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Water potential  = Solute potential + Pressure                                                             potential 

${\mathrm{\psi }}_{\mathrm{w} }= {\mathrm{\psi }}_{\mathrm{s} }+ {\mathrm{\psi }}_{\mathrm{p}}$
where 
${\mathrm{\psi }}_{\mathrm{w} }= \mathrm{Water} \mathrm{potential}$

$$\begin{gathered} {\mathrm{\psi }}_{\mathrm{s} }= \mathrm{Solute} \mathrm{potential} \\ \mathrm{\psi p} = \mathrm{Pressure} \mathrm{potential} \end{gathered}$$

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Osmostic potential is called solute potential. Both are numerically equal. If the solute concentration is increased in the cell, then i.e. osmotic potential is lowered and thus water potential is also lowered. Because of this the water enters from outside into the cell. Thus, decrease or increase in osmotic pressure determines water potential and regulates it.

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Water flows from high water potential to low water potential. 

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Plasmolysis : It is the shrinkage of protoplast from cell wall when plant cell is placed in hypertonic solution.

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The water potential of the following will be 
a. Pure water - Zero
b. Fully turgid cells - Zero
c. Concentrated solution - Negative.