Phloem

12 Key excerpts on "Phloem"

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  eBook - ePub

  Marschner's Mineral Nutrition of Higher Plants

  • Horst Marschner(Author)
  • 2011(Publication Date)
  • Academic Press

    (Publisher)

  Phloem loading) and water is sucked into the Phloem, creating a positive internal pressure. This pressure induces a mass flow in the Phloem to the sites of lower positive pressure caused by removal of solutes from the Phloem. Therefore, flow rate and direction of flow are closely related to Phloem unloading at the sink. This type of pressure-driven mass flow in the Phloem differs from that in the xylem in three important ways: (i) organic compounds are the dominant solutes in the Phloem sap, (ii) transport takes place in living cells, and (iii) the unloading of solutes at the sink plays an important role.

  Figure 3.9 Cross-section of a vascular bundle from the stem of maize. Inset: Sieve tube with sieve plate pores and ‘P-protein’. Redrawn from Eschrich (1976).

  For nutrients, the main sites (sources) for Phloem loading are located in the stem and the leaves. These supply nutrients to growth sinks (shoot apices, fruits, roots) and allow nutrient cycling within the plant. An example of sink-regulated transport of a nutrient is shown in Fig. 3.10 for P. After application to one of the two mature primary leaves, the labelled P is transported to the shoot apex and the roots whereas transport to the other primary leaf is negligible. In contrast, Na is not transported to the shoot apex but moves exclusively downwards (basipetally) to the roots where it is confined to the basal zones (Fig. 3.10 ). From here a considerable net efflux of Na takes place ( Lessani and Marschner, 1978 ). This example reflects the role of Phloem transport in cycling elements within the plant and specifically in prevention of Na accumulation in the shoots of natrophobic plant species. The capacity of bidirectional, ion-specific, long-distance transport is based on the physio-logy and anatomy of the Phloem and its elements.

  Figure 3.10 Re-translocation of labelled P ( 32 P) and Na ( 22 Na) after application to the tip of a primary leaf of bean. Autoradiogram, 24 h after application.

  Within the Phloem, the sieve tube elements are associated with companion cells and parenchyma cells (Fig. 3.9 ). Some of these individual sieve tube elements are stretched end to end in a long series, forming the sieve tubes which are connected by pores (inset, Fig. 3.9

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  Phloem

  Molecular Cell Biology, Systemic Communication, Biotic Interactions

  • Gary A. Thompson, Aart J.E. van Bel(Authors)
  • 2012(Publication Date)
  • Wiley-Blackwell

    (Publisher)

  Section A Introduction 1 Phloem, the Integrative Avenue 2 Cell Biology of Sieve Element–Companion Cell Complexes 3 Fundamentals of Phloem Transport Physiology 1 Phloem, the Integrative Avenue Aart J.E. van Bel 1 and Gary A. Thompson 2 1 Institut für Allgemeine Botanik, Justus-Liebig-University, Germany 2 College of Agricultural Sciences, Pennsylvania State University, USA By the end of the nineteenth 1 century, plant biologists recognized the paramount importance of Phloem transport for plant growth. They suspected that plant growth strongly relies on the Phloem-mediated supply of photosynthates and other organic compounds. These initial studies culminated in 1930 with the pressure flow hypothesis proposed by Ernst Münch, which offered a solid theoretical and unifying platform to understand the fundamental mechanism of Phloem translocation. For decades following the general acceptance of Münch's concept, Phloem research predominantly focused on the movement and distribution of photoassimilates. Source supply and sink demand combined with the concepts of donor and receiver organs were seen as key factors in determining plant productivity and, hence, the agricultural yield. The field of Phloem physiology became well established as new tools were developed that allowed researchers to quantifiably measure translocation and to visualize the Phloem tissue at high resolution. Many studies of photoassimilate movement throughout the plant were conducted using 14 C-labeled carbohydrates. These approaches were widely used in the 1970s and early 1980s to learn about carbohydrate metabolism and sugar carrier activities in source and sink tissues. From the 1960s, transmission electron microscopy provided views unparalleled at the time into the ultrastructure of Phloem cells. Great strides were made in detailing the variation and development of sieve element–companion cell complexes and other Phloem cell types in different plant taxa

