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

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12 1 Chemistry and Structure of Cuticles as Related to Water and Solute Permeability different chain lengths within each substance class. A representative wax sample can be composed of 50 individual compounds and more. Often, the most prominent wax compounds are identified, whereas compounds present only in traces remain unidentified. Identification of 90–95% of the compounds occurring in a specific wax sample is considered a successful analysis, and this can take a fairly long time, when time needed for identifying unknown mass spectra is included. Quantification is normally carried out adding an internal standard (e.g., an alkane) of known amount to the wax samples. Ideally, for each individual wax compound, varying in chain length and functionalization, the best standard would of course be the identical compound. However, most wax compounds are not commercially available as standards. Furthermore, in view of the large number of wax molecules which are normally present in a typical wax sample, it would be unrealistic running a separate standard for each wax molecule, even if it was available. Therefore, in most cases an alkane, representing a linear long-chain aliphatic molecule as they are typically found in wax samples, is used as internal standard. Alkanes of even chain lengths (e.g., C24) are preferred, since alkanes of uneven chain length are dominant in plant waxes. In addition to these limitations in quantitative wax analysis, there is another problem which has rarely been addressed in the past. The total amount of cuticular wax determined gravimetrically is generally larger than wax amounts determined by GC–MS. Various reasons might contribute to this observation. Weighing does not discriminate between wax compounds and non-wax compounds, which may contribute to total weight loss. On the other hand, analysis by GC is highly specific, and permits exact identification of compounds. Wax and other compounds (e.g., sugars) can be distinguished. Esters with very high molecular weight can present formidable problems in GC due to their very long retention times (Santos et al. 2007) and the tendency to produce broad and blurred peaks. Esters with high molecular weight (>700–800) also approach the detection limit of the MS. This could lead to an underestimation of wax amounts, especially in wax samples with high amounts of esters. Compounds with very high molecular weights have been detected in wax from Hedera helix leaves (Hauke and Schreiber 1998). The large difference between wax amounts determined gravimetrically and by GC–MS was mainly due to a limited resolution of the analytical approach. The difference in wax amounts depended on leaf age and season (Fig. 1.5a). When wax extracts from ivy CM were separated on a column packed with silica gel using solvents of different polarity and subjected to GS–MS, two distinct wax fractions were obtained. The long-chain aliphatics as described above were eluted from the column with the apolar solvent (diethylether), and this could be analysed directly by gas chromatography. This fraction yielded alkanes, alcohols, aldehydes, acids and esters. The eluate of the polar solvent (chloroform/methanol/water in ratios of 60:25:5) resulted in an additional wax fraction, which could not be analysed directly by gas chromatography but only after transesterification (Fig. 1.5b). Analysis of this transesterified polar wax fraction revealed that it was composed of di- and presumably trimers of ω-hydroxy fatty acids in the chain lengths C12–C16, which were linked by ester bonds to linear long-chain alco-
1.3 Soluble Cuticular Lipids 13 Wax amount (µg/leaf) a 800 apolar wax fraction 600 gravimetric determination 400 200 0 800 b polar wax fraction 600 gravimetric determination 400 200 0 800 c apolar + polar wax fraction 600 gravimetric determination 400 200 0 0 30 60 90 120 150 180 Leaf age (days) Fig. 1.5 Quantitative determination of wax in ivy (Hedera helix) CM over one season from bud break at the end of April to leaf senescence in November. Apolar and polar wax fractions were separated as described in the text hols with chain lengths from C22 to C32. Compounds of similar oligomeric structure have also been found in gymnosperms, and they are called estolides (Bianchi 1995). Molecular weights and polarity of these compounds are too high for direct GC analysis. They do not move on the column, and are overlooked if not transesterified. When apolar and polar wax fractions are combined, total amounts of wax are close to the wax amounts determined gravimetrically (Fig. 1.5c). Based on these results, it appears that gravimetric determination of wax amounts is more reliable. We don’t know if the problem occurs with waxes of all species, but we suggest that wax amounts determined by GC should always be compared to those determined gravimetrically. If a significant difference is observed, fractionation of waxes by column chromatography is indicated. Deterioration of the ability of a column to separate homologues, broadening of peaks and discolouration of the column entrance could also indicate the presence of estolides in the sample which failed to move on the column. As a corollary, it should be realised that it is not a good practice to analyse waxes from leaves of unspecified age and to use only one sampling time. There simply exists no typical wax composition of leaves and fruits.
Page 2 and 3: Water and Solute Permeability of Pl
Page 4 and 5: Professor Dr. Lukas Schreiber Ecoph
Page 6 and 7: vi Preface This is not a review abo
Page 8 and 9: Contents 1 Chemistry and Structure
Page 10 and 11: Contents xi 4.6.3 Diffusion Coeffic
Page 12 and 13: Contents xiii 7.4.2 Effects of Plas
Page 14 and 15: 2 1 Chemistry and Structure of Cuti
Page 22 and 23: 10 1 Chemistry and Structure of Cut
Page 42 and 43: Chapter 2 Quantitative Description
Page 44 and 45: 2.1 Models for Analysing Mass Trans
Page 50 and 51: 2.3 Steady State Diffusion Across a
Page 52 and 53: 2.4 Steady State Diffusion of a Sol
Page 54 and 55: 2.4 Steady State Diffusion of a Sol
Page 56 and 57: 2.5 Diffusion Across a Membrane wit
Page 58 and 59: 2.5 Diffusion Across a Membrane wit
Page 60 and 61: 2.6 Determination of the Diffusion
Page 62 and 63: Solutions 51 2.7 Summary In this ch
Page 64 and 65: Chapter 3 Permeance, Diffusion and
Page 66 and 67: 3.1 Units of Permeability 55 3.1.1
Page 68 and 69: 3.1 Units of Permeability 57 identi
Page 70 and 71: 3.3 Partition Coefficients 59 The d
Page 72 and 73: 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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206 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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