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

4.6 Water Permeability of Isolated Astomatous Cuticular Membranes 105
Water loss (mg)
60
50
40
30
20
10
AgCl
precipitation
0
0 5 10 15 24 28 32
Time (h)
Fig. 4.19 Water permeability of an isolated Populus canescens CM before and after counter diffusion
of NaCl and AgNO 3 . The slope of the linear transpiration kinetic is significantly decreased by
64% after the formation of AgCl precipitations in the CM. Data were taken from Schreiber et al.
(2006)
4.6.2.5 Effect of AgCl Precipitates on Water Permeance
Polar functional groups contribute to the sorption of water to the MX (Sect. 4.3)
and to the formation of aqueous pores (Sect. 4.4). Water permeability of some CM
increases with humidity (Sect. 4.6.2, subsection: “Effect of Partial Vapour Pressure
(Humidity) on Permeability of CM”) and if they are traversed by aqueous pores
they are permeable to hydrated ions (Chap. 5). As hydrated ions diffuse exclusively
in aqueous pores, it was attempted to selectively block these pores in CM membranes
with insoluble AgCl precipitates. CM were inserted in the transpiration cups
(Fig. 9.2), and water permeance was determined gravimetrically with 0.01moll −1
NaCl as donor solution. Partial pressure of the receiver was practically zero. With
NaCl still in the donor, the outer surface of the CM was treated with 0.01moll −1
AgNO 3 in deionised water for 24 h. This was done by adding AgNO 3 solution to
the outer side of the CM, sealing the chambers with adhesive tape and incubating
them on a rolling bench, which guaranteed homogeneous wetting of both sides of
the CM. After blotting off the silver nitrate solution, water permeability was again
determined gravimetrically (Schreiber et al. 2006). When Ag + and Cl − diffusing
towards each other in aqueous pores of the CM meet, insoluble precipitates are
formed. These precipitates could at least partially block aqueous pores, and water
permeability should decrease. This is exactly what happened (Fig. 4.19). If water
loss was plotted vs time, slopes decreased after the formation of AgCl precipitates.