What is DPD?

Hint: DPD stands for diffusion pressure deficit. It is the pressure difference between the solution and pure water. It is simply defined as the thirst of the cell for water. It can be depicted as osmotic pressure subtracted from turgor pressure. A cell that is fully plasmolyzed has zero DPD.

Complete answer:
DPD was first mentioned by B.S. Meyer in 1938. The diffusion pressure deficit is expressed as the potential of molecules to diffuse from the higher concentration area to a lesser concentration area. It was originally named suction pressure. This is because it was thought to be the capability of the cell to absorb water in its surroundings. Later it was regarded as the difference between the diffusion pressure of a pure solvent and solution. It depends on the osmotic pressure and turgor pressure of the cell. It can be measured as $DPD=OP-TP$, where OP is the osmotic pressure of the cell and TP is the turgor pressure of the cell. The turgor pressure is the amount of pressure experienced by a cell due to its water content. Osmotic pressure is the pressure required to inhibit the inward flow of pure solvent across the cell membrane. Water always moves from the region of lower DPD to higher DPD.
Except for the turgor pressure and osmotic pressure, the DPD depends on the type of cell, the height of the cell in the plant, atmospheric humidity, wind, and the substrate.
Zero diffusion pressure deficit indicates that the cell has enough of the water it requires. As it is a measure of the thirst of the cell for water, it can be used to infer the irrigation needs of crops.

Note: The diffusion pressure deficit is measured in atmospheres. It usually fluctuates between $0.1$ and 100 atmospheres. When the DPD exceeds 10 atmospheres, the crops need to be irrigated as it indicates less water in cells. DPD alone cannot be used to characterize water relations of plant cells. It is always measured by taking into account the turgor pressure and osmotic pressure.