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

162 6 Diffusion of Non-Electrolytes
the dark, and temperature was 25 ◦ C. It is important to establish this temperature
of the donor before closing the vessel. If cold solutions are warmed up in a closed
vessel, the pressure increases above ambient pressure, and this may cause stomatal
infiltration (Schönherr and Bukovac 1972a). When studying very lipophilic chemicals
it is essential to use glass tubes, as they may be sorbed to and in the walls of
plastic vessels.
Barley leaves and conifer needles are covered profusely with microcrystalline
surface waxes which render them very difficult to wet by aqueous solutions as long
as no surfactant is added. The water is in contact only with the tips of the surface wax
crystallites. When leaves are withdrawn from the donor solution, they are not covered
by water films and appear dry. After incubation the leaves were lightly blotted
with soft tissue paper, combusted and radioactivity was measured by scintillation
counting. Results obtained with pentachlorophenol (PCP) are shown in Fig. 6.5.
Donor concentration was 10 −7 mol l −1 and the pH of the donor was 3.0, that is, 98%
of the PCP was non-ionised. Penetration is expressed in moles penetrated per cm 2
of leaf surface, which is the sum of upper and lower surfaces. Amounts of chemical
are calculated by dividing radioactivity (Bq) in the leaf by specific radioactivity
(Bq mol −1 ).
The plot marked “measured” (Fig. 6.5) refers to amount penetrated per cm 2 leaf
surface. Permeance is calculated by dividing this slope by leaf area and donor concentration.
Permeance is 1.3 × 10 −7 m s −1 , which is larger by a factor of 3.25 than
permeance of Citrus CM (Fig. 6.3). The plot is straight, but it intersects the y-axis at
about 12 × 10 −12 mol cm −2 . This may appear strange, since with CM plots amount
Amounts penetrated x 10 12 (mole/cm 2 )
40
35
30
25
20
15
10
5
0
measured
calculated from desorption
0 50 100
Time (min)
150 200
Fig. 6.5 Foliar penetration at 25 ◦ C of PCP into barley leaves. Amounts penetrated increased
linearly with time, but measured penetration (red symbols) was biphasic and the regression line
intersects the ordinate at a positive value