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

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158 6 Diffusion of Non-Electrolytes concentrations of non-ionised molecules. The partition coefficients CM/water for the non-ionised species varied from 83 (4-nitrophenol) to 7.24 × 10 7 (diethylhexyl phthalate), which is a factor of almost one million. It is not trivial to measure extremely high partition coefficients (Sect. 6.1) and permeances. With hexachlorobenzene (HCB), perylene and diethylhexyl phthalate (DEHP), Kerler and Schönherr (1988b) measured steady state diffusion using solid residues of the solutes in the donor to prevent rapid depletion in the donor solutions. When log P was plotted vs log Kcw, a positive dependence could be seen (red circles). From 4-nitrophenol to DEHP, permeance increased by a factor of nearly 2,000. This is considerably less than the difference in partition coefficients. One of the reasons for this is the fact that molecular weights of the compounds were not constant but ranged from 139 (4-nitrophenol) to 390g mol −1 (DEHP), and diffusion coefficients in CM and waxes greatly depend on size of solutes. This will be dealt with quantitatively in Sect. 6.3. There are various ways to estimate the volumes of molecules. With liquids it is not difficult to determine the volume of one mole of substance, but these figures greatly depend on temperature, and it is not possible to obtain good estimates for solids. We have been using the method of Abraham and McGowan (1987) to characterise size of solutes. Molar volumes (in cm 3 mol −1 ) are obtained by adding the volumes of the atoms and correcting for the influence of bonds on molar volumes. Volumes of all relevant atoms have been tabulated, and the molar volumes (Vx) calculated are characteristic volumes at zero Kelvin. For this reason characteristic volumes are smaller than molecular weights, but for the purpose of comparing permeances or diffusion coefficients of compounds having different molecular weights, Vx turned out to be well-suited. Dividing log Kcw by Vx corrects for differences in size of solutes, and a good correlation with log Pcw/Vx is obtained (green squares in Fig. 6.3) if data for DEHP and HCB were disregarded. The fact that permeances for DEHP and HCB are much higher than predicted by the regression line is probably related to their plasticising activity (Chap. 7). Plasticisers render solid polymers more flexible and increase solute mobility (Buchholz 2006). DEHP is a typical plasticiser used commercially in synthetic polymers. Some ethoxylated alcohols are very effective plasticisers (Schönherr 1993a, b). Pentafluorophenol is also a very effective plasticiser (Schönherr and Baur 1996). Plasticising activities of PCP and HCB seem not to have been studied, but in view of structural similarity with pentafluorophenol it is likely that they are plasticisers as well. The effect of plasticisers is concentration-dependent, but concentration of solutes in the CM cannot be calculated, as donor concentrations were not given in the original publication. Their concentration in the CM probably differed, and their intrinsic activity probably as well. It should be realised that plasticiser activity of solutes would have gone unnoticed had penetration been measured using only one time interval, and results would have been expressed as percent penetration in an arbitrary time interval. Prediction of permeances of Citrus CM at 25 ◦ C to other solutes is possible using the equation logP = 192 logKcw Vx − 13.2. (6.7)
6.2 Steady State Penetration 159 Equation (6.7) returns permeances (in m s −1 ) of Citrus CM for lipophilic solutes which are inert, that is solutes which have no plasticiser activity. The dimension of Vx is cm 3 mol −1 , and with the solutes used it ranged from 95 (4-nitrophenol) to 340 (DEHP). The equation is valid over a very wide range of partition coefficients. Since Kcw of neutral solutes varies much more than molar volumes of solutes, it is more important than size in determining permeance. Since log Kcw can be calculated from octanol/water partition coefficients (Sect. 6.1) which have been tabulated or can be calculated using various methods, prediction of permeance of Citrus CM is possible with good precision without having to conduct experiments. Equation (6.7) should not be used with compounds that are better soluble in water than in cutin (K < 1). Such polar neutral solutes (i.e., urea, glycerol, glucose) diffuse in the water of the polar cuticular polymer fraction. There are no reliable data on permeances of small neutral polar solutes, but based on the arguments of Sect. 4.4 it is not likely that valid estimates of using partition coefficients and (6.7) can be obtained. Likewise, prediction of permeances of ionic solutes (inorganic salts or organic electrolytes such as glyphosate) is not possible (Chap. 5). The steady state flux (J) of a solute across cuticles is proportional to permeance and driving force, that is, solute concentration of the donor (6.6). Fluxes of different compounds are proportional to Kcw provided Vx and driving force are the same. In this case, very lipophilic solutes penetrate faster than slightly lipophilic ones. However, rates of penetration must not be confused with the amounts that penetrate. Amounts are limited by the dose contained in the donor (Cdonor ×Vdonor), and given sufficient time the total dose will penetrate (see Sect. 6.3). It is of practical relevance that Cdonor cannot be varied arbitrarily, because aqueous solubility (Swater) of very lipophilic solutes can be extremely low. In fact, high partition coefficients are not the result of high lipid solubility but rather of low water solubility, and log Kcw is proportional to −log Swater. With some simplifying assumptions, Schönherr and Riederer (1989) derived the (6.3) which is repeated here for convenience: logK CM/water = 1.118 − 0.596 × logSwater(r 2 = 0.96). (6.8) This equation can be used to calculate Kcw from the aqueous solubility (mol l −1 ). The data base includes solutes shown in Fig. 6.3, and aqueous solubilities ranged from 0.17 to 2.4 × 10 −11 mol l −1 . 6.2.2 Steady State Penetration into Detached Leaves: The Submersion Technique In the experiments discussed above, concentrations in donor and receiver were monitored and permeance was calculated using (6.6). Concentration of the donor was constant, while concentration of the receiver was kept close to zero by constantly changing the solutions in the receiver compartment. When studying foliar
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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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208 7 Accelerators Increase Solute
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Chapter 8 Effects of Temperature on
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8.1 Sorption from Aqueous Solutions
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8.1 Sorption from Aqueous Solutions
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8.2 Solute Mobility in Cuticles 239
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8.3 Water Permeability in CM and MX
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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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