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

6.2 Steady State Penetration 173
Half time (s)
1e+6
1e+5
1e+4
1e+3
Droblet radius (µm)
1e+2
1e−10 1e−9 1e−8
Permeance (m/s)
1e−7
133
100
67
33
278 h
27.8 h
2.78 h
16.7 min
1.67 min
Fig. 6.11 Half-times for solute penetration from hemispherical droplets of different size as a
function of permeance
penetration of a constant dose is aimed at, and we demonstrate this in Chap. 5 and
Sects. 6.3.1–6.3.4.
In an attempt to circumvent problems associated with droplet experiments, glass
wells have been glued to leaf surfaces using silicone rubber (Fig. 6.12). Relatively
large volumes of up to 1 ml donor can be pipetted into these wells, and both contact
area and donor volume can be kept constant. Problems may arise if the glue is phototoxic
or when the solute is sorbed in the glue. Schönherr (1969) and Schönherr and
Bukovac (1978) have used this approach for studying foliar penetration of succinic
acid-2,2-dimethyl hydrazide (Alar), which is a zwitterion. Small glass tubes (10 mm
diameter and 7 mm height) were attached to leaf discs using silicon rubber and a
non-toxic catalyst. Silicon rubber provides a good seal even over veins, and surfactant
solutions did not leak out. At the end of the experiment the rubber remained
attached to the glass and the leaf disk could be peeled easily and did not interfere
with subsequent processing of leaf disks (autoradiography and counting radioactivity).
Rates of penetration were constant, as penetration plots were linear with all
treatments. Plots intersect the origin, that is, there was no measurable hold-up time
and no positive y-intercepts due to sorption in wax and cutin. From the slopes and
the donor concentration, permeance can be calculated. Permeance was very low,
depending on treatment. It ranged from 5 × 10 −11 to 25 × 10 −11 m s −1 (Fig. 6.13).
Permeability of the lower leaf surface was higher than that of the upper one
(Fig. 6.13). Light increased permeability of both leaf surfaces, and the wetting agent
Tween 20 (polyoxylethylene sorbitane monolaurate) increased rates of penetration.
This wetting agent did not cause stomatal infiltration, and its effect on rates of penetration
was related to increased contact areas between donor and leaf. The light