Driving forces for long-distance transport

Abstract

It is now well established that the transpirational flow of water and solutes through the xylem conduits is a bulk or mass flow of solution. Ringing experiments, sap analysis, and tracer movement in intact plants have all confirmed that the upward movement of sap from the roots is through the xylem. According to the cohesion theory the driving force for this long-distance transport during transpiration is the gradient of negative hydrostatic pressure (tension) between one end of the system and the other. The negative pressures develop in the xylem of the leaves as water loss by transpiration reduces the leaf water potential. As a result of liquid phase continuity, and the fact that the osmotic pressure of the xylem sap is very small, the water potential of the xylem will be nearly equal in magnitude to the water potential of the leaf, resulting in a negative pressure in the xylem. This negative pressure is transmitted through the liquid continuity within the xylem between leaves and root, the breaking of the liquid columns being prevented by cohesion between adjacent water molecules and adhesion between the water molecules and the xylem wall.