Such water loss in its liquid phase is known as guttation . Root pressure can only provide a modest push in the overall process of water transport. Its greatest contribution maybe to reestablish the continuous chains of water molecules in the xylem which often break under the enormous tensions created by transpiration. 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.