Phloem

Phloem is the vascular tissue that distributes sugars through a plant. It connects parts that supply these substances, called sources, with parts that consume or store them, called sinks. Sucrose is the main sugar carried over long distances, although some plants also transport sugar alcohols or sugars of the raffinose family. Mature leaves supply carbon fixed through photosynthesis.

Phloem sap also contains nitrogen compounds, mineral ions, proteins and RNA. The presence of a mobile protein or RNA does not by itself establish a signalling function.

Structure and development

In flowering plants, elongated sieve elements join through perforated end walls, called sieve plates, to form conducting tubes. Plasmodesmata connect them laterally with companion cells, which provide metabolic support. Mature sieve elements remain alive but lose their nuclei and much of their internal machinery. They depend on their companion cells to maintain transport. Parenchyma and supporting fibres can surround this conducting tissue.

Early primary phloem, or protophloem, develops at the growing tips, the meristems. Later primary phloem, called metaphloem, persists in mature organs. During secondary growth, which thickens stems and roots, the vascular cambium produces secondary phloem. Its dividing cells form xylem towards the inside and phloem towards the outside.

Sources and sinks

The balance of supply and demand changes as a plant grows. Roots and young leaves receive sugars during vegetative growth; developing fruits, seeds and tubers become major destinations during reproduction. Storage tissues can change roles: stems and tree trunks may accumulate carbon temporarily, then supply it to other organs. Environmental conditions affect sugar production, transport and use.

Transport

The pressure-flow model proposed by Ernst Münch explains long-distance movement of phloem sap. Loading sugars into the conducting tissue increases its solute concentration, drawing in water by osmosis and raising pressure. Unloading sugars at sinks helps establish a pressure difference along the tube. This difference drives the solution as a bulk flow; long-distance movement is not simply diffusion of individual sugar molecules.

The pathway also supplies tissues beside it. Sugars leave the conducting tubes to support local growth and maintenance, and some are taken back up. Phloem therefore exchanges material along its length as well as delivering it to distant sinks.

Loading pathways

Phloem loading follows several routes. In apoplasmic loading, sugar crosses cell membranes through transport proteins after entering the cell-wall space. In symplasmic loading, it travels between cells through plasmodesmata. This can be passive, with movement down a concentration gradient. Alternatively, sucrose entering companion cells is converted into larger sugars whose return through the narrow connections is restricted. This polymer trapping allows sugars to accumulate in the phloem. More than one loading strategy can operate in a species, so a single route cannot be assigned to all trees or all herbaceous plants.

Attribution

Adapted from English Wikipedia's Phloem, revision 1379224769 (contributors and history), under the Creative Commons Attribution-ShareAlike 4.0 licence. The prose has been rewritten and condensed, and the references replaced with independently checked sources.