This gradient is necessary for plants to transport water . Pure water has the highest potential while a saturated solution of ions etc. would have the lowest potential. Water potential is highest when water is a liquid and lowest when water is a gas in air. See below for my explanation of water transport. The system of xylem vessels from root to leaf vein can supply the needed water.
Several forces combine to overcome the pull of gravity.
These combined forces culminate in a process called transpiration . Ultimately water is pulled, molecule by molecule into the leaf. Each force can be communicated to the next because water forms a strong continuous chain from root to leaf. The gradient is highest in the water surrounding the roots and lowest in the air space within the spongy parenchyma of the leaf. The energy of evaporation is needed to to pull molecules away from the film of water coating air spaces within the spongy parenchyma.