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

6.2 Steady State Penetration 163
penetrated vs time has a positive intersection with the time axis (extrapolated holdup
time seen in Fig. 6.2), because it takes some time before the first solute molecules
appear in the receiver and penetration becomes steady. When penetration across CM
is measured, concentration of the receiver is monitored and sorption in the CM goes
unnoticed. During penetration into leaves, solutes are first sorbed in epicuticular
waxes and in cuticles before they reach the tissue (apoplast and the symplast). Penetration
of PCP into barley leaves was biphasic. During the first 30 min it was much
faster than later on. The rapid penetration represents soprtion to the leaf surface and
diffusion into epicuticular wax, while slow penetration marks cuticular penetration
into the leaf apoplast. Compartmental analysis substantiates this conclusion.
6.2.2.3 Compartmental Analysis
From leaves preloaded with PCP in an experiment as shown in Fig. 6.5, the PCP
was desorbed by submerging the leaves in borax buffer of pH 9. At this pH all PCP
is ionised, because the pKa is 4.73. PCP sorbed in wax and in cutin is non-ionised,
and the concentration of non-ionised PCP in borax buffer is zero. Hence, there is
a large driving force favouring efflux of PCP from the wax. Initially, desorption is
very rapid and large amounts of PCP are desorbed during the first 5 min (Fig. 6.6).
As time progresses, desorption curves approach a plateau which is lower the longer
preloading lasted.
M t/M 0
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
30 min
90 min
180 min
0 60 120 180 240 300 360
Time (min)
Fig. 6.6 Desorption of PCP from barley leaves preloaded with PCP for 30, 90 or 180 min. During
preloading, PCP penetrated from the donor solution at pH 3 into leaves. After being briefly blotted
between soft tissue paper, the leaves were submerged in borax buffer at pH 9. This changed the
direction of the PCP concentration difference, and PCP diffused out of the leaves. (Redrawn from
Schreiber and Schönherr 1992a)