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

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8.3 Water Permeability in CM and MX 247 crystalline wax layers, which are separated by layers of cutin. A wax monolayer composed of a C40 hydrocarbon is 5 nm thick and has a mass of about 0.5µg cm −2 . Total wax in Citrus CM is about 12µg cm −2 , and about one third of it is crystalline, i.e. 4µg cm −2 . This would be enough for eight superimposed wax monolayers. It is not at all obvious how they could be accommodated in cutin as large continuous platelets. Waxes also occur in cuticular layers and as surface waxes (Chap. 1). There is good evidence that waxes in the cuticular layers have very little effect on water permeance and solute mobility (Chaps. 4 and 6). When discussing effects of waxes on water permeability (Sect. 4.6), some evidence was discussed showing that one or two monolayers on top of the limiting layer could account for the effect of waxes on water permeability. A similar effect on solute mobility of superficial wax monolayers cannot be ruled out. 8.3 Water Permeability in CM and MX In the previous sections, we have taken a look at the thermodynamics of sorption and diffusion of lipophilic solutes. For water, there are no studies of sorption or diffusion in cuticles as affected by temperature. Only permeance has been studied. Permeance depends on partition coefficients (sorption) and diffusion coefficients (2.18). This is repeated here for convenience: Pℓ = P = DK (8.14) The effect of temperature on Pwv or Pwv can be analysed using Arrhenius plots such as Pwv = Po exp(−EP/RT) (8.15) where Po is the pre-exponential factor and EP is the Arrhenius activation energy of permeability or permeance. As long as EP is constant and sorption isotherms are linear, we can further write D = Do exp(−ED/RT) (8.16) which is identical to (8.6), and for the partition coefficients we can write K wv = Ko exp(−∆HS/RT) (8.17) For a complete analysis of the effects of temperature on water permeability at least two of the above energies of activation are needed, and the remaining one can be calculated using the relationship EP = ED + ∆HS (8.18) where ∆HS is the heat of sorption as explained in Sect. 8.1. EP has been determined for cuticles of a number of species. Later we shall take a look at the
248 8 Effects of Temperature on Sorption and Diffusion of Solutes and Penetration of Water relationship between EP, ED and ∆HS as determined with homogeneous synthetic polymer membranes. Using the system buffer/CM/buffer and tritiated water, Pw was measured at various temperatures (Schönherr et al. 1979). Water permeance of CM increased with temperature, and Arrhenius plots ln Pw vs 1/T had two linear portions which intersected at about 44 ◦ C. EP was 52kJ mol −1 (5–40 ◦ C) and 185kJ mol −1 (50–65 ◦ C) respectively. The plot for MX was slightly convex to the abscissa, and Pw was much higher. The differences between the plots for CM and MX decrease with increasing temperature. If the straight line for the CM is extrapolated to 72 ◦ C, the plots for CM and MX converge (Fig. 8.6). The melting range of Citrus wax is 72–77 ◦ C (Reynhardt and Riederer 1991), and when waxes are molten and fluid the waxy transport barrier breaks down completely. Above 44 ◦ C the slope of the Arrhenius plot for the CM suddenly increases, and a kink is clearly visible. There is no discontinuity or kink in the Arrhenius plot for the MX. Similarly, the Arrhenius plot for solute mobility in CM of Citrus (IAA) is linear up to 50. With Hedera CM/bifenox, linearity was maintained up to 75 ◦ C. The kink or bend in the Arrhenius plot occurred only with Pw and CM, but not with Pw for MX or with solute mobility in CM. ln P w (m/s) -7 -8 -9 -10 -11 -12 -13 -14 -15 -16 -17 -18 72 60 50 40 30 21 13 5 Hedera k ∗ (bifenox) Citrus CM (P w) Temperature (∞C ) 2.8 2.9 3.0 3.1 3.2 3.3 3.4 3.5 3.6 3.7 1/T x 1000 (Kelvin -1 ) Citrus MX (P w) Citrus k ∗ (IAA) Fig. 8.6 Arrhenius plots showing the effects of temperature on water permeance (Pw) of Citrus aurantium CM and MX. The membranes separated aqueous buffers at pH 6 containing CaCl2 (0.01mol l −1 ) and water fluxes were measured using tritiated water. For comparison rate constants (k ∗ ) determined with Hedera/bifenox and Citrus IAA were included. Water permeance data were taken from Schönherr et al. (1979). Rate constants were taken from Fig. 8.3 -8 -10 -12 -14 -16 -18 ln Rate constant k ∗ (s -1 )
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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 206 and 207: 196 6 Diffusion of Non-Electrolytes
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Page 242 and 243: Chapter 8 Effects of Temperature on
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Page 248 and 249: 8.2 Solute Mobility in Cuticles 239
Page 258 and 259: 8.3 Water Permeability in CM and MX
Page 260 and 261: 8.4 Thermal Expansion of CM, MX, Cu
Page 262 and 263: 8.5 Water Permeability of Synthetic
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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
Page 286 and 287: 278 Appendix A Symbols (continued)
Page 288 and 289: 280 Appendix B Physico-chemical pro
Page 290 and 291: Appendix C Index of Plants Botanica
Page 292 and 293: References Abraham MH, McGowan JC (
Page 294 and 295: References 287 Doty PM, Aiken WH, M
Page 296 and 297: References 289 Kolattukudy P (2004)
Page 298 and 299: References 291 Sabljic A, Güsten H
Page 300 and 301: References 293 Schreiber L, Schönh
Page 302 and 303: Index 2,4-D, 46 2,4-Dichlorophenoxy
Page 304 and 305: Index 297 leaf disks, 130 leaf surf
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Index 299 water molar volume, 110 p
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