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

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226 7 Accelerators Increase Solute Permeability of Cuticles be calculated from (7.9). A reasonable positive correlation between effects and concentrations of the respective alcohol ethoxylates sorbed in Citrus MX is obtained (Fig. 7.15a). Unfortunately, partition coefficients Kww have not been determined for Citrus wax. Citrus wax is largely amorphous, and we expect Kww to be smaller than Kmxw. Using (7.4), concentrations of alcohol ethoxylates sorbed in barley wax instead of Citrus wax can be calculated. A plot of the effects on mobilities in Citrus as a function of the plasticiser concentration in barley wax gives a reasonable correlation as well (Fig. 7.15b). This confirms that intrinsic effects of these plasticisers are similar, and that effects on solute penetration across cuticles are only a function of the concentration of accelerators sorbed in the transport-limiting barrier of wax. When comparing the effects of alcohol ethoxylates with those of n-alkyl esters on diffusion in wax (Sect. 7.3.3), it was shown that n-alkyl esters were five times more efficient in plasticising waxes at the same internal concentrations in the wax (Fig. 7.10). A similar tendency can be observed with CM (Schönherr et al. 2001; Shi et al. 2005b). This is not too surprising, since above we have presented evidence showing that transport properties of cuticles and effects of plasticisers can be attributed to sorption and diffusion properties of wax. Comparing intrinsic effects of alcohol ethoxylates with those of the n-alkyl esters tributyl phosphate (TBP) and diethyl suberate (DESU) on 2,4-DB (2,4dichlorophenoxybutyric acid) mobility in Stephanotis cuticles shows that highest effects were obtained with DESU, intermediate effects with TBP, and lowest but still significant effects with alcohol ethoxylates (Table 7.4). At 15 ◦ C the n-alkyl ester DESU is about 4.4 times more effective than alcohol ethoxylates in plasticising the waxy transport barrier. This effect gradually decreases with increasing temperature from 15 to 25 ◦ C. At 30 ◦ C the difference disappears completely (Table 7.4). Activation energies for mobility of 2,4-DB in Stephanotis CM are 105kJ mol −1 in the absence of a plasticiser. They significantly decrease to 26 and 36kJ mol −1 in the presence of DESU and of the alcohol ethoxylates respectively (Schönherr et al. 2001). TBP shows the same tendency of decreasing effects with increasing temperature (Table 7.4), and the activation energy decreases to 64kJ mol −1 (Schönherr et al. 2001). Effects of plasticisers are highest at low temperatures. Increasing temperature or adding plasticisers have similar effects on fluidity of the waxy transport-limiting barrier of cuticles. ESR experiments (Sect. 7.2) support this conclusion. Table 7.4 Effect of alcohol ethoxylates (CxEy), tributyl phosphate (TBP) and diethyl suberate (DESU) on 2,4-DB (2,4-dichlorophenoxybutyric acid) mobility k ∗ in Stephanotis CM at 4 different temperatures Plasticiser Concentration in CM Effect on k ∗ (g kg −1 ) 15 ◦ C 20 ◦ C 25 ◦ C 30 ◦ C CxEy 85 11 9 9 9 TBP 90 35 17 17 14 DESU 90 48 25 15 9 Data from Schönherr et al. (2001)
7.4 Effects of Plasticisers on Transport in Cuticles 227 7.4.3 Effects of Plasticisers on the Mobility of Polar Solutes in CM UDOS is a very useful tool for analysing the plasticising effect of chemicals on the transport-limiting barrier of cuticles, and this has been confirmed by many investigations (Schönherr 1993a, b; Schönherr and Baur 1994; Schönherr et al. 2001; Shi et al. 2005b; Buchholz 2006). Unfortunately, UDOS is limited to the investigation of lipophilic solutes, since UDOS requires that the 14 C-labelled solute is sufficiently soluble in the inner sorption compartment of the cuticle (Sect. 6.3). For studying cuticular penetration of polar solutes and for testing the effect of plasticisers on rates of cuticular penetration, SOFP is the method of choice (Sect. 5.2 and Fig. 9.5). SOFP was used to investigate the plasticising effect of n-alkyl esters and alcohol ethoxylates on cuticular penetration of the highly water soluble sugar methyl glucose, in comparison to the lipophilic molecules metribuzin and iprovalicarb, which have log Kow values of 1.6 and 3.18 respectively (Shi et al 2005a). Methyl glucose has a log Kow of −3.0, indicating a much higher solubility in water than in the cuticle. Droplets of 5µl of the 14 C-labelled solutes dissolved in water were applied to the physiological outer side of Stephanotis CM and were allowed to dry. Labelled solutes were desorbed from the physiological inner side of the CM using the plasticisers DESU, TBP and alcohol ethoxylates of the C12Ey series with y increasing from 2 to 8 (Shi et al. 2005a). All three types of plasticisers significantly increased rate constants k of cuticular penetration of the two lipophilic solutes metribuzin and iprovalicarb. The logarithms of the effects (k measured in the presence of the plasticizer/kcontrol measured without a plasticizer) linearly increased within increasing concentration of the plasticizer in the wax, as shown for iprovalicarb and various ethoxylated alcohols (Fig. 7.16). Thus, using SOFP as a method for measuring the effect of plasticisers on rates of cuticular penetration confirms the results obtained with reconstituted waxes (Sect. 7.3) and with UDOS experiments (Sect. 7.4.2). At identical internal plasticizer concentrations in the wax, intrinsic effects of DESU were again fourto five-fold higher than for the alcohol ethoxylates, whereas the effect of TBP was intermediate (Shi et al. 2005a). Surprisingly different results were obtained with methyl glucose. Neither DESU nor TBP had an effect on k of methyl glucose (Shi et al. 2005a). With the alcohol ethoxylates of the series C12En, exactly the opposite sequence as with iprovalicarb was observed. (Fig. 7.16). The largest and most polar alcohol ethoxylate C12E 8 had the largest effect on k of methyl glucose rates of penetration, and effects decreased with decreasing degree of ethoxylation (Fig. 7.16), that is with increasing concentration in wax. There was no statistically significant effect with C12E 4 and C12E2. We have shown in previous chapters that the plant cuticle is a heterogeneous membrane containing aqueous pores (Fig. 4.11),which are a prerequisite for penetration of ionic compounds (Chap. 5). In parallel there is a lipophilic waxy path in cuticles (Fig. 4.13), open to lipophilic solutes which are soluble in amorphous waxes (Chap. 6). All efficient plasticisers (DESU, TBP and the alcohol ethoxylates tested) are lipophilic, and they increase fluidity of the waxy pathway. They dissolve and
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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 186 and 187: 176 6 Diffusion of Non-Electrolytes
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Page 248 and 249: 8.2 Solute Mobility in Cuticles 239
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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
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Page 278 and 279: 9.8 Measuring Solute Permeability 2
Page 284 and 285: 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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