Xylem vessels

Xylem Tracheids, vessel elements, Conduction of water and minerals ... 

Fig. 5. Part of a video image of xylem vessels, exaggerating the size of the pixels and plotting them on a Laseijet printer. 

Taiz and Zeiger (2002) give a full account of this topic. Mineral ions absorbed from solution outside the root surface must be transported across the root to the main long-distance transport vessels in the xylem, through which they reach the shoot. This process is highly specific for different ions and molecules and is closely regulated. The regulation is in part a fnnction of the anatomy of the varions root tissues and in part a fnnction of active transport processes in root cells. The pathways and transport processes are affected by root adaptations to anoxia. 

Finally the ion mnst leave the symplast of the xylem and be loaded into the xylem s long-distance conducting vessels. The mechanism of xylem loading apparently involves both passive and active transfer from the xylem parenchymal cells. 

Lignin is found in plant cell walls of supporting and conducting tissue, mostly the trac-heids and vessel parts of the xylem. It is largely found in the thickened secondary wall but can occur elsewhere close to the celluloses and hemicelluloses. 

The induction of PAL activity at the onset of vascular differentiation can be shown by the use of plant tissue cultures (37-39). Xylem cells with secondary and lignified walls are differentiated over a time course of 3-14 days by the application of the plant growth factors naphthylene acetic acid (NAA) and kinetin in the ratio 5 1 (1.0 mg/liter NAA, 0.2 mg/liter kinetin) to tissue cultures of bean cells (Phaseolus vulgaris) (37,40). The time for differentiation varies with the type of culture, solid or suspension, and with the frequency and duration of subculture, but for any one culture it is relatively constant (37,41,42). At the time of differentiation when the xylem vessels form, the activity of PAL rises to a maximum. The rising phase of the enzyme activity was inhibited by actinomycin D and by D-2,4-(4-methyl-2,6-dinitroanilino)-N-methylpropionamide (MDMP) applied under carefully controlled conditions (42). This indicated that both transcription and translation were necessary for the response to the hormones. Experiments using an antibody for PAL and a cDNA probe for the PAL-mRNA have also shown that there is an increase in the amount of transcript for PAL during the formation of lignin when Zinnia mesophyll cells are induced to form xylem elements in culture (Lin and Northcote, unpublished work). 

Special characteristics Mainly in warm and arid regions Stems often sympodial Stems joined at the nodes Xylem with vessels Tracheids and fibres Hairs... 

Figure 2. Transverse section of coalified Persea secondary xylem for comparison with Figure I. At least three coalu products are shown dark colored cell inclusions, vessel wall derivatives, and pber-tracheid wall derivatives. 304X...

Copper is an essential element. Copper plays a significant role in several physiological processes - photosynthesis, respiration, carbohydrate distribution, nitrogen reduction and fixation, protein metabolism, and cell wall metabolism. Many plant metalloenzymes contain copper. It also influences water permeability of xylem vessels and thus controls water relationships. It is mainly complexed with organic compounds of low molecular weight and with proteins (Henze and Umland, 1987). Kabata-Pendias and Pendias (1984) have compiled data on the Cu concentrations in... 

Typical mature roots have different shapes (conical, conical-cylindrical, cylindrical, fusiform) and different sizes (3 to 15 centimeters in diameter), depending on variety, age and growth conditions. The color of the outer peel varies from white to dark brown. The cross-section of cassava roots shows the two major components which are the peel and the central pith (Figure 12.1). The peel is composed of the outer layer (called the periderm) and the inner layer (called the cortical region or cortex), which contains sclerenchyma, cortical parenchyma and phloem tissue. The large central pith of the roots is the starch-reserve flesh, comprised of cambium and parenchyma tissue and xylem vessels. 

The movement of water and nutrients from the soil to the upper portions of a plant occurs primarily in the xylem. The xylem sap usually contains about 10 mol m-3 (10 mM)2 inorganic nutrients plus organic forms of nitrogen that are metabolically produced in the root. The xylem is a tissue of various cell types that we will consider in more detail in the final chapter (Section 9.4B,D), when water movement in plants is discussed quantitatively. The conducting cells in the xylem are the narrow, elongated tracheids and the vessel members (also called vessel elements), which tend to be shorter and wider than the tracheids. Vessel members are joined end-to-end in long... 

The intermolecular attraction between like molecules in the liquid state, such as the water-water attraction based on hydrogen bonds, is called cohesion. The attractive interaction between a liquid and a solid phase, such as water and the walls of a glass capillary (a cylindrical tube with a small internal diameter), is called adhesion. When the water-wall adhesion is appreciable compared with the water-water cohesion, the walls are said to be wettable, and water then rises in such a vertical capillary. At the opposite extreme, when the intermolecular cohesive forces within the liquid are substantially greater than is the adhesion between the liquid and the wall material, the upper level of the liquid in such a capillary is lower than the surface of the solution. Capillary depression occurs for liquid mercury in glass capillaries. For water in glass capillaries or in xylem vessels, the... 

Although Equation 2.2 refers to the height of capillary rise only in a static sense, it still has important implications concerning the movement of water in plants. To be specific, let us consider a xylem vessel having a lumen radius of 20 pm. From Equation 2.2b, we calculate that water will rise in it to the following height ... 

Such a capillary rise would account for the extent of the ascent of water in small plants, although it says nothing about the rate of such movement (to be considered in Chapter 9, Section 9.4D). For water to reach the top of a 30-m-tall tree by capillary action, however, the vessel would have to be 0.5 pin in radius. This is much smaller than observed for xylem vessels, indicating that capillary rise in channels of the size of xylem cells cannot account for the extent of the water rise in tall trees. Furthermore, the lumens of the xylem vessels are not open to air at the upper end, and thus they are not really analogous to the capillary depicted in Figure 2-3. 

The numerous interstices in the cell walls of xylem vessels form a mesh-work of many small, tortuous capillaries, which can lead to an extensive capillary rise of water in a tree. A representative radius for these channels in the cell wall might be 5 nm. According to Equation 2.2b, a capillary of 5 nm radius could support a water column of 3 km—far in excess of the needs of any plant. The cell wall could thus act as a very effective wick for water rise in its numerous small interstices, although such movement up a tree is generally too slow to replace the water lost by transpiration. 

Because of the appreciable water-wall attraction that can develop both at the top of a xylem vessel and in the numerous interstices of its cell walls, water already present in the lumen of a xylem vessel can be sustained or supported at great heights. The upward force, transmitted to the rest of the solution in the xylem vessel by water-water cohesion, overcomes the gravitational pull downward. The key to sustaining water already present in the xylem vessel against the pull of gravity is the very potent attractive... 

Besides vessel members and tracheids, parenchyma cells and fibers also occur in the xylem (see Fig. 1-3). Xylem fibers, which contribute to the structural support of a plant, are long thin cells with lignified cell walls they are generally devoid of protoplasts at maturity but are nonconducting. The living parenchyma cells in the xylem are important for the storage of carbohydrates and for the lateral movement of water and solutes into and out of the conducting cells. 

For application of Poiseuille s law to a complex tissue such as the xylem, care must be taken to ensure that particular vessel elements or tracheids are conducting (e.g., not blocked by embolisms), the actual radii must be determined (note the r4 dependence iiiEq. 9.11a), and corrections may be necessary for lumen shape, tracheid taper, and cell wall characteristics including pits (Calkin et al., 1986 Schulte et al., 1989a). For instance, if the lumen is elliptical with major and minor axes of a and b, respectively, then r4 in Equation 9.11a should be replaced by a3b3/ (8a2 + 8b2). 

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