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

148 6 Diffusion of Non-Electrolytes
6.1.3 Wax/Water Partition Coefficients Kww
Waxes greatly contribute to barrier properties of the CM (Sects. 4.6 and 6.3), and
solubility of penetrants in cuticular wax is important when analysing transport
properties of cuticles. Wax/water partition coefficients Kww have been determined
using reconstituted cuticular waxes extracted from leaves of Hordeum vulgare and
from isolated cuticles of Prunus laurocerasus, Ginkgo biloba and Juglans regia
(Burghardt et al. 1998; Schreiber and Schönherr 1992a; Kirsch et al. 1997). Cuticular
wax was extracted using chloroform and was recrystallised on thin aluminium
disks (Sect. 9.6). Wax amounts (200 ± 30µg) were calculated from differences in
weight of the aluminium disks before and after applying the wax. Wax samples
were incubated in solutions of the radio-labelled compounds, and radioactivity in
both phases was determined after equilibration.
Kww values were always significantly lower than Kcw, by factors of 3–10
(Table 6.2). Cuticular waxes are mixtures of linear long-chain aliphatic molecules
(Table 1.3) and in some species pentacyclic triterpenoids are abundant (Fig. 1.4).
These waxes are solids at physiological temperatures, and about 20–50% of the
waxes are crystalline at room temperature (Reynhardt and Riederer 1991, 1994).
Crystals are impermeable and inaccessible for solutes. Only the amorphous wax
fraction can sorb lipophilic solutes. Barley leaf wax is about 50% crystalline, and
Table 6.2 Wax/water partition coefficients (log Kww) measured with cuticular wax isolated from
Hordeum vulgare, Prunus laurocerasus, Ginkgo biloba and Juglans regia, mean log Kww values
and mean log Kcw values of ten compounds. Values in brackets indicate log Kww values of
Hordeum corrected for the crystallinity of 50%, assuming that the crystalline wax fraction does not
contribute to sorption
Substance log Kww log Kww log Kww log Kww log Kww log Kcw
Hordeum Prunus Ginkgo Juglans mean mean
MET 0.54 (0.84) a 1.11 c – – 0.82 1.48 c
4-NP – 1.15 c – – 1.15 1.87 d
BA – 1.32 c 1.35 c 1.34 c 1.34 1.71 c
AT – 1.45 c – – 1.45 2.16 d
SA 1.48 (1.78) a 1.66 c 1.67 c 1.68 c 1.62 2.03 c
TRI 1.51 (1.81) b – – – 1.51 2.88 b
2,4-D 1.66 (1.96) b 2.13 c 2.14 c 2.16 c 2.02 2.61 d
TB 2.81 (3.11) a – – – 2.81 3.54 e
BIT 3.02 (3.32) b – – – 3.02 4.05 b
PCP 3.55 (3.85) b – – – 3.55 4.56 d
Metribuzin (MET), 4-nitrophenol (4-NP), benzoic acid (BA), atrazine (AT), salicylic acid (SA),
triadimenol (TRI), 2,4-dichlorophenoxyacetic acid (2,4-D), tebuconazole (TB), bitertanol (BIT)
and pentachlorophenol (PCP).
aBurghardt et al. (1998)
bSchreiber and Schönherr (1992a)
cKirsch et al. (1997)
dKerler and Schönherr (1988a)
eBaur et al. (1996b)