Translocation in the Phloem

This chapter studies translocation in the phloem of angiosperms, because most of the research has been conducted on that group of plants. The phloem is the tissue that transports (translocates) the products of photosynthesis, particularly sugars, from mature leaves to areas of growth and storage, including the roots. Along with sugars, the phloem also transmits signals in the form of regulatory molecules and redistributes water and various compounds throughout the plant body. All of these molecules appear to move with the transported sugars. The compounds to be redistributed, some of which initially arrive in the mature leaves via the xylem, can be either transferred out of the leaves without modification or metabolized before redistribution. The fluid that flows through the phloem—the water plus all its solutes—is called phloem sap. The chapter also considers the pressure-flow model, phloem loading and unloading, photosynthate distribution, and the transport of signaling molecules.