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  Cambridge O Level Biology 5090

  • Azhar ul Haque Sario(Author)
  • 2023(Publication Date)
  • tredition

    (Publisher)

  Picture the journey of sucrose and amino acids as a relay race. The leaves, after producing these substances, pass them to the Phloem. The Phloem then races through the plant's structure, passing through stems, reaching down into the roots, or climbing up to the fruits and seeds. This journey isn't always a downhill ride. Sometimes, the Phloem has to transport these substances up against gravity, like delivering food to the top floor of a tall building.

  The fascinating part of this process is the pressure flow hypothesis, which explains how the movement happens. It's like a pressure cooker situation. The loading of sucrose into the Phloem at the source creates a high pressure. Water follows, rushing in, and this creates a flow. The sucrose and amino acids are swept along in this flow, moving from the high-pressure area (the source) to a lower pressure area (the sink).

  When they reach their destination, these nutrients are offloaded. The sink uses the sucrose for energy or stores it for later use. The amino acids are used to build proteins that help the plant grow, fight diseases, or even prepare for adverse conditions.

  In summary, translocation is not just a movement; it's a lifeline for the plant. It ensures that every part of the plant gets what it needs, when it needs it, much like a well-coordinated delivery system ensuring that every part of a city gets its essential supplies. This remarkable process showcases the intricate and efficient ways in which plants sustain themselves and thrive.

  Understanding the arrangement of tissues in transverse sections of non-woody dicotyledonous roots and stems provides valuable insight into plant biology and function. Let's explore the positions of the xyleem, Phloem, and cortex in these plant parts.

  Non-Woody Dicotyledonous Roots

  Cortex: The outermost layer of the root, the cortex, plays a crucial role in the storage of food and the absorption of water and nutrients from the soil. It is a region with loosely packed cells, allowing for easy passage of water and minerals.

  Phloem: Situated inside the cortex, the Phloem forms a part of the vascular system of the root. This tissue is responsible for transporting organic nutrients, primarily sugars, from the leaves to other parts of the plant. In dicotyledonous roots, Phloem is typically found in patches interspersed around the central core of xylem.

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  The Physiology of Flowering Plants

  • Helgi Öpik, Stephen A. Rolfe(Authors)
  • 2005(Publication Date)
  • Cambridge University Press

    (Publisher)

  Various fluorescent dyes, such as fluorescein and its derivatives, can be directly observed under the microscope to move in the Phloem. At first this observation was regarded with caution, since these are artificial compounds and might move along paths different from those of natural metabolites. Final confirmation has come from radioactive labelling. When radio-active CO 2 is supplied to photosynthesizing leaves, the radioactivity soon appears in the Phloem of the petiole and the stem, as radio-active sugars. Here there has been no introduction of any foreign substance, nor any interference with the plant’s normal activity, and the data prove that the products of photosynthesis move from their sites of production in the Phloem. Radiolabelling has shown that the Phloem is also the pathway of translocation out of non-photosyn-thetic storage organs. Since it has been established that naturally transported metabolites and numerous fluorescent dyes move along the same pathway, the dyes are now frequently used as tracers for Phloem transport. 5.2.2 The structure of Phloem The Phloem of flowering plants consists of several types of cell. Tracer experiments have shown that, at the cellular level, transloca-tion proceeds through the sieve tubes , built of longitudinal files of individual sieve tube elements (sieve tube cells). The sieve tube diameter usually lies between 10 and 50 m m and the length of individual cells is 150–1000 m m, but in palms diameters of 400 m m and lengths of 5000 m m have been reported; in minor veins, however, sieve tubes can be very narrow, below 2 m m. The transverse or oblique end walls between the individual sieve tube cells, some 0.5–2 m m thick, are pierced by pores giving them a sieve-like appearance and are known as the sieve plates , hence the cells’ name (Fig. 5.1 ). The diameter of the sieve plate pores is extremely variable between species and in different parts of a plant. The narrowest are only 134 TRANSLOCATION OF ORGANIC COMPOUNDS

