Phloem is the complex tissue, which acts as a transport system for soluble organic compounds within vascular plants.
The phloem is made up of living tissue, which uses turgor pressure and energy in the form of ATP to actively transport sugars to the plant organs such as the fruits, flowers, buds, and roots; the other material that makes up the vascular plant transport system, the xylem, moves water and minerals from the root and is formed of non-living material.
Function of Phloem
Through the system of translocation, the phloem moves photoassimilates, mainly in the form of sucrose sugars and proteins, from the leaves where they are produced by photosynthesis to the rest of the plant.
The sugars are moved from the source, usually the leaves, to the phloem through active transport. The next step, translocation of the photoassimilates, is explained by the pressure-flow hypothesis.
When there is a high concentration of an organic substance (in this case sugar) within the cells, an osmotic gradient is created. Water is drawn passively from the adjacent xylem over the gradient to create a sugar solution and a high turgor pressure within the phloem.
The high turgor pressure causes the water and sugars to move through the tubes of the phloem, into the ‘sink tissues’ (e.g. the roots, growing tips of stems and leaves, flowers, and fruits). When the sink receives the sugar solution, the sugars are used for growth and other processes.
As the concentration of sugars reduces in the solution, the amount of water influx from the xylem also drops; this results in low pressure in the phloem at the sink. Where there are areas of high and low pressure, the photoassimilates and water are consistently moved around the plant in both directions.
Structure of Phloem
The structure of the phloem is made up of several components. Each of the components works together to facilitate the conduction of sugars and amino acids, from a source, to sink tissues where they are consumed or stored.
The Sieve Elements
The sieve elements are elongated, narrow cells, which are connected together to form the sieve tube structure of the phloem. The sieve element cells are the most highly specialized cell type found in plants.
They are unique in that they do not contain a nucleus at maturity and are also lacking in organelles such as ribosomes, cytosol, and Golgi apparatus, maximizing available space for the translocation of materials.
There are two main types of sieve elements: the ‘sieve member’, which is found in angiosperms, and the more primitive ‘sieve cells’, which are associated with gymnosperms; both are derived from a common ‘mother cell’ form.
At the connections between sieve member cells are sieve plates, which are modified plasmodesmata. Sieve plates are relatively large, thin areas of pores that facilitate the exchange of materials between the element cells.
The sieve plates also act as a barrier to prevent the loss of sap when the phloem is cut or damaged, often by an insect or herbivorous animal. After an injury, a unique protein called “P-protein” (Phloem-protein), which is formed within the sieve element, is released from its anchor site and accumulates to form a ‘clot’ on the pores of the sieve plate and prevent loss of sap at the damage site.
In gymnosperms, the sieve elements display more primitive features than in angiosperms, and instead of sieve plates, have numerous pores at the tapered end of the cell walls for material to pass through directly.
The Companion Cells
Each sieve element cell is usually closely associated with a ‘companion cell’ in angiosperms and an albuminous cell or ‘Strasburger cell’ in gymnosperms.
Companion cells have a nucleus, are packed with dense cytoplasm contain many ribosomes and many mitochondria. This means that the companion cells are able to undertake the metabolic reactions and other cellular functions, which the sieve element cannot perform as it lacks the appropriate organelles. The sieve elements are therefore dependent upon the companion cells for their functioning and survival.
The sieve tube and companion cells are connected via plasmodesmata, a microscopic channel connecting the cytoplasm of the cells, which allows the transfer of the sucrose, proteins, and other molecules to the sieve elements.
The companion cells are thus responsible for fuelling the transport of materials around the plant and to the sink tissues, as well as facilitating the loading of sieve tubes with the products of photosynthesis, and unloading at the sink tissues. Additionally, the companion cells generate and transmit signals, such as defense signals and phytohormones, which are transported through the phloem to the sink organs.
The parenchyma is a collection of cells, which make up the ‘filler’ of plant tissues. They have thin but flexible walls made of cellulose. Within the phloem, the parenchyma’s main function is the storage of starch, fats, and proteins as well as tannins and resins in certain plants.
The sclerenchyma is the main support tissue of the phloem, which provides stiffness and strength to the plant. Sclerenchyma comes in two forms: fibers and sclereids; both are characterized by a thick secondary cell wall and are usually dead upon reaching maturity.
The bast fibers, which support the tension strength while allowing flexibility of the phloem, are narrow, elongated cells with walls of thick cellulose, hemicellulose, and lignin and a narrow lumen (inner cavity).
Sclereids are slightly shorter, irregularly shapes cells, which add compression strength to the phloem, although somewhat restrict flexibility. Sclereids act somewhat as a protective measure from herbivory by generating a gritty texture when chewed.