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

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198 6 Diffusion of Non-Electrolytes logD = −15.76 − 0.0076 ×Vx. (6.23) This equation can be very helpful when D is not easily measured due to experimental limitations. As mentioned above, solubility of small polar molecules, such as water, urea or glucose, is too low in the amorphous phase of reconstituted cuticular wax, and D cannot be determined experimentally. However, using (6.23), D of these compounds in wax can be estimated. For example, for water having a Vx of 18cm 3 mol −1 , a D of 1.23 × 10 −16 m 2 s −1 can be calculated. Size selectivities measured with UDOS experiments for a similar set of lipophilic molecules and CM isolated from 11 species ranged from −0.007 to −0.012 (Table 6.8). Mean size selectivity for CM was −0.0095 (Table 6.8), which is somewhat larger than mean size selectivity of isolated wax, which had a value of −0.0076 (6.23). Nevertheless, this comparison shows that size selectivities in reconstituted cuticular wax are in a similar range to that of size selectivity for cuticular membranes. This is important evidence that D values obtained in diffusion experiments or predicted from (6.23) are reliable since they are comparable to values obtained from isolated CM. Size selectivities measured for linear long-chain aliphatic molecules in reconstituted wax of Hordeum, Fagus and Picea were somewhat higher, ranging from −0.015 to −0.020 (Schreiber and Schönherr 1993b; Schreiber et al. 1996a). However, these experiments included a very different set of compounds, and radiolabelled probes were always added to the wax prior to reconstitution. Therefore, a direct comparison with the results obtained for lipophilic molecules as they are listed in Table 6.9 should not be made. 6.5.3 Relationship Between D and P The relationship between diffusion coefficients D (m 2 s −1 ) and permeances P (m s −1 ) is given by (2.18). This equation relates to a homogeneous barrier, and it is assumed that the cuticular transport barrier of wax represents a homogeneous phase. Equation (6.18) states that P can be calculated from D if the partition coefficient K and the thickness of the barrier ℓ are known. Instead of cuticle/water partition coefficients Kcw, wax/water partition coefficients Kww should be used here, since we deal with diffusion in a waxy barrier. Kww have been measured and are available (Table 6.2). The thickness of the waxy barrier in the limiting skin is used, rather than thickness of CM. This may be estimated from the total amount of wax deposited on the CM (Table 4.8) if we assume that all wax contributes to the formation of the barrier. Equation (2.18) can be rearranged to P ×ℓwax D = . (6.24) Kww Equation (6.24) states that D (m 2 s −1 ) and P/Kww(m s −1 ) should only differ by the thickness of the wax layer ℓwax (m). D (Table 6.9), Kww (Table 6.2) and P have been
6.5 Diffusion in Reconstituted Isolated Cuticular Waxes 199 log P cm /K ww (m/s) −10.0 −10.2 −10.4 −10.6 −10.8 −11.0 −11.2 BA 4-NP SA V x (cm 3 /mol) 2,4-D AT MET 90 100 110 120 130 140 150 160 170 Fig. 6.27 Logarithms of the ratios Pcm/Kww of lipophilic molecules in Prunus laurocerasus CM as a function of the molar volumes Vx of the molecules. Benzoic acid (BA), 4-nitrophenol (4-NP), salicylic acid (SA), 2,4-dichlorophenoxyacetic acid (2,4-D), atrazine (AT), metribuzin (MET). Data from Kirsch et al. (1997) measured for cuticles of Prunus, Ginko and Juglans (Kirsch et al. 1997). Isolated CM (Pcm) or leaf disks (Pleaf) as described in Sect. 6.2.3 were used. As shown for Prunus laurocerasus CM, a plot of Pcm/Kww vs. Vx gives a reasonable correlation (Fig. 6.27), as was also observed with diffusion in reconstituted Prunus laurocerasus wax (Fig. 6.26). Using (6.22), regression equations similar to those obtained for diffusion in reconstituted wax (Table 6.10), were derived for transport across isolated CM and leaf disks of the three species (Table 6.11). From these regressions, size selectivities ′ were obtained for the transport across CM cuticles and for non-isolated cuticles of leaf disks. Size selectivities ranged from −0.0074 to −0.013, and large differences between intact leaves and isolated CM were not observed. This is good evidence that isolation of the cuticle does not alter transport properties. The mean value for all β ′ in Table 6.10 is 0.0073. This is similar to the mean β ′ obtained with CM (Table 6.8). Cuticles appear to be very robust, since comparable results were obtained with four different experimental approaches: (1) diffusion in wax (Sect. 6.5.1), (2) UDOS (Sect. 6.3.2), (3) steady state penetration across the isolated cuticle (Sect. 6.2.1), and (4) steady state penetration into leaf disks (Sect. 6.2.3). Path length of diffusion ℓcalc can be calculated dividing D0 in wax (Table 6.10) by D0 obtained from transport experiments across CM and intact leaf surface (Table 6.11). Values obtained for the thickness of the transport-limiting wax barrier range from 50 nm (Juglans CM) to 800 nm (Prunus CM). These are reasonable values, since they are of the same order of magnitude as those determined from wax
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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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8.3 Water Permeability in CM and MX
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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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