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

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200 6 Diffusion of Non-Electrolytes Table 6.11 Slopes β ′ , y-intercepts D0 and coefficients of determination r 2 of the regression equations fitted to plots of log Pcm/Kww and Pleaf/Kww vs Vx. Path length of diffusion ℓcalc are calculated by dividing D0 obtained in wax (Table 6.10) by D0 measured for CM and leaf respectively. Path length of diffusion ℓmeas are calculated from wax coverage determined for the same set of cuticles used in transport experiments (Kirsch et al. 1997) Species β ′ (mol cm −3 ) D0 × 10 10 r 2 ℓcalc(nm) ℓmeas(nm) ℓcalc/ℓmeas (m 2 s −1 ) Prunus laurocerasus −0.0074 1.93 0.93 839 1,600 0.52 CM Prunus laurocerasus −0.012 5.51 0.99 294 1,600 0.18 Leaf Ginkgo biloba CM −0.0095 13.01 0.94 164 210 0.78 Ginkgo biloba Leaf −0.010 20.20 0.94 106 210 0.50 Juglans regia CM −0.013 30.13 0.97 49 570 0.09 Juglans regia Leaf −0.011 13.14 0.93 113 570 0.20 coverage of the CM (Table 6.11). However, it is obvious that path length of diffusion ℓmeas, obtained from total wax coverage is always higher than ℓcalc. This could indicate that only a fraction of the total wax (0.1–0.8) contributes to the limiting barrier, while the remainder is deposited as intracuticular wax in cuticular layer(s). Results of diffusion experiments in reconstituted wax agree fairly well with results of transport experiments using isolated cuticles and leaf disks. Size selectivities are comparable, and reasonable values for the thickness of the transport limiting barrier of the CM are obtained. This justifies the following conclusions: (1) The transport-limiting barrier of the CM for lipophilic molecules is formed by cuticular waxes deposited in/on the limiting skin, (2) water penetrates cuticles using two parallel pathways, the waxy pathway and aqueous pores (Chap. 4), and (3) penetration of ionic compounds is restricted to aqueous pores, and the waxy pathway cannot be accessed (Chap. 5). Several micrometres away from the living epidermal cell, wax molecules spontaneously arrange themselves, which leads to the formation of an efficient transport barrier. Cuticular waxes deposited at the outer surface of the CM follow the rules of self-organisation. When wax is reconstituted on an artificial surface, barrier properties of reconstituted waxes are very similar as those in isolated CM and intact leaves. Sorption and diffusion in waxes can give valuable insights into the structure and function of the cuticular transport barrier. This experimental approach has been used to analyse the effect of plasticisers on solute diffusion in CM and waxes. This is the topic of Chap. 7.
Problems 201 Problems 1. The mean log Kcw of PCP is 4.56 and the mean log Kww is 3.55 (Table 6.1). How much (mol) of PCP is sorbed in 1 mg CM and in 100µg waxes after equilibration in a large volume of an external aqueous PCP solution of 1mg l −1 ? What is the ratio of the amounts of PCP in CM and wax? 2. At 25 ◦ C, water solubility of PCP is 20mg l −1 and the mean Kow is 11,749 (Table 6.1). Calculate Kcw from Kow (6.2) and from water solubility (6.3). 3. Let the Kcw of two solutes be 30 or 0.03. Pieces of 1 mg CM are equilibrated aqueous solutions of 1mg l −1 . How much is sorbed in the CM, and which amount is contained in a thin water film of 0.3 mg which remains on the CM after equilibration. How much of the solutes is sorbed in the CM and in the water film? 4. A piece of CM having a mass of 1 mg is equilibrated in 1ml (=1g) aqueous buffer which contains 20mg kg −1 non-dissociated 2,4-D. Calculate the final amounts of 2,4-D in the CM and in the solution. What is the final amount of PCP in the cuticle and in the solution, assuming the same experimental conditions? 5. In Fig. 6.2, hold-up times of up to 45 h are shown, while in Fig. 6.5 the plot amount penetrated vs time intersects the y-axis at a positive value. What is the reason? 6. Diffusion coefficients in leaf CM (Table 6.3) are much lower than in leaf MX membranes (1.3 × 10 −14 m 2 s −1 ). In Citrus this ratio is 241, while the effect of extraction on permeance was 1,767. Which other factors might have contributed to the very low P of the CM? 7. Using (6.7), data of Table 6.1 and an equivalent molar volume of 2,4-D of 138cm 3 mol −1 , calculate permeance for 2,4-D of Citrus CM. How good is this estimate? 8. With compartment sizes given in Table 6.5, calculate Mt/M0 for compartments 1, 2 and 3 after 180 min of desorption. Use (6.10). 9. Rates of penetration of lipophilic solutes into conifer needles are proportional to sorption in surface wax and cuticles (Fig. 6.9). How can this be explained? 10. When calculated using the specific surface area of the needles, permeance increased with partition coefficient of the solutes (Table 6.6). What does this imply? 11. Half-time of penetration from a sessile droplet depends on the volume of the droplet and the contact area between droplet and cuticle (6.12) You measured permeance using droplets having contact angles of 90 ◦ . In a second experiment you added a small amount of wetting agent, such that the droplet spread on the cuticle. Half-time was reduced by factor of 0.5. Did the wetting agent increase permeance of the cuticle and how would you test this? 12. How would you interpret the autoradiographs seen in Fig. 6.14? 13. The average diffusion coefficient calculated from a desorption experiment of 2,4-D in the inner sorption compartment of the Citrus CM is about 2 × 10 −15 m 2 s −1 . With a steady state penetration experiment, D of 5.4 ×
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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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148 6 Diffusion of Non-Electrolytes
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8.4 Thermal Expansion of CM, MX, Cu
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Problems 257 E D (kJ/mol) DH S (kJ/
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Chapter 9 General Methods, Sources
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9.2 Testing Integrity of Isolated C
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9.4 Distribution of Water and Solut
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9.6 Cutin and Wax Analysis and Prep
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9.7 Measuring Water Permeability 26
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9.8 Measuring Solute Permeability 2
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276 Appendix A Abbreviations (conti
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278 Appendix A Symbols (continued)
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280 Appendix B Physico-chemical pro
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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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