How is root pressure developed?

Young cabbage plants (Brassica oleracea L. var. capitata) that were exposed to an atmosphere at 50% relative humidity transpired freely and accumulated significant quantities of 45Ca in the leaves. Plants that were enclosed by plastic bags to stop transpiration from all leaves exhibited guttation with the development of root pressure and also accumulated significant quantities of 45Ca in the leaves. 45Ca accumulation increased in the leaves and tended to decrease in roots and stems with increasing quantities of water transpired or guttated by the plant. When plants were only partially enclosed so that some leaves were covered and the remainder exposed, only the exposed leaves that were transpiring accumulated significant quantities of 45Ca. The covered leaves of partially enclosed plants exhibited no guttation and accumulated little 45Ca with no measurable 45Ca at the margins of the leaves. The results demonstrate that root pressure flow is required to transport adequate amounts of Ca to those tissues in plants that are not undergoing transpirational water loss.

The above answer is a bit over done.  I think this answers your question.  When transpiration is low, ions pumped into the stele decrease water potential and cause water uptake by the stele. This uptake force is called root pressure. It Cannot keep pace with transpiration, and can only force water up a few meters.