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

168 6 Diffusion of Non-Electrolytes
Table 6.6 Amounts of surface wax, projected and specific surface areas and permeances of conifer
needles from Abies koreana, Abies alba, Picea pungens and Pinus sylvestris (Schreiber and
Schönherr 1992b)
Species Surf. wax Aprojected Aspecific Solute Pspecific Pprojected
(µg mm −1 ) (mm 2 mm −1 ) (mm 2 mm −1 ) (m s −1 ) (m s −1 )
A. koreana 7.10 5 683 2,4-D 2.71 × 10 −11 3.70 × 10 −9
A. alba 2.50 5 254 2,4-D 3.11 × 10 −11 1.58 × 10 −9
P. pungens – 3 65.7 2,4-D 2.24 × 10 −11 4.91 × 10 −10
P. sylvestris – 3 41.3 2,4-D 3.78 × 10 −11 5.22 × 10 −10
P. abies 1.56 3 29.4 2,4-D 3.51 × 10 −11 3.43 × 10 −10
A. koreana 7.10 5 683 Triadim 2.54 × 10 −11 3.47 × 10 −9
A. alba 2.50 5 254 Triadim 5.58 × 10 −11 2.83 × 10 −9
P. pungens – 3 65.7 Triadim 7.00 × 10 −11 1.53 × 10 −9
P. abies 1.56 3 29.4 Triadim 4.00 × 10 −11 3.92 × 10 −9
P. pungens – 3 65.7 Lindane 4.23 × 10 −10 9.26 × 10 −9
P. sylvestris – 3 41.3 Lindane 5.01 × 10 −10 6.91 × 10 −9
A. koreana 7.10 5 683 Bitertanol 4.23 × 10 −10 5.78 × 10 −8
A. alba 2.50 5 254 Bitertanol 6.84 × 10 −10 3.47 × 10 −8
P. pungens – 3 65.7 Bitertanol 7.95 × 10 −10 1.74 × 10 −8
P. abies 1.56 3 29.4 Bitertanol 4.36 × 10 −10 4.27 × 10 −9
A. koreana 7.10 5 683 PCP 4.20 × 10 −9 5.74 × 10 −7
A. alba 2.50 5 254 PCP 3.90 × 10 −9 1.98 × 10 −7
P. pungens – 3 65.7 PCP 6.63 × 10 −9 1.45 × 10 −7
P. sylvestris – 3 41.3 PCP 3.30 × 10 −9 4.55 × 10 −8
P. abies 1.56 3 29.4 PCP 5.00 × 10 −9 4.90 × 10 −8
why both amounts penetrated and y-intercepts were highest (Fig. 6.8). The other
chemicals had smaller partition coefficients than PCP, and for this reason sorption
is smaller, and rates of penetration as well (Fig. 6.8).
This correlation between rates of penetration and sorption in surface wax and
cuticles is not really surprising, since amounts sorbed and rates of penetration are
proportional to concentration in the donor (6.6). After loading compartments 1 and
2 from the aqueous solution, the concentration in waxes and cuticles is the driving
force for loading the mesophyll, at least as long as concentration of non-ionised
PCP in the mesophyll remains small and insignificant and external concentration
does not change.
6.2.2.4 Projected and Specific Surface Area
Whenever permeances are calculated, rates of penetration are divided by area and
donor concentration (6.6). Glaucous leaves have large amounts of epicuticular wax,
which can greatly increase surface area. With regard to foliar penetration, the question
arises as to how much of this surface wax area is in contact with the aqueous
donor solution. This in turn depends on wetting, and in absence of surfactants it
seems plausible that only the tips of the wax rodlets typical for barley and conifers