According to the pressure flow hypothesis of major force responsible for mass transfer of solute from source to sink is

Pressure flow hypothesis is a theory which helps to define the flow of sap through the phloem. A high concentration of organic substances or solute such as sugar, inside cells of the phloem present at the leaf, creates an osmotic gradient which helps to draw water into the cells from the adjacent xylem.

This creates hydrostatic pressure also called as turgor pressure in the phloem. By the pressure flow movement of phloem sap occurs from sugar sources to sugar sinks.

it takes about 30 days for it to break down