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Water potential of full turgid cells is zero. Fully turgid cell has the highest water potential . As water enters the cell the water apllies pressure against the walls of the cell. In the case of a fully turgid cell, no more water can enter the cell, thus it has the highest water potential. The pressure potential becomes equal to the solute potential in magnitude. The solute potential is negative and the pressure potential is postive. Water potential is given by the equation -
 ${\mathrm{\psi }}_{\mathrm{w} } = {\mathrm{\psi }}_{\mathrm{s} } + {\mathrm{\psi }}_{\mathrm{p}}$ 
Therefore the water potential becomes zero when solute potential becomes equal to the pressure potential.