PHLOEM TRANSPORT: The plant body consists of organs specialized for various functions. Its roots are meant for absorption and anchoring the plant in the soil and its leaves are responsible for photosynthesis. These plant parts contain specialized conducting tissues, the xylem, and the phloem.
The water and minerals absorbed by roots are transported to aerial portions of the plant (leaves) by the xylem. The phloem translocates the products of photosynthesis from mature leaves to areas of growth and storage. The phloem also serves to redistribute water and minerals that reach the leaves. Similarly, certain hormones synthesized in specific parts of the plant move to other parts via the phloem.
The Pathway of Translocation of Organic Solutes Girdling
The experiments that helped us to understand the transport of organic solutes were first performed by an Italian anatomist Marcello Malpighi (1686) and later by Hales (1727), and Mason and Maskell (1926). They removed the bark of a tree in a ring around the trunk (a phenomenon called girdling) and found that there was no immediate effect on the movement of water as transpiration continued to take place.
However, the sugar transport in the trunk was blocked at the site where the bark has been removed. Sugars accumulate above the girdle (the side towards the leaves) and the bark below the girdle dries up and eventually died.
The experiments suggested that sugar transport occurs in the bark (phloem) of the tree. Later on, experiments with dyes, especially dye fluorescein, confirmed movement in phloem cells, especially sieve elements.
Experiments with Radioactive Tracers
In the 1940s more sophisticated experiments on phloem translocation became possible when radioactive tracers became available for scientific research. Labeled organic compounds (14CO2) were introduced during carbon dioxide fixation reactions and labeled sugars were found to be transported in the sieve elements.
In another experiment labeled sucrose was applied to the leaf surface directly after removing the epidermis. The labeled Sugar appeared in the sieve elements.
A Junction can often be understood by understanding the structure in which it occurs. For example; an examination of valves and chambers of the heart makes its function as a pump clear to anyone. Similarly, the mechanism of phloem transport can be understood by studying phloem anatomy.
Phloem is a heterogeneous tissue composed of four different types of cells (sieve elements, companion cells. Phloem parenchyma cells, and phloem fibers) that are similar in origin and major function but differ in structure.
Sieve elements are elongated living cells, usually without nuclei at maturity. These are the cells in which translocation actually takes place. In angiosperms, they are connected end to end, forming long tube-like series of cells known as sieve tubes. The sieve elements are characterized by sieve areas, portions of cell walls where pores interconnect the conducting cells. A mature sieve element is highly vacuolated with cytoplasm in the form of a thin layer closely appressed to the cell wall.
Minute bodies called slime bodies formed of P-proteins are present in the cytoplasm. The apparent function of P- proteins are sealing off damaged sieve elements by plugging the sieve-plate pores.
The smooth endoplasmic reticulum is present as a continuous network in the cytoplasm. Mitochondria and plastids sometimes containing starch, protein, or both are also present in the cytoplasm. Walls of mature elements are non-lignified, rich in cellulose, and thick.
Each sieve element is associated with one or more companion cells, arising from the same mother cell as the sieve element. Both the cells are connected through plasmodesmata, suggesting a close functional relationship between the two cells.
The companion cells are nucleated at maturity and have dense cytoplasm. Companion cells are thought to be concerned with the supply of ATP to sieve elements and in some cases flow of sugars from mesophyll cells to sieve elements.
The phloem parenchyma cells are thin-walled. Cells that are different from other parenchyma cells in that they are elongated. They may transport solutes and water or act as storage cells. Phloem fibers are thick-walled cells that are usually grouped in a bundle. The major function of phloem fibers is to provide strength. In some species these act as storage cells.
Vascular Anatomy of Leaves
The vascular anatomy of the minor veins in leaves is especially important to an understanding of phloem transport. The large veins in a leaf branch into smaller veins and eventually into the minor leaf veins.
Each minor vein may contain only one vessel representing the xylem and one or two sieve tubes surrounded by companion cells. Phloem parenchyma cells are intermingled with the companion cells and it separates phloem tissue from the xylem. Photosynthesizing mesophyll cells are in close contact with the minor vein.
Role of Sieve Elements in Translocation
There is a consensus, of opinion among specialists in the field of phloem translocation that a sieve element becomes capable of translocation of organic solutes when it reaches maturity. The lumen provides space for accommodating larger amounts of sugar and sieve pores in the end walls permit free flow of sugar solution from one sieve element to the other.
The thin layer of cytoplasm lining the vertical walls provides a differentially permeable membrane across which a difference in sugar concentration (between apoplast and symplast) is maintained, permits water movement (osmosis) across it and allows active transport of sugar molecules during loading and unloading.
Sources and Sinks
The girdling experiments revealed another important fart. When the bark between a leafy branch and the developing fruit is removed, the sugars accumulate in the bark on the side of the leafy branch.
Similarly, the bark was removed below the shoot tip i.e. above the leaves; the sugars accumulate in the bark on the side of the leaves again, at the bottom of the girdle. This suggests that gravity does not govern the movement of materials in the bark, rather the assimilates move from source to sink.
The regions supplying sugars to the phloem are referred to as sources. Primary sources are green leaves because of their photosynthetic capacity. The sugars produced in these leaves are transported to other parts of the plant. Also, the plant parts, for example, root (beetroot that stores food one season and supplies it next growing season), tubers, endosperm, cotyledons, etc., in which food materials are stored, serve as sources.
The regions of the plant that utilize sugars translocated in the phloem are referred to as sinks. Any growing, storing, or metabolizing tissue might be a sink, for example growing fruits, stems, roots, corms, tubers, flowers, or young leaves. As a general rule sinks are supplied mainly by nearby sources.
Aphids, small insects that feed by inserting their mouthparts consisting of four tubular stylets into a sieve element of leaf or stem, helped in the identification of material translocated in the phloem.
The high turgor pressure in the sieve element forces the cell contents into the insect’s gut, where amino acids are removed and some sugars are metabolized. The excess sap, rich in carbohydrates, is excreted as honeydew, which can be collected and chemically analyzed.
This method is advantageous as the aphids puncture a single sieve element, so there is no problem of contamination from other cell types. The exudation from severed stylets continues for hours suggesting that aphids prevent normal sealing mechanisms.
Analysis of phloem sap suggested that water is the most abundant substance transported in the phloem. Solutes dissolved in the water are carbohydrates mainly. Sucrose is the most common sugar transported in sieve elements. The concentration of sucrose ranges from 0.3 to 0.9 M.
Other organic solutes moving in the phloem are amino acids and amides, especially glutamic acid and aspartic acid; plant hormones such as auxins, gibberellins, cytokinins, and abscisic acid; nucleotides and enzymes. Inorganic solutes moving in the phloem include potassium, magnesium, phosphate, and chloride.