Water and Solute Permeability of Plant Cuticles: Measurement and ...

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Chapter 8 Effects of Temperature on Sorption and Diffusion of Solutes and Penetration of Water Several processes are involved in penetration of a polymer membrane separating two aqueous solutions. Water and dissolved solutes leave the aqueous phase (donor) and enter the membrane phase in which they are sorbed. These sorbed molecules then diffuse in the membrane via a random hopping mechanism. Eventually they arrive at the opposite membrane interface facing the receiver. In cuticles, penetration is complicated, because they consist of more than one phase (cutin, waxes, polysaccharides, polypeptides and phenolic compounds) and they contain up to 8% water, part of which is clustered in aqueous pores. The polymer matrix is an ion exchanger, and concentration of counter ions depends on pH. Sorption, diffusion, swelling and ion exchange vary with temperature. In addition, swelling and phase transitions of membrane components can affect membrane structure and function. 8.1 Sorption from Aqueous Solutions Riederer and Schönherr (1986a) studied sorption of 4-nitrophenol at various concentrations in isolated cuticles (CM) and MX membranes at temperatures ranging from 5 to 50 ◦ C. The choice of the sorbate and types of cuticles was deliberate. 4-nitrophenol (4-NP) is a weak acid having a molecular weight of 139g mol −1 and a pKa of 7.25. It is a planar molecule with two permanent dipoles. The dipole moment of the C- + OH group is 1.65 Debye units, while the C + -NO2 function has 3.1–3.8 Debye units. These polar groups can form hydrogen bonds with water and with dipoles of the membranes. Van der Waals forces related to the aromatic ring affect interactions between 4-NP, water and polymer chains. The permanent dipoles are attached to the phenol ring in para position, and since sorption was studied at pH3, ionisation of the acidic hydroxyl group of 4-NP was suppressed. Thus, 4-NP was neutral, and interactions with positive fixed charges of the MX did not occur (Sects. 4.2 and 4.3). The non-ionised 4-NP is sufficiently water soluble to permit sorption to be studied at concentrations ranging from 0.1 to 10 −6 mol l −1 . L. Schreiber and J. Schönherr, Water and Solute Permeability of Plant Cuticles. © Springer-Verlag Berlin Heidelberg 2009 233
234 8 Effects of Temperature on Sorption and Diffusion of Solutes and Penetration of Water Tomato fruit cuticles (Lycopersicon esculentum) contain 7% waxes and 62% cutin. Polar polymers amount to 24% (Table 1.1). The ester cutin is of the C16 type, and dihydroxyfatty acids are the major constituents. Adaxial rubber leaf cuticles (Ficus decora) are composed of 56% cutin, 19% polar polymers and 25% waxes (Table 1.1). The cutin contains C16 and C18 hydroxyfatty acids, and 18-hydroxy- 9,10-epoxystearic acid is a major constituent, which results in the formation of non-ester cutin (cutan) similarly to what happens in Clivia cutin (Fig. 1.2). 8.1.1 Sorption Isotherms and Partition Coefficients Small pieces of cuticular material were equilibrated at constant temperature as described in Sect. 6.1. At equilibrium, the molal 4-NP concentrations (mol kg −1 ) in the aqueous phase (Caqueous) and in the CM or MX (internal phase Cinternal) were determined. The partition coefficients K were calculated as the ratio of the molal concentrations Cinternal/Caqueous. When 4-nitrophenol concentration in the cuticle (Cinternal) was plotted against concentration in water (Caqueous), linear sorption isotherms were obtained at all temperatures and concentrations of 10 −3 –10 −6 mol kg −1 . Alternatively, the natural logarithms of these concentrations can be plotted. The slopes of these plots (k) are related to the intensity of sorption and the number of sorption sites. The parameter n characterises the slope of the isotherm. The linear portions of the plots fit the Freundlich isotherm or in logarithmic form Cinternal = kC 1/n aqueous logCinternal = logk+ 1 n logCaqueous (8.1) (8.1a) Freundlich isotherms are frequently encountered when solutes interact with heterogeneous substrates. At low concentrations the parameter n is not far from 1.0, but at higher sorbate concentrations the plots become increasingly convex to the Caqueous axis. At low sorbate concentrations this type of isotherm can be classified as constant partitioning, while at higher concentrations (>10 −3 mol kg −1 ) it resembles a Langmuir type isotherm. Linear isotherms are obtained under the following conditions: (1) the substrate consists of flexible molecules and has regions of differing accessibility to the sorbate, (2) the solute has a higher affinity to the substrate than to the solvent water, and (3) the solute is able to penetrate into initially less accessible (highly ordered) regions of the solid. At all concentrations and temperatures, partition coefficients (K) and the Freundlich parameter k were >1. K ranged from about 15 to 250 (Fig. 8.1), and k varied between 14 and 190 (Riederer and Schönherr 1986a); that is, 4-NP is better soluble in CM and MX than in water. At low internal concentration, K is greater with the MX than with the CM. The only lipid compartment of the MX is cutin,
