Water potential (ΨW) is calculated using the following equation: ΨW = ΨP + ΨS. Where ΨP is the pressure potential and ΨS is the solute potential. What would occur if ΨP decreases in one phloem cell in comparison to the ΨP in a neighboring phloem cell?

Water will flow from phloem cell with higher ΨW into the phloem cell with the lower ΨW until ΨW was the same in both cells.

Explanation:

As we know that water potential of cell is equal to the water existing in its surrounding. If this is not the case, then the water will move from one cell to the other cell based on water potential difference till the water potential is the same in both the cell.  

In the given question, two neighboring phloem cells have unequal water potential and thus Water will flow from phloem cell with higher ΨW into the phloem cell with the lower ΨW until ΨW is the same in both cells.