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Water potential is a potential energy of water per unit volume relative to pure water(tendency of water move from one area to another area).Normally water potential of a pure water is zero.Because there is no other solute present in it.But if we add any mineral or if any mineral is present in pure water it may form complex with the ions present in water(H+,OH-) resulting in formation of any solute in water.So new pressure due to solute is created.So water potential will become negative.

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