Xylem water flow

A non-invasive measurement of phloem and xylem water flow in castor bean seedlings by nuclear magnetic resonance microimaging), Planta 201, 53-63. 

Before discussing the characteristics of flow in the xylem, we will briefly review some of its anatomical features [see Chapter 1, Section 1.1C (e.g., Fig. 1-3) for an introduction to the xylem]. In general, the conducting xylem elements have thick, lignified secondary cell walls and contain no protoplasts. Indeed, the xylem cells serve their special function of providing the plant with a low-resistance conduit for water flow only when they are dead Because these conducting cells are essentially membraneless hollow pipes, water in... 

In fact, membranes generally serve as the main barrier to water flow into or out of plant cells. The interstices of the cell walls provide a much easier pathway for such flow, and hollow xylem vessels present the least impediment to flow (such as up a stem). Consequently, the xylem provides a plant with tubes, or conduits, that are remarkably well suited for moving water over long distances. The region of a plant made up of cell walls and the hollow xylem vessels is often called the apoplast, as noted above (Chapter 1, Section 1.1D and in Section 9.4A). Water and the solutes that it contains can move fairly readily in the apoplast, but they must cross a membrane to enter the symplast (symplasm), the interconnected cytoplasm of the cells. 

Although Equation 9.12 can be used to describe certain overall characteristics of water flow in the soil-plant-atmosphere continuum, A F does not always represent the driving force on water. For instance, a change in the osmotic pressure component of has no direct effect on the flow along the xylem or the phloem. Also, such a relation is not useful for a gas phase because the resistance IV then depends on the concentration of water vapor (see Chapter 8, Section 8.IF). 

S. Sargnkool Na Ayutthaya, J. Junjittakarn, F. C. Do, K. Pannengpetch, J. L. Maeght, A. Rocheteau and D. Nandris, Drought and trunk phloem necrosis (TPN) effects on water status and xylem sap flow of Hevea brasiliensis, RRDB International Natural Rubber tree latex, Siam Reap, Cambodia, 2007, p. 75 4. 

Evidence was obtained recently that pesticide vapors may enter the air by still another mechanism, involving plant circulation and water loss (57). Rice plants were found to efficiently transport root-zone applied systemic carbamate insecticides via xylem flow to the leaves, eventually to the leaf surface by the processes of guttation and/or stomatal transpiration, and finally to the air by surface volatilization. Results from a model chamber showed that 4.2, 5.8, and 5.7% of the residues of carbaryl, carbofuran, and aldicarb, respectively, present in rice plants after root soaking vaporized within 10 days after treatment. The major process was evaporation of surface residues deposited by guttation fluid. 

In contrast, a dPIdx of only -0.02 MPa m-1 is needed for the same Jv in the xylem element with a 20-pm radius. Thus, the dPIdx for Poiseuille flow through the small interstices of a cell wall is over 107 times greater than that for the same flux density through the lumen of the xylem element. Because of the tremendous pressure gradients required to force water through the small interstices available for solution conduction in the cell wall, fluid cannot flow rapidly enough up a tree in its cell walls — as has been suggested — to account for the observed rates of water movement. 

Water is conducted to and across the leaves in the xylem. It then moves to the individual leaf cells by flowing partly apoplastically in the cell walls and partly symplastically (only short distances are involved, because the xylem ramifies extensively in a leaf). The water potential is usually about the same in the vacuole, the cytosol, and the cell wall of a particular mesophyll cell (see values in Table 9-3). If this were not the case, water would redistribute by flowing energetically downhill toward lower water potentials. The water in the cell wall pores is in contact with air, where evaporation can take place, leading to a flow along the cell wall interstices to replace the lost water. This flow can be approximately described by Poiseuille s law (Eq. 9.11), which indicates that a (very small) hydrostatic pressure decrease exists across such cell walls. 

The resistance for water movement along the stem can be separated into (1) a quantity expressing some inherent flow properties of the xylem and (2) the geometrical aspects of the conduits. By analogy with Ohm s law, where the resistance R equals pAx/A (Chapter 3, Section 3.2), we obtain... 

Continuity of Crude Sap Flow.—The crude sap (water with mineral salts in solution) penetrates the thin walls of the root hairs by osmosis and passes into the interior of hairs, thence into the root xylemand through this to stem xylem, thence through stem xylem into the leaves. 

The properties of water are different from those of sodium chloride and carbon dioxide. Water is the only one of the three compounds that occurs in Earth s environment in aU three states of matter, as shown in Figure 4.7. At sea level, liquid water boils into gaseous water (steam) at 100°C and freezes to solid water (ice) at 0°C. Pure water does not conduct electricity in any of its states. Water is also excellent at dissolving other substances. It is often called the universal solvent in recognition of this valuable property. Water plays a vital role in the transport of dissolved materials, whether the aqueous solution is flowing down a river up the xylem in a tree or through the veins, capillaries, and arteries of your circulatory system. 

The capillary and porous system of the body exists in vascular tissue and intercellular spaces. Xylem forms an open conduit of relatively low hydraulic resistance that is filled with diluted mineral solution. Phloem exists in cells with a width ranging from 10 to 70 pm and a length from 100 to 500 pm in dicotyledons [4]. Their turgor is around 2 MPa (beetroot is 1.83 MPa) with a pressure gradient of 0.02-0.03 MPa/m [5,6]. As phloem transports substances of very different molecular weight, shape, charge, and surface activity along with water, it is presumed that the mechanism is an osmotically driven solution flow [6]. 

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