Water potential.
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   Water  moves across a selectively permeable membrane from higher water
   potential to lower by osmosis.
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   Water  potential  (MegaPascal,  or Q) is the sum of pressure potential
   (Q[P]) and solute potential (Q[S]):
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   Q = Q[P] + Q[S].
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<lI> The  addition  of  solutes  reduces water potential (to a negative
value).
<lI> Application of physical pressure increases water potential.
<lI> The  increased  physical  pressure  can reverse the effects of the
solutes .
<lI> A negative pressure reduces water potential (on the other side).
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