BIOLOGY Volume 2 - A Guide to General Biology - 2004

13. PLANT TRANSPORT

13.8. Translocation of Organic Substances in the Phloem

Photosynthesis does not take place in all parts of a plant. Plant Organs located far from photosynthetic structures, such as the roots, require a dedicated transport system to supply them with assimilates. In vascular plants, organic products are distributed from the primary sites of photosynthesis—the leaves—to all other PARTS OF THE plant via the phloem. Fig. 13.21 illustrates the general relationship between autotrophic Cells that produce organic nutrients and the cells that consume them. As this diagram shows, organic substances in plants can move both upward and downward through the shoots. This distinguishes the phloem from the xylem, where transport is strictly unidirectional (upward). It is also worth noting that storage organs can function alternately as sources or sinks of assimilates at different times.

Class="center">

Fig. 13.21. Movement of organic substances in a green plant.

Typically, about 90% of the dissolved nutrients transported in the phloem consist of the disaccharide sucrose. It is a relatively inert, highly soluble carbohydrate that plays almost no role in METABOLISM, making it an ideal transport molecule since it is unlikely to be consumed en route. The primary purpose of sucrose is to be converted back into more reactive Monosaccharides—glucose and fructose. Its high solubility allows it to reach very high concentrations in the phloem sap; for example, in sugarcane, it can make up to 25% (w/v).

The phloem also transports various mineral nutrients in different forms, such as nitrogen and sulfur as Amino Acids, phosphorus as inorganic phosphate and phosphorylated sugars, and potassium as ions. It may additionally contain trace amounts of Vitamins, Plant HORMONES (such as Auxins and Gibberellins), Viruses, and Other Compounds.

Carbon Circulation within a plant can be clearly demonstrated by allowing leaves to absorb carbon dioxide labeled with the radioactive isotope ¹⁴C. This radioactive Carbon dioxide is fixed during photosynthesis, incorporating ¹⁴C into Organic compounds, including sucrose. The subsequent movement of the isotope through the plant can then be tracked using established techniques, such as autoradiography, measuring radiation at the plant surface with a Geiger counter, or extracting the isotope from various plant Tissues. Ultimately, both the phloem and xylem participate directly in carbon cycling. For instance, upon reaching the roots as part of a sucrose molecule, carbon can be utilized there to synthesize amino acids from nitrates and CARBOHYDRATES; these newly synthesized amino acids containing labeled carbon can then be transported upward through the stem in the xylem sap.

13.8.1. Features of Phloem Translocation

Before examining potential mechanisms of phloem translocation, it is helpful to outline several key facts that any proposed hypothesis must account for.

1. The amount of solutes transported by the phloem is extremely large. It has been estimated, for example, that up to 250 kg of sugar moves down the trunk of a large tree during a single growing season.

2. The rate of translocation is high, typically ranging from 20 to 100 cm/h, with maximum recorded values exceeding 600 cm/h.

3. Transport can occur over very long distances. Eucalyptus trees can grow over 100 m tall. Since their leaves are located predominantly near the crown, assimilates must travel downward along almost the entire length of the trunk, and frequently over considerable distances within The ROOT System as well.

4. The relative mass of the phloem is small. The functional phloem layer encircling a tree trunk is roughly as thick as a postcard. The phloem forms the innermost layer of the bark (specifically the bast) in woody stems and roots, while older phloem layers become stretched and eventually die as the plant organs grow and increase in diameter.

5. In flowering plants, phloem sap moves through sieve tubes with very small diameters—no more than 30 µm (comparable to the finest human Hair). At regular intervals, these tubes are divided by sieve plates perforated with even finer pores. The smaller the diameters of the tubes and pores, the greater the resistance to fluid flow, and the greater the force required to drive it. The hydrostatic pressure inside the sieve tubes is high.

6. In addition to sieve plates, sieve tubes possess other structural features that must also be taken into consideration (see the following section).

13.24. How many sieve plates will a sucrose molecule pass through if it travels 1 km through sieve tubes composed of individual elements each 400 µm in length? (Sieve tube elements, or segments, are elongated cells connected end-to-end, forming sieve plates at their junctions.)