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  eBook - ePub

  Principles of Horticulture: Level 3

  • Charles Adams, Mike Early, Jane Brook, Katherine Bamford(Authors)
  • 2015(Publication Date)
  • Routledge

    (Publisher)

  6    

  CHAPTER

  Level 3

  Transport in the plant

  Figure 6.1 A blue dye has been used to show the pathway of water movement in the xylem of a celery stem and leaves. Note the staining of the xylem tissue in the vascular bundles of the stem cross section, the continuity of xylem vessels up the stem itself (cortex removed) and the distribution of xylem in the veins of the leaves

  This chapter includes the following topics:

  • Movement of substances in the plant

  • Water movement between cells

  • Water movement through the plant

  • Factors affecting transpiration

  • Plant adaptations to low water environments

  • Stomata

  • Uptake and movement of mineral nutrients

  • Phloem translocation and the movement of sugars

  • The balance between photosynthesis, respiration and transpiration

  Principles of Horticulture. 978-0-415-85909-7 © C.R. Adams, M.P. Early, J.E. Brook and K.M. Bamford. Published by Taylor & Francis. All rights reserved.

  Without effective uptake and transport of gases, water and dissolved substances such as minerals, sucrose and plant growth regulators, plants would not be able to function. The exchange of gases necessary for photosynthesis and respiration is described in Chapter 5 . The movement of water and dissolved substances (translocation) takes place in two specialized tissues, the xylem and the Phloem, whose structures are described in Chapter 3

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  The Molecular Life of Plants

  • Russell L. Jones, Helen Ougham, Howard Thomas, Susan Waaland(Authors)
  • 2012(Publication Date)
  • Wiley-Blackwell

    (Publisher)

  w in the sieve tube and lowers it in sink cells, causing water to leave the unloading end of the Phloem. Experiments with aphids, insects that feed directly on Phloem contents, have established the occurrence of sugar gradients in Phloem as well as the presence of positive pressure.

  Figure 14.17 Electron micrographs showing (A) an unblocked sieve plate and (B) an open sieve tube element that is not occluded. The asterisk marks the open channel through which solute transport occurs. The tissue was prepared for microscopy by ultra-rapid freezing.

  14.6 Phloem Loading, Translocation and Unloading

  The movement of sugars into the Phloem at the source is referred to as loading, and the delivery of translocated sugars to the sink is termed unloading. Studies of the mechanisms of loading and unloading have focused on the relative contributions of symplasmic and apoplastic processes and have revealed marked species-to-species variations in strategies for moving photosynthate from sources to sinks.

  14.6.1 At the Source, Phloem Loading can Occur from the Apoplast or through the Symplasm

  Phloem loading has been almost exclusively studied in photosynthetic leaves. There are two mechanisms by which sugars may be loaded into the Phloem. In plants with apoplastic loading, source cells release sugar into the apoplast; from there it is actively loaded into the SE/CC complex. In plants with symplasmic loading, there are uninterrupted symplasmic connections between photosynthesizing mesophyll cells and sieve tubes so that sugars diffuse via plasmodesmata from source cells to the SE/CC complex. Species with apoplastic loading have ordinary companion cells. These cells have many plasmodesmatal connections to their partner sieve elements, but few or none to surrounding cells (Figure 14.18 A). The cell wall of an ordinary companion cell is often highly modified by the presence of ingrowths that are found in the cell wall facing away from the sieve element. These wall ingrowths have the effect of increasing the surface area of the plasma membrane to facilitate uptake of assimilates. This type of companion cell is referred to as having transfer cell characteristics. The plasmodesmata connecting the companion cell to the sieve element are complex and often highly branched on the companion cell side (Figure 14.18 B). The absence of plasmodesmata between the SE/CC complex and neighboring cells ensures that they are symplasmically isolated and the SE/CC complex is referred to as having a closed configuration

