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

6.2 Steady State Penetration 161
M x/M o
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
6 h
24 h
0.0
0 3 6 9 12 15
Distance from cut edge (mm)
Fig. 6.4 Distribution of 14 C-triadimenol in barley leaves which had been dipped with their cut
edges 1 mm deep for 6 or 24 h into radio-labelled aqueous triadimenol. Pink symbols are experimental
values, while cyan symbols were calculated using a D of 2×10 −10 m 2 s −1 Vertical bars are
95% confidence intervals. Redrawn from Schreiber and Schönherr (1992a)
transpiration was absent. D in water decreases with increasing molecular weight
(MW), but since it is proportional to √ MW, the effect of MW is small for most
solutes of interest. Thus, results depicted in Fig. 6.4 will be similar, with many
compounds diffusing in the apoplast of barley leaves. If leaves are submerged in
solutions for up to 24 h, discarding the lower15 mm portion of the leaf eliminates
practically all radioactivity which entered through the cut edge, und the remaining
radioactivity in the leaf has penetrated through the cuticle. This method has
been used with conifer needles and other lipophilic compounds, and distribution of
radioactivity along the needle was the same except for the first 3 mm segment near
the cut edge. This segment was discarded prior to combusting the needles (Schreiber
and Schönherr 1992b, 1993a).
6.2.2.2 Cuticular Penetration
Cuticular penetration into barley leaves and conifer needles was studied by submerging
them in large volumes of water or buffer containing radiolabelled solutes.
Depending on partition coefficient, the mass of the donor solution should be about
20–50 times larger than the mass of the leaves. This will keep donor concentrations
constant, but this should be checked by taking samples at the end of the experiments.
During incubation the glass test tubes were closed with screw caps lined
with aluminium foil, and they were lightly agitated for mixing. Incubation was in