pull water in more strongly by endosmosis, Renner (1935) independently gave the very same quantity a second name, Suction Pressure (SP) - the two terms are used interchangeably in plant physiology. DPD takes three characteristic values depending on a cell's turgor state: in an ordinary cell that is neither fully turgid nor fully flaccid, DPD equals osmotic pressure minus turgor pressure (DPD = OP − TP); in a fully turgid cell, osmotic pressure exactly equals turgor pressure (OP = TP), so DPD falls to zero, since such a cell can absorb no further water; and in a fully flaccid cell, turgor pressure is zero altogether, so DPD simply equals the full osmotic pressure (DPD = OP), giving the cell its strongest possible pull for water. Whenev …