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  Contemporary Problems in Plant Anatomy

  • Richard White(Author)
  • 2012(Publication Date)
  • Academic Press

    (Publisher)

  Typically, the companion cells of the secondary Phloem abut the rays (Evert, 1963a), presumably acting as intermediaries in the transfer of assimilates from the sieve-tube member to the ray parenchyma cells and vice versa. In the functional Phloem, the parenchyma cells and com-panion cells apparently have unlignified primary walls. After the tissue ceases to be involved with long-distance transport, the parenchyma cell may remain relatively unchanged or become sclerified. Two instances have been recorded of sclerifica-tion of companion cells in old Phloem, one in the secondary Phloem of Tilia americana (Evert, 1963b), and the other in the metaPhloem of the perennial monocotyledon Smilax rotundifolia (Ervin and Evert, 1967). C. Minor Vein Phloem In most parts of the plant the sieve elements commonly are wider than the contiguous parenchyma cells. By contrast, the minor vein Phloem of dicotyledonous leaves is characterized by the presence of diminutive sieve-tube members and large parenchyma cells, many of which have dense, organelle-rich protoplasts (FIG. 20; Evert, 1980; Fisher and Evert, 1982). 178 Ray F Evert IV Comparative Structure of Phloem 179 The dense parenchyma cells have numerous plasmodesmatal con-nections with their associated sieve-tube members, and com-monly are considered to be companion cells. Fischer (1884) called these dense cells Übergangszellen or intermediary cells (Esau, 1969) because he thought they served as inter-mediaries in the transfer of photosynthates between mesophyll cells and sieve elements of the leaf. Less dense parenchyma cells of the minor vein Phloem may also be involved in the transfer or loading of photosynthates into the sieve tubes. Although these cells may also be termed intermediary (Esau, 1977), they commonly are called Phloem parenchyma cells to distinguish them from companion cells.

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  Transport of Nutrients in Plants

  • A. J. Peel(Author)
  • 2013(Publication Date)
  • Butterworth-Heinemann

    (Publisher)

  6 Up to this point, with the exception of the section on ion movement across the root cortex in Chapter 1, we have been considering aspects of solute transport in which there is a fair degree of unanimity in the interpretation of the experimental data. Even though there is a divergence of opinion as to the relative merits of velocity and mass transfer as useful parameters of Phloem transport, at least most workers on Phloem would agree with the values for these parameters quoted in Chapter 4. Now, however, we enter an area where the available data are subject to considerable controversy as to their interpretation. This area is, of course, that concerned with the mechanism responsible for movement in sieve tubes, and the following five chapters will to all intents and purposes be devoted to a discussion of this problem. The purpose of the present chapter is to provide a review of investigations with the optical and electron microscopes into the structure of Phloem cells. This subject is so intimately bound up with the possible mechanisms of movement that it seems inconceivable that the mechanism can be elucidated until the structure of the functioning sieve element is understood. Indeed, when we come to consider the hypothesis of the mechanism of transport, we shall see that each relies on a particular type of structure being present in the sieve element, and therefore any may be discarded if the required structure is not realised. For an excellent account of the relationships between structure and function, the reader is referred to the review by Weatherley and Johnson (1968). The type of structure assigned to the presumed mature sieve element can vary widely between different workers. This variability in the interpretation of structure has frequently been so pronounced 97 The structure of Phloem cells

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  Vascular Transport in Plants

  • N. Michelle Holbrook, Maciej A. Zwieniecki(Authors)
  • 2011(Publication Date)
  • Academic Press

    (Publisher)

  Plasmodesmata are present at every interface in a leaf along the photoassimilate transport pathway, from mesophyll cells to sieve elements. Although transport routes are often referred to as symplastic (through the plasmodesmata-connected cytosol of cells) or apoplastic (involving at least one extracellular step), we should perhaps think of these as quantitative, rather than strictly qualitative, differences. It is possible that both pathways contribute to photoassimilate flux, more so at some interfaces than at others. It is also possible that the relative distribution of photoassimilate from one pathway to another is subject to regulation in response to nutrient demands or environmental changes.

