Cell wall water relations

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. 

Pritchard, J., Tomos, A.D. Wyn Jones, R.G. (1987). Control of wheat root elongation growth. I. Effects of ions on growth rate, wall rheology and cell water relations. Journal of Experimental Botany, 38, 948-59. 

On this work the main changes that take place on three pectic fractions, water, imidazoIe/HCl and carbonate soluble polysaccharides, of olive cell wall are described and related with modifications of the fruit s texture. 

The surface wettability of heat-treated wood decreases due to a reduction in the hydroxyl content of the modified wood (Pdtrissans etal., 2003). There is a reduction in the water-sorption capacity, which is related to a reduction in the number of primary sorption sites (OH groups) within the wood cell wall, largely as a result of the removal/degradation of the hemicellulosic component. As remarked upon earlier, hygroscopic properties are strongly influenced by the treatment method employed. Podgorski etal. (2000) heated... 

Ethambutol is a water-soluble, heat-stable compound that acts by inhibition of arabinosyl transferase enzymes that are involved in cell wall biosynthesis. Nearly all strains of M tuberculosis and M. kansasii and most strains of Mycobacterium avium-intracellulare are sensitive to ethambutol. Drug resistance relates to point mutations in the gene (EmbB) that encodes the arabinosyl transferases that are involved in mycobacterial cell wall synthesis. 

The role of LCC s in living plant tissues is presumed to be related to the prevention of water-soluble hemicelluloses from dissolving out of the cell wall by the formation of micelles or aggregates that immobilize sugar chains, and the solubilization of water-insoluble material such as lignin, thereby enabling it to move to any place in the cell. 

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... 

The rate of water vapor diffusion per unit leaf area, Jw> equals the difference in water vapor concentration multiplied by the conductance across which Acm occurs (// = g/Ac - Eq. 8.2). In the steady state (Chapter 3, Section 3.2B), when the flux density of water vapor and the conductance of each component are constant with time, this relation holds both for the overall pathway and for any individual segment of it. Because some water evaporates from the cell walls of mesophyll cells along the pathway within the leaf, is actually not spatially constant in the intercellular airspaces. For simplicity, however, we generally assume that Jm, is unchanging from the mesophyll cell walls out to the turbulent air outside a leaf. When water vapor moves out only across the lower epidermis of the leaf and when cuticular transpiration is negligible, we obtain the following relations in the... 

Therefore, the water-dependent glass transition of the cell walls is a critical parameter in predicting the expanded structure, and this must be related to the material at the die exit, not the properties of the starting material. Similar arguments relating expansion directly to the moisture content dependence of extrudate Tg have been presented by Donald et al. (1993). 

Protoplasm is in intimate relation to water. The reaction of the cytoplasm to a bounding film of water between it and the cell wall forms the outer plasma membrane or ectoplasm, a clear homogeneous outer band of cytoplasm the reaction of cytoplasm to the water... 

The water content of the wood cell wall has a strong influence on the wood s mechanical properties, and a higher moisture content, at least below the fiber saturation point, and normally is inversely related to most strength properties see Chapter 5). This situation is easily reconciled if one considers that the takeup of water below the... 

Wood strength is related to the amount of water in the wood fiber cell wall (7-JO). At moisture contents from oven-dry (OD) to the fiber-saturation point, water accumulates in the wood cell wall... 

The amount of moisture adsorbed by the cell walls is directly related to the humidity of the surrounding air. Relative humidity or relative vapour pressure is defined as the ratio of the amount of water vapour in the atmosphere to the amount... 

Wood only shrinks when water is lost from the cell walls and it shrinks by an amount that is proportional to the moisture lost below fibre saturation point. To a first approximation the volumetric shrinkage is proportional to the number of water molecules that are adsorbed within the cell wall, and that in turn is related to the number of accessible hydroxyls on the cellulose, hemicelluloses and lignin, and to the amount of cell wall material, i.e. the basic density of the wood (Figure 4.2). 

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