|MadSci Network: Earth Sciences|
Water potential is the sum of three components; pressure potential, matric potential and solute (osmotic) potential. Sine PEG 6000 is a moderately large but soluble molecule my bet is that it lowers the water potential by acting as a solute that is small enough to influence the osmotic potential but large enough to avoid being taken up by the plants. The more concentrated a solute is the more negative is the water potential.
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