Question

A plant can store starch in many areas, from the leaves down to the roots .What does this tell us about the transport of sugar solution in phloem tubes?

Answer 1

A high concentration of sugar at the source creates a low solute potential (Ψs), which draws water into the phloem from the adjacent xylem. This creates a high pressure potential (Ψp), or high turgor pressure, in the phloem. The high turgor pressure drives movement of phloem sap by “bulk flow” from source to sink, where the sugars are rapidly removed from the phloem at the sink. Removal of the sugar increases the Ψs, which causes water to leave the phloem and return to the xylem, decreasing Ψp.