Ah, the phloem! Isn’t that the thing that carries sugar? Or is that the xylem? While the vascular system of a plant can be confusing, these transportation highways play an integral role in plant metabolism. One major blockage without rectification and the plant will perish.

This is particularly evident in the method of girdling the outside of a tree. Obstruct the phloem from supplying the roots with energy, and the whole plant dies. For the sake of distinguishing the plant vascular system, the xylem is a series of tubes comprised of non-living cells. Water and solutes (nutrients) travel in one direction up the xylem via the transpirational stream.

In other words, evaporation of water from the leaves creates a force not unlike a person sucking on a straw that draws these solutes up. This is in stark contrast to the phloem, which carries sugar and other photosynthates (products of photosynthesis, mostly sap) from the leaf source to the root sink. So, with that in mind, let’s first look at the anatomy of the phloem.

Composition and orientation of a plant's phloem

Simplistically, the phloem consists of sieve tube elements, sieve plates, companion cells and parenchyma cells. The sieve tube elements are longitudinal in shape and are oriented end to end forming a consecutive tube. These tube cells are comprised of living cells, albeit not quite fully functional without help.

Once the network of tube cells reaches maturity, several organelles (organs of the cell) including the vacuole (storage) and nucleus (brain center of a cell), which would otherwise block the flow of sugar and other chemicals, disintegrates. Other repackaging or modification organelles remain on either side of the cell to help with various facets of plant metabolism. Imagine how dysfunctional a tube would be when it contains numerous blockades.

Consequently, these living tube cells are like a computer without a hard-drive. So, how do they operate? Or more importantly, live? Each sieve tube element depends on companion cells for life-support to help offset the functions of the lost organelles.

At either end of the cell are screen-like sieve plates that allow passage of the sap from tube cell to tube cell. Lastly there are the parenchyma cells located on the outside of the tube cell that function as storage vessels for the various compounds that pass by.

Exceptions notwithstanding, the phloem resides in the vascular bundle closest to the outer epidermis/cortex layers of the stem. In-between the phloem is a layer referred to as the cambium followed by the xylem that is closest to the pith or inner portion of the stem. Now it is easy to see why a wound to the outside of the plant stem can impact the overall health of the plant. Interestingly, plants have the ability to close off the sieve plate quickly in the event of mechanical damage, thus preventing the sap of one tube from flowing into adjacent tissue.

Going up or down?

While the title of this article may be a bit of a misnomer, unlike the xylem, flow of sap is not unidirectional but rather bidirectional. It is important to note that within the same tube at a particular moment, the movement is unidirectional.

Don’t think of the phloem as one tube, but rather a system of many tubes operating like a series of elevators in a hotel. Some are going up, some are going down, and it just depends on where the cargo is needed. Shoot meristems or new growth, roots, and flowering and fruiting bodies are considered energy sinks. That is to say they consume more than they produce.

Therefore, downward flow in the phloem supplies energy to the roots so they can in turn uptake additional water and nutrients. However, actively growing regions of the plants also require more energy (sugar) than mature leaves, so certain elevators of the phloem can go up to meet the metabolic demands. The most widely accepted theory explaining this movement involves the change in osmotic or water potential driving the sap up or down.

Water potential

The force is not so clear-cut in this one.As previously described, phloem movement is dictated by a difference between source and sink. But how does that occur? Source cells (mature leaves, high concentration) load all compounds that will comprise the sap into the tube cells. This action favors water entering the tube cell as well, creating hydrostatic pressure against the wall of the tube cell driving the sap toward the sink (roots, low concentration).

The offloading of solutes into the sink tissue likewise favors water to leave the tube cell where it is redistributed to adjacent cells and back into the xylem. Therefore, gradients are created in the source such as a high concentration of sucrose. And, once loaded into the tube cells of the phloem, they are propelled toward the area of lower concentration found in the sink (roots and fruit).

What is traveling in the elevator?

The phloem is often credited with being the transporter of sugar manufactured in the leaves. While sugar is arguably one of the most important passengers, the phloem is also responsible for the transportation of other compounds.

This includes nitrogen in the form of amino acids and amides, organic acids, proteins and various other solutes. While there are several constituents compressing the sap, sugar is the focal point. There is credence to this claim in that the sap consists of 16 to 25% carbohydrates, making it the single largest contributor to this chemical cocktail.

In conclusion

The phloem is a series of tubes consisting of sieve tube elements (tube cells), sieve plates (screens), companion cells (life support) and parenchyma cells (storage). The phloem differs from the xylem in that all cells of the xylem are non-living, and movement is only upward as dictated by transpiration.

The phloem transportation highway is responsible for delivering sugars, nitrogenous compounds and other assimilates from the source (leaves) to the sink (roots, flowers, fruit and newly developing leaves).

The movement of these materials is bidirectional; movement is down to supply to roots or upward to supply flower, fruit or new growth. Hopefully this article has illuminated what the phloem is and how integral it is to delivering energy and nutrients throughout the plant.