The pressure in the stele at the root is a sum of the tension of transpiration and the osmotic potential of active transport of minerals. Phloem loading a. secondary cotransport coupled to proton pumps uses the potential of charge gradient and the concentration gradient created by moving protons out of the cell. The pressure in the sieve tube member is increased as the walls of the sieve tube push back. Phloem unloading - active transport of sucrose into the sink. Reduces the osmotic pressure in the sieve tube member.
Water flows out reducing the pressure of the sieve tube member wall pushing back.
Phloem flow is by bulk flow as the water carrying the sucrose flows to the low pressure at the sink from the high pressure at the source.