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Water and Solute Permeability of Pl
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Professor Dr. Lukas Schreiber Ecoph
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vi Preface This is not a review abo
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Contents 1 Chemistry and Structure
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Contents xi 4.6.3 Diffusion Coeffic
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Contents xiii 7.4.2 Effects of Plas
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2 1 Chemistry and Structure of Cuti
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10 1 Chemistry and Structure of Cut
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Chapter 2 Quantitative Description
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2.1 Models for Analysing Mass Trans
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2.3 Steady State Diffusion Across a
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2.4 Steady State Diffusion of a Sol
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2.4 Steady State Diffusion of a Sol
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2.5 Diffusion Across a Membrane wit
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2.5 Diffusion Across a Membrane wit
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2.6 Determination of the Diffusion
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Solutions 51 2.7 Summary In this ch
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Chapter 3 Permeance, Diffusion and
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3.1 Units of Permeability 55 3.1.1
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3.1 Units of Permeability 57 identi
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3.3 Partition Coefficients 59 The d
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Chapter 4 Water Permeability All te
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4.1 Water Permeability of Synthetic
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4.2 Isoelectric Points of Polymer M
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4.2 Isoelectric Points of Polymer M
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4.3 Ion Exchange Capacity 69 The hi
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4.3 Ion Exchange Capacity 71 has an
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4.3 Ion Exchange Capacity 73 Counte
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4.4 Water Vapour Sorption and Perme
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4.4 Water Vapour Sorption and Perme
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4.5 Diffusion and Viscous Transport
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4.6 Water Permeability of Isolated
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Problems 121 Our present knowledge
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Solutions 123 5. Ca 2+ , because se
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126 5 Penetration of Ionic Solutes
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146 6 Diffusion of Non-Electrolytes
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Page 192 and 193: 182 6 Diffusion of Non-Electrolytes
Page 216 and 217: 206 7 Accelerators Increase Solute
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Page 248 and 249: 8.2 Solute Mobility in Cuticles 239
Page 256 and 257: 8.3 Water Permeability in CM and MX
Page 258 and 259: 8.3 Water Permeability in CM and MX
Page 260 and 261: 8.4 Thermal Expansion of CM, MX, Cu
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Page 266 and 267: Problems 257 E D (kJ/mol) DH S (kJ/
Page 268 and 269: Chapter 9 General Methods, Sources
Page 270 and 271: 9.2 Testing Integrity of Isolated C
Page 272 and 273: 9.4 Distribution of Water and Solut
Page 274 and 275: 9.6 Cutin and Wax Analysis and Prep
Page 276 and 277: 9.7 Measuring Water Permeability 26
Page 278 and 279: 9.8 Measuring Solute Permeability 2
Page 284 and 285: 276 Appendix A Abbreviations (conti
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Page 288 and 289: 280 Appendix B Physico-chemical pro
Page 290 and 291: Appendix C Index of Plants Botanica
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References Abraham MH, McGowan JC (
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References 287 Doty PM, Aiken WH, M
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References 289 Kolattukudy P (2004)
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References 291 Sabljic A, Güsten H
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References 293 Schreiber L, Schönh
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Index 2,4-D, 46 2,4-Dichlorophenoxy
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Index 297 leaf disks, 130 leaf surf
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Index 299 water molar volume, 110 p
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