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

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130 5 Penetration of Ionic Solutes be realised by applying small droplets to the outer surface of CM or to leaf discs. After evaporation of water, the solid salt residue is exposed to high humidity such that salts deliquesce. The point of deliquescence (POD) is defined as the humidity at which solid salt, saturated salt solution and relative humidity of the mixture are in equilibrium. POD depends only on temperature, but fortunately it varies little at physiological temperatures. For instance, humidity over a saturated solution of CaCl2 × 6 H2O at 10 ◦ C, 15 ◦ C, 20 ◦ C, 25 ◦ C and 30 ◦ C is 38%, 35%, 32%, 29% and 26% (Kolthoff et al. 1969). If humidity is below POD the salt loses all its water and solidifies, and penetration ceases. With humidity higher than POD the salt takes up water and deliquesces, and penetration proceeds from a concentrated salt solution. Hence, accurate control of temperature and humidity of the salt residue are essential when studying cuticular penetration of salts. Cuticular penetration of salts may be measured using isolated CM and the simulation of foliar penetration technique (SOFP) which is described in Sect. 6.4. CM are inserted into chambers shown in Fig. 5.4a. A finite dose of the radio-labelled ionic solute is placed as a small drop (5 µl) on the outer surface of the cuticle and allowed to dry. After drying, time-dependent penetration of the radio-labelled compound occurring from this finite dose on the cuticle surface is measured at controlled temperature and humidity by repeatedly changing the receiver and counting radioactivity. Non-linear rates of uptake are observed when relative amounts of penetration are plotted vs time. As an example, the penetration of 14 C-labelled isopropylamine salt of glyphosate (IPA glyphosate) across poplar CM (Populus canescens) at different humidities is shown (Fig. 5.5). Rates of penetration of the salt (slopes) are highest at the beginning and level off with time (Fig. 5.5a). This non-linearity is a consequence of the fact that the amount of IPA glyphosate in the deposit on the cuticle surface is decreasing in proportion to the amount which penetrated the cuticle. Hence, driving force decreases with time (Sect. 2.5). SOFP has great advantages, because the same set of CM can be used repeatedly with different humidities or solutes. Also, the time course can be followed for each CM individually, because penetration is followed by changing the receiver and assaying it for salt which has penetrated. By reference to Fig. 5.5a, it is clear that determining amount penetrated only at a single time interval is likely to lead to erroneous conclusions. The effect of guard cells and glandular trichomes cannot be studied using isolated CM, because of the stomatal pores between guard cells. These holes would be filled with donor solutions and serve as parallel pathways for ionic solutes. In these cases, leaf discs must be used (Fig. 5.4b). Leaf disks are punched out and are inserted into the apparatus. The lower part is a hollow cylinder made of Teflon. It serves as a spacer to prevent contact of the leaf disk with water in the Petri dish. In this way, leaching of radio-labelled solutes from the leaf disk is prevented. The lid with an orifice in its centre is secured with silicon grease on the leaf disk, which prevents water loss from the cut edges of the disc. Ten of these penetration units are placed in a Petri dish lined with moist filter paper. A 5µL droplet of donor solution is pipetted onto the leaf surface. After evaporation of the water, the Petri dish is closed and placed in a growth chamber either in light or in the dark. Humidity inside the Petri dish is 100% and it cannot be varied. This type of experiment has an
5.2 Experimental Methods 131 Fig. 5.4 (a) Transport chambers used for studying penetration of salts and other electrolytes across isolated CM. The chambers are positioned in wells of a thermostated aluminium block. Air of constant humidity is blown on the salt residues (arrows) at about ten air changes per second. Temperature and humidity can be varied (taken from Schönherr 2006). (b) Penetration unit used to study salt penetration into leaf disks. The leaf disk is positioned on a short hollow Teflon tube and covered with a stainless steel lid. Teflon and lid are coated with silicon grease. The units are placed on moist filter paper in Petri dishes. Humidity is always 100%
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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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180 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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