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

6.2 Steady State Penetration 167
desorbed from epicuticular waxes of conifers at similar rates to those from barley
leaf wax. Rate constants of desorption (k2) from the second compartment (i.e., the
cuticle underneath the epicuticular wax) differed somewhat depending on species.
They were a little higher with Picea abies and Taxus baccata (2.4 × 10 −4 s −1 ) than
with Abies koreana and Abies alba (1.5 × 10 −4 s −1 ). These rate constants are also
similar to k2 measured with barley leaves (1.4 × 10 −4 s −1 ). As with barley leaves,
the size of the second compartment (CPT2) did not depend on time of loading and
was similar for all conifers. About 7% of PCP in needles was sorbed in compartment
2, that is in the cuticle. Fractions of PCP in compartment 1 decreased and
compartment 3 increased with time, as was the case with barley leaves.
Permeances obtained from penetration were not significantly different from permeances
obtained from desorption experiments. When rates of penetration were
plotted against amount sorbed in compartments 1 and 2, a straight line was obtained.
Hence, rates of penetration into compartment 3 (mesophyll) were proportional to
amounts of chemicals sorbed in waxes and cuticles (Fig. 6.9). The amounts of chemicals
sorbed are proportional to mass of wax, mass of cutin and partition coefficients.
The mass of epicuticular wax per mm needle length was determined by dipping the
needles for 1–2 s into chloroform. This method can be questioned (Sect. 2.2) but it
is likely that most epicuticular waxes were dissolved without extracting too much
embedded wax. Since compartments 1 and 2 were added (Fig. 6.9), the problem is
not crucial. Amounts of surface wax varied among species (Table 6.6) and ranged
from 1.56 to 7.1µg mm −1 needle length (Schreiber and Schönherr 1993b). Abies
koreana and A. alba had the highest amount of surface wax, and this is the reason
log Rates of penetration (mol s −1 mm −1 )
−15
−16
−17
−18
Abies koreana
Abies alba
Picea pungens
Pinus sylvestris
Picea abies
−13 −12 −11 −10
log Sorption (mol mm −1 )
Fig. 6.9 Dependence of rate of penetration of 2,4-D, triadimenol, bitertanol, lindane and PCP into
conifer needles on amounts of chemicals sorbed reversibly in compartments 1 and 2. (Redrawn
from data of Schreiber and Schönherr 1992b)