  In this chapter, the potential for symplastic and apoplastic transport across each interface is discussed to provide a framework for further exploration of the mechanisms and regulation of Phloem loading. Beebe and Russin (1999) have described the details of plasmodesmatal structure along the loading route. Dicots are emphasized here since monocots are dealt with by Botha (see Chapter 6 ) in this volume.

  Minor Veins

  Three cell types are most commonly found in minor vein Phloem: sieve elements, companion cells, and Phloem parenchyma cells. Since sieve elements and companion cells are ontogenetically related (Behnke and Sjolund, 1990 ) and symplastically coupled (see later), they are frequently considered together as the sieve element/companion cell complex (SE/CCC). Companion cells can often be distinguished from Phloem parenchyma cells on the basis of their greater cytoplasmic density. However, in some plants the distinction is difficult to make, even in electron micrographs. Minor veins are bounded by a sheath of cells that may be more or less mesophyll-like in structure. This bundle sheath separates the air spaces of the mesophyll from the very limited air spaces within the veins. In some plants the bundle sheath is connected, at the level of the Phloem, to a specialized layer of mesophyll cells called the paraveinal mesophyll that is specialized for delivery of photoassimilates to the veins (Lansing and Franceschi, 2000

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  Plant Physiology 9

  A Treatise: Water and Solutes in Plants

  • F.C. Steward(Author)
  • 2012(Publication Date)
  • Academic Press

    (Publisher)

  , Gorham, P. R., Srivastava, L. M., and Swanson, C. Α., eds. (1975). Phloem Transport, NATO Adv. Study Inst. Ser., Ser. A, Life Sci., Vol. 4. Plenum, New York. 9. Baker, D. Α., Malek, F., and Denvar, F. D. (1980). Phloem loading of amino acids from the petioles of Ricinus leaves. Ber. Dtsch. Bot. Ges. 93, 2 0 3 -2 0 9 . 10. Barclay, G. F., and Fensom, D. S. (1973). Passage of carbon black through sieve plates of unexcised Heracleum sphondylium after micro-injection. Acta Bot. Neerl. 22, 2 2 8 -232. 11. Barlow, H . W . B . ( 1 9 7 9 ) . Sectorial patterns on leaves of fruit tree shoots produced by radioactive assimilates and solutions. Ann. Bot. (London)[N.S.] 43, 5 9 5 -6 0 2 . 12. Baset, Q. Α., and Sutcliffe, J . F. (1975). Regulation of the export of potassium, nitro-gen, phosphorus, magnesium and dry matter from the endosperm of etiolated oat seedlings (Avena sativa, cv Victory). Ann. Bot. (London) [N.S.] 39, 3 1 -4 1 . 13. Bauermeister, Α., Dale,J. E., Williams, E., andScobie,J. (1980). Movement of 1 4 C and u C-labeled assimilate in wheat leaves: The effect of IAA./. Exp. Bot. 31, 1 1 9 9 -1 2 0 9 . 14. Behnke, H. D. (1971). The control of the sieve plate pores in Aristolochia.J. Ultrastruct. Res. 36, 4 9 3 -4 9 8 . 15. Behnke, H. D. (1975). Companion cells and transfer cells. In Phloem Transport (S. Aronoff et ai, eds.), pp. 1 5 3 -1 7 5 . Plenum, New York. 16. Bennet-Clark, T. A. (1959). Water relations of cells. In Plant Physiology: A Treatise (F. C. Steward, ed.), Vol. 2, pp. 105 - 191. Academic Press, New York. 17. Bennett, C. W. (1940). The relation of virus to plant tissue. Bot. Rev. 6, 4 2 7 -4 7 3 . 18. Bentwood, B. J . , and Cronshaw, J . (1978). Cytochemical localisation of adenosine triphosphatase in the Phloem of Pisum sativum and its relation to the function of transfer cells. Planta 140, 1 1 1 -1 2 0 . 19. Biddulph, O. (1956). Visual indications of 3 6 S and 3 2 P translocation in the Phloem. Am. J. Bot. 43, 1 4 3 -1 4 8 .

