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

160 6 Diffusion of Non-Electrolytes
penetration, it is not possible to manipulate the receiver. Donor concentrations can
be adjusted and held constant, but the influx is measured by analysing the entire leaf
after blotting or rinsing off the donor solution. The entire leaf serves as receiver,
and it is not known a priori where the solute is located. Depending on water and
lipid solubility it will be distributed in aqueous and/or lipid compartments, including
cuticular wax and cutin. With intact leaves, permeances can be estimated from timecourse
experiments, and using a desorption technique sorption compartments can
be characterised. We will demonstrate this with barley leaves and conifer needles.
Slender leaves as typical for conifer needles and some monocot species can be submerged
in large volumes of donor solutions in glass test tubes. They are maintained
at constant temperature, and are lightly agitated for mixing to minimise unstirred
boundary layers.
6.2.2.1 Penetration into Cut Edges
Whenever leaves or needles are removed from the plants a wound is generated, and
solutes can penetrate into the wound. This necessitates corrections for solute penetration
into the wound. Wounds may be closed by dipping into paraffin wax, grease,
glue or other materials. In these cases, it must be established that the seal is effective
and that the materials do not sorb solutes. With barley leaves and conifer needles we
have used a different approach. The cut edge was dipped into a shallow (1 mm)
aqueous solution of 14 C-labelled triadimenol (Kcw = 760), the vessels were closed
to establish 100% humidity, and they were incubated in the dark to minimise transpiration.
After 6 or 24 h the leaves were removed from the bath, the base was blotted
dry, and leaves were dissected in 1 mm-wide strips. These strips were combusted,
14 CO2 was trapped and radioactivity was determined by scintillation counting. If
M0 is the radioactivity associated with the first mm of leaf and Mx is the amount
contained in segments above, the concentration gradient along the leaf is proportional
to Mx/M0. With increasing distance from the solution, radioactivity (that is,
Mx/M0) decreased, and in the segments 9 (6 h incubation) to 15 from the cut edge
(24 h incubation) practically no radioactive triadimenol had arrived (Fig. 6.4).
This approach is superior to trying to seal the wound, as it gives a result that can
be generalised to other non-volatile compounds which are not readily metabolised.
Cyan symbols represent Mx/M0 values calculated from (6.9)
�
Mx
x
= 1 − erf
2 √ �
, (6.9)
Dt
M0
where x is the distance from the cut edge and t is the duration of incubation. The
error functions of the term in brackets are tabulated (Crank 1975, Table 2.1, p. 375).
With x and t given, various D values must be tried which result in the best fit to
the data. With triadimenol, the best fit was obtained with a diffusion coefficient
of 2 × 10 −10 m 2 s −1 . This is a value typical for diffusion in water at 25 ◦ C. Hence,
triadimenol diffused up the leaf in an aqueous compartment, and viscous flow by