Cell wall water potential

Plant cells come into contact with air where the cell walls are adjacent to the intercellular air spaces (see Fig. 1-2). Thus, the water potential in the cell walls must be considered with respect to T 1W in the adjacent gas phase. The main contributing term for T in cell wall water is usually the negative hydrostatic pressure arising from surface tension at the numerous ail-liquid interfaces of the cell wall interstices near the cell surface. In turn, Z 11 wal1 can be related to the geometry of the cell wall pores and the contact angles. 

Any procedure for wood specific gravity or density based on measuring weight and volume can be considered accurate only if the wood sample has been first extracted with suitable organic solvents to remove extraneous resins, oils, fats, gums, etc. (27). These materials bulk cell walls, block potential sites for water adsorption/ab-sorption, alter potential wood swelling/shrinkage, and thereby interfere with the accurate characterization of wood tissue. 

Cell enlargement occurs when a demand for water is created by relaxation of the cell walls under the influence of turgor pressure and wall-loosening factors. Water enters the cell down a water potential gradient, extending the cell walls (Lockhart, 1965 Boyer, 1985 Tomos, 1985). 

Bozarth, C.S., Mullet, J.E. Boyer, J.S. (1987). Cell wall proteins at low water potentials. Plant Physiology, 85, 261-7. 

While the lipid bilayer has a very low water content, and therefore behaves quite hydrophobically, especially in its core (see Chapter 2 of this volume), the cell wall is rather hydrophilic, with some 90% of water. Physicochemically, the cell wall is particularly relevant because of its high ion binding capacity and the ensuing impact on the biointerphasial electric double layer. Due to the presence of such an electric double layer, the cell wall possesses Donnan-like features, leaving only a limited part of the interphasial potential decay in the diffuse double layer in the adjacent medium. For a detailed outline, the reader is referred to recent overviews of the subject matter [1,2]. 

Blue-green algae (cyanobacteria) can occur in surface water bodies used for water supply Some species of cyanobacteria contain toxins of concern to human health (e.g. microcystins), and these can be released when algal cell walls are ruptured, There is a wide range of potential toxins and it appears that not all of the possible toxins have been identified. 

Because of their rigid cell walls, large hydrostatic pressures can exist in plant cells, whereas hydrostatic pressures in animal cells generally are relatively small. Hydrostatic pressures are involved in plant support and also are important for the movement of water and solutes in the xylem and in the phloem. The effect of pressure on the chemical potential of water is expressed by the term VWP (see Eq. 2.4), where Vw is the partial molal volume of water and P is the hydrostatic pressure in the aqueous solution in excess of the ambient atmospheric pressure. The density of water is about 1000 kg m-3 (1 g cm-3) therefore, when 1 mol or 18.0 x 10-3 kg of water is added to water, the volume increases by 18.0 x 10-6 m3. Using the definition ofV,., as a partial derivative (see Eq. 2.6), we need to add only an infinitesimally small amount of water (dnw) and then observe the infinitesimal change in volume of the system (dV). We thus find that Vw for pure water is 18.0 x 10-6 m3 mol-1 (18.0 cm3 mol-1). Although Vw can be influenced by the solutes present, it is generally close to 18.0 x 10-6 m3 mol-1 for a dilute solution, a value that we will use for calculations in this book. 

A loss of water from plant shoots—indeed, sometimes even an uptake — occurs at cell-air interfaces. As we would expect, the chemical potential of water in cells compared with that in the adjacent air determines the direction for net water movement at such locations. Thus we must obtain an expression for the water potential in a vapor phase and then relate this P to for the liquid phases in a cell. We will specifically consider the factors influencing the water potential at the plant cell-air interface, namely, in the cell wall. We will find that vFcel1 wal1 is dominated by a negative hydrostatic pressure resulting from surface tension effects in the cell wall pores. 

When Equation 2.26 is applied to cells, Y° is the water potential in the external solution, and Y1 usually refers to the water potential in the vacuole. Lw then indicates the conductivity for water flow across the cell wall, the plasma membrane, and the tonoplast, all in series. For a group of barriers in series, the overall water conductivity coefficient of the pathway, L,., is related... 

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