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  Biology Workbook For Dummies

  • Rene Fester Kratz(Author)
  • 2022(Publication Date)
  • For Dummies

    (Publisher)

  Phloem tissue is surrounded by strong cells called fibers (a type of sclerenchyma) and parenchyma cells that form the cortex. » Outside the Phloem ring is the bark, a ring of boxy, waterproof cells that help protect the stem. Bark includes the stem’s outermost cells and a layer of cork cells just beneath that outermost layer. The cork cambium is a layer of parenchyma cells that divides to produce new cork cells, increasing the woody stem’s diameter. 330 PART 5 Going Green with Plant Biology 40 The cells that make the strings in celery thicken their cell walls with extra cellulose. 41 Sieve cells connect end to end to transport sugary sap through a tree trunk. Use the following terms to identify which type of tissue would perform the function in each question. a. Parenchyma b. Sclerenchyma c. Collenchyma d. Xylem e. Phloem 39 A leaf cell does photosynthesis. 39 – 43 42 The cells that make the gritty texture in pears thicken their cell walls with lignin. 43 Hollow, open-ended cells called vessels con- duct water through a flower stem. CHAPTER 18 Studying Plant Structures 331 Use the following terms to label the woody dicot stem in Figure 18-4. a. Pith b. Annual ring c. Phloem d. Cortex e. Cork cambium f. Vascular cambium g. Secondary xylem h. Primary xylem i. Summer wood j. Spring wood k. Cork 44 – 54 FIGURE 18-4: Internal anatomy of a woody stem. Illustration by Kathryn Born, M.A. 332 PART 5 Going Green with Plant Biology Growing Like a Weed: Plant Reproduction Plants are very successful organisms, growing in almost every environment on Earth. Part of their success is because they can reproduce both asexually and sexually (see Figure 18-5). » When plants reproduce asexually, they use mitosis to produce offspring that are genetically identical to the parent plant. (See Chapter 5 for more on mitosis.) The advantage of asexual reproduction is that it allows successful organisms to reproduce quickly.

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  Mechanisms of Plant Growth and Improved Productivity Modern Approaches

  • Amarjit Basra(Author)
  • 2021(Publication Date)
  • CRC Press

    (Publisher)

  This plethora of information emanating from a wide range of disciplines, now requires careful evaluation to provide an understanding of the key processes involved in the partitioning and compartmentation between source and sink tissues within the plant. Thus the subject of Phloem transport has undergone major shifts in emphasis from the time when a few researchers hotly debated the pros and cons of hypothetical long-distance transport mechanisms in a manner reminiscent of the scientists in Gulliver’s Travels who sought to trap sunlight in bottles when surrounded by photo-synthesizing plants. There are several specific areas in assimilate transport where future research should now appropriately be concentrated. These include the major enzyme-mediated processes which regulate the export and import of assimilates by source and sink tissues, respectively. Here the identification, isolation, and purification of key enzymes, utilizing biochemical techniques, will allow the genes to be identified and cloned, using molecular genetics. Ectopic expression of these enzymes in transgenic plants then provides a powerful means of resolving the question of whether the enzymes which play the major roles in these regulatory processes can themselves be manipulated to adjust the sourcersink balance, both in a qualitative and a quantitative sense. However, a number of features of the long-distance transport system still need to be elucidated. In this sense “long-distance” refers to cell to cell distances and greater. We do not yet understand the regulation of plasmodesmata: are there real pores from cell to cell which can be opened and closed on demand? It is also pertinent to reflect on the fact that sieve-plate pores of the sieve tubes, which seem to be under somewhat similar control, are very poorly understood. We know that their “porosity” seems to increase in response to demand under stable conditions favorable to growth

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