Phloem elements

Protophlo em.—The first-formed phloem elements in a fibrovascular bundle. 

During the division and enlargement phases of wood cell development, the cell wall is a thin, deformable, and extensible envelope of material referred to as the primary wall. Near the cessation of cell enlargement, however, a secondary wall may begin to be manufactured to the lumen side of the primary wall. Wood fibers, vessel elements, and certain other xylem or phloem elements that function in passive conduction and/or support normally develop a secondary wall (Figure 5). 

Doi, Y., Teranaka, M., Yora, K. and Asuyama, H. (1967). Mycoplasma or PLT group-like micro-organisms found in the phloem elements of plants infected with mulberry dwarf, potato witches, broom, aster yellow or Paulownia witches broom. Annals of Phytopathological Society of Japan, 33 259-266. 

Phloem sap may be obtained by a variety of methods ranging from the use of aphid stylets to the cutting of vascular tissue. Recent techniques have involved the use of EDTA to prevent the resealing of phloem elements (King and Zeevaart, 1974) and successive cutting of the pod tip of Lupinus albus (Pate et al., 1974) and Glycine max (Fellows et al., 1978). A summary of some... 

As the embryo matures, a procambial system differentiates throughout the hypocotyl and cotyledons. This system later gives rise to the components of the vascular system. This is foreshadowed by the development of differences in staining characteristics. Most commonly embryos have only differentiating xylem or phloem elements (Bisalputra and Esau 1964). 

We have already mentioned that wound tissue, a callus, first forms on the cut surface of explants from higher plants. Let us now consider an explant from the xylem. It also first forms a callus out of undifferentiated cells (Fig. 209). Then pockets of cambium develop in this callus. This cambium then differentiates, xylem elements being formed on the side towards the original xylem explant and phloem elements on the other side. Now let us do the same experiment with an explant from the phloem (Fig. 209). In this case the cambium forms phloem on the side oriented towards the phloem explant and xylem elements on the other. Evidently there exists a gradient of material from the explant to the callus which specifies and directs the differentiation of the vascular tissue. 

Kuhn AJ, Schroder WH, Bauch J. On the distribution and transport of mineral elements in xylem, cambium and phloem of spruce (Picea abies [L.] Karst.). Holz-forschung 1997 51 487-496. 

Mineral elements are not synthesized by the plant. They must be acquired from the soil solution by the plant roots (Figure 5.3). Although tubers have associated roots, with the possible exception of Ca, these roots appear to supply only small amounts of minerals to tubers (Kratzke and Palta, 1985, 1986 Busse and Palta, 2006 Sowokinos, 2007). Most of the minerals present in potato tubers appear to have been taken up originally by the main roots that deliver them first to the shoot via the xylem. From the shoot, these mineral elements must be loaded into the phloem for... 

To best describe the ability of a plant to create xylem, there are some technical indicators that can be used. Figure 1 illustrates that an SSP skeletal stem is basically tubular. The "tube" itself is primarily xylem because the epidermal phloem and pith cell walls are usually very thin. For energy crops, it is best to have this tube as large and substantial as possible. In monocot plants and some dicots, the xylem and phloem are not created in tubes but are "bundled" together as highly efficient string elements that are located in the pith. There is not much mass to these bundles and, therefore, not much mass to their skeletal remains. SSPs typically are the dicots that do not form xylem bundles but, rather, xylem rings. 

The phloem tubes consist of adapted cells that transport the products of photosynthesis, and other compounds such as growth regulators, to the growing parts of the plant including the roots. Since phosphate concentration in the phloem is high, those elements with insoluble phosphates are not usually transported in the phloem. Na+, K+, Rb +, Cs+ and Mg2+ are easily transported, whereas, Pb2+, Ca2+, Sr2+ and Ba2+ are not. 

Figure 4.1 The mustard oil bomb in flower stalks of Arabidopsis thaliana consists of S-cells (with glucosinolates) and adjacent myrosin cells (with myrosinase). This is illustrated by transverse (A,C) and longitudinal (B) sections of a pedicel, analyzed by light microscopy (A,B) and transmission electron microscopy (C). The myrosin cells (m) are in contact with the S-cells (S-c), situated inside the starch sheath ( ) (A,B,C). The myrosin cells are located peripherally in the phloem tissue other cells of the phloem include sieve elements (s) and companion cells (cc, in (C) only).

Parenchyma cells have the function of storing nutrients and are located between the sieve elements in the inner bark. Both vertical parenchyma cells and horizontal phloem rays are present. The latter are direct continuations of the xylem rays, but much shorter. 

The functions of potassium in the plant are manifold. This element serves to activate or catalyze a host of enzyme actions, to facilitate the transport of nutrients and assimilates in the xylem and phloem, to maintain the structural integrity of the plant cell, to regulate turgor pressure, to mediate the fixation of nitrogen in leguminous plant species, and to protect plants to some degree from certain plant diseases. 

Figure 1. Source leaf minor vein phloem. (A) Autoradiograph of leaf tissues following l C-sucrose accumulation showing radioactivity (white) in veins. (B) Tracing of an electron micrograph of a cross section of minor vein, x, xylem, vp, vascular parenchyma cc, companion cell se, sieve element pp, phloem parenchyma, me, mesophyll cell. Reproduced with permission from Ref. 6. Copyright 1983. Annual Reviews.

Rather than the solute speed in the phloem, we are sometimes more interested in how much matter is translocated. For example, if the sieve elements contain 0.5 m (500 mol m-3) sucrose moving at an average speed of 0.6 m hour-1, what is the transfer rate of sucrose in kg m-2 hour-1 By Equation 3.7 (Jj = vjcj), the flux density of sucrose is... 

In the current case, slightly more than half of the hydrostatic pressure drop along the phloem is necessary to overcome the resistance of the sieve plate pores. When the end walls are steeply inclined to the axis of the sieve element, the pores of the sieve plate can occupy an area that is greater than the cross section of the sieve tube. This causes Jv in the pores to be less than in the lumen and tends to reduce the resistance to flow in the phloem. 

Water enters and leaves the phloem by passively moving toward regions of lower water potential ( P = P - n + pwgh Eq. 2.13a). The conducting cells of the xylem generally have a low and relatively constant osmotic pressure (here 0.1 MPa). Solutes either diffuse or are actively transported into and out of the sieve elements, leading to a high phloem osmotic pressure of 1.7 MPa in the leaf and a decrease to 0.7 MPa in the root the much lower n in the sink leads to a lower P there, which favors the delivery of more solutes. 

Such a large osmotic pressure, caused by the high concentrations of sucrose and other solutes, suggests that active transport is necessary at some stage to move certain photosynthetic products from leaf mesophyll cells to the sieve elements of the phloem. From the definition of water potential, = P — II + pwgh (Eq. 2.13a), we conclude that the hydrostatic pressure in the phloem of a leaf that is 10 m above the ground is... 

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