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

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8.2 Solute Mobility in Cuticles 243 increases. This general feature has been observed with CM of all species and solutes (Baur et al. 1997a). When the logarithms of average k ∗ values are plotted over a wider temperature range, these plots are often linear up to 70 ◦ C, as with the combinations Hedera/IAA and Hedera/bifenox (Fig. 8.3b). With some CM/solute combinations, plots departed from linearity at temperature above 50 ◦ C, when the melting points of the waxes were approached and the diffusion was no longer totally determined by properties of the limiting skin (Sect. 6.3.2). When rate constants for the limiting skin and the sorption compartment are similar, diffusion in the sorption compartment can become rate-limiting, as was shown for the combination Citrus MX/PCP (Fig. 6.20). Further, flattening of slopes is often observed with homogeneous rubbery polymers where it indicates the onset of viscous flow of polymer chains (Schlotter and Furlan 1992). For these reason, most studies have been limited to temperatures below 50 ◦ C, which is the range experienced by most plant species under natural conditions. Generally, only linear portions of Arrhenius plots were utilised for data analysis, as shown for the combinations Ilex/bifenox and Hedera/cholesterol. In the upper right hand corner of Fig. 8.3b, regression equations for the linear portions of the Arrhenius plots are given. Slopes vary between −6,460 and −9,910, and using (8.7), ED in the range of 123.5–185.5kJ mol −1 can be calculated. As seen in Fig. 8.3b, differences in rate constant among species also tend to decrease with increasing temperature, and above 60 ◦ C differences have practically disappeared. Thus, differences among individual CM and differences between species/solute combinations decrease with increasing temperature, and above 50–60 ◦ C they are small or nil. These observations are the consequences of the fact that size selectivity β ′ , which amounts to 0.095mol cm −3 at 25 ◦ C (Table 6.8), decreases with temperature. Buchholz et al. (1998) measured k ∗ for ivy leaf CM and the solutes IAA and tebuconazole, which had molar volumes of 130 and 241cm 3 mol −1 respectively. The ratios of the two rate constants decreased with temperature, and amounted to 24.5 at 25 ◦ C, 10.5 at 35 ◦ C and 2.2 at 55 ◦ C. 8.2.2 Thermodynamics of Solute Diffusion in CM Slopes and y-intercepts of Arrhenius plots vary with types of cuticle and solute, and k ∗ infinite is related to entropy. Before discussing the physical meanings of slopes and intercepts, we take a look at their dependence on solutes and species. This was studied using leaf CM from 14 different species and varieties (Buchholz and Schönherr 2000). Barrier properties of CM can be classified based on bifenox mobility in CM at 25 ◦C (Fig. 6.21). Populus canescens and pear are species with relatively high solute mobilities. Ilex paraguariensis and Hedera helix are species having very low bifenox mobility. The other species and cultivars tested (Malus domestica, Prunus laurocerasus, Pyrus communis, Schefflera actinophylla, Strophantus gratus) have intermediate mobilities. All solutes were lipophilic, and their partition coefficients varied by seven orders of magnitude. They were (log Kcw in parentheses) IAA (1.16), NAA (2.43), tebuconazole (3.54), bifenox (4.44) and cholesterol (8.0).
244 8 Effects of Temperature on Sorption and Diffusion of Solutes and Penetration of Water From Arrhenius plots as shown in Fig. 8.3, y-intercepts (pre-exponential factor k ∗ infinite ) and the slopes (ED/R) were obtained for individual CM, and results were plotted (Fig. 8.4). Data obtained with four species and the smallest and least lipophilic solute IAA (Appendix B) are depicted in Fig. 8.4a. There is an excellent correlation between k ∗ infinite and the Arrhenius slopes (ED/R). CM with steep slopes, that is, high energies of activation, have large y-intercepts and vice versa. With all species, k ∗ infinite varied among individual CM. For instance with Citrus CM, k ∗ infinite varied between 10 and 20 and (ED/R) between 11 × 10 −3 and 19 × 10 −3 Kelvin. Large variability in permeance and solute mobility is a typical property of CM. In Fig. 8.4b, all available species/solute combinations are plotted for individual CM. An excellent linear correlation was again obtained, and both y-intercept and slope are very similar to those in Fig. 8.4a. This linear free energy relationship is good evidence that in CM from all species all solutes diffuse in a similar environment. This is astounding, since wax amounts and composition differ greatly among species (Tables 1.1 and 4.8) and partition coefficients ranged from 14 to 10 8 . The results characterise the lipophilic or waxy pathway, which in CM consists of methylene groups donated by amorphous waxes and cutin environment. The most polar solute included in the study is IAA; it is 14 times more soluble in CM than in water, and is still sufficiently soluble in cutin and waxes. Its solubility in water is smaller than in membrane lipids, and since water content of fully swollen CM is below 8% (Fig. 4.7) the concentration of IAA in lipid compartments is much larger. IAA exclusively diffused in a lipophilic matrix, otherwise these data would not have fallen on the straight line seen in Fig. 8.4b, and slopes and y-intercepts in Fig. 8.4a and 8.4b would have differed. It was not possible to include highly water soluble solutes in these studies, because the UDOS method does not work with them (Sect. 6.3). However, there is no reason why results should not apply to more polar non-electrolytes, as long as they are not ionised and soluble in amorphous waxes. As shown in Sect. 4.6, water can penetrate cuticles using two parallel pathways, in aqueous pores located between polar polymers and in cutin and in amorphous waxes. ED/R and y-intercepts for the fraction of water which diffuses in cutin and waxes most probably would fit the regression equation shown in Fig. 8.4b, but data are not available (see below). There are a few data available for MX membranes. Extraction of waxes greatly increases solute mobility (k ∗ ) and activation energies are much lower. Plots log k ∗ vs ED/R have similar slopes than shown in Fig. 8.4 for CM, but y-intercepts were higher, that is, they had smaller negative numbers (Fig. 8.5). With the combination Hedera/IAA, this parallel displacement amounted to a factor of 1.97 (log scale), that is, with the MX entropy was higher by a factor of 94. With Strophanthus/IAA the factor was 50, and with Schefflera/IAA entropy was 5,011 times higher with MX than with CM. With Pyrus/bifenox, the MX had a 25-times higher entropy than the CM (Buchholz 2006; Buchholz and Schönherr 2000). These y-intercepts represent the entropy, when ED/R is zero (Fig. 8.5), that is when there is no free energy and no driving force for diffusion.
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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 202 and 203: 192 6 Diffusion of Non-Electrolytes
Page 216 and 217: 206 7 Accelerators Increase Solute
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
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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
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Page 274 and 275: 9.6 Cutin and Wax Analysis and Prep
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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
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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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