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

116 4 Water Permeability
in the CM and on the surface of the CM. The onion bulb scale CM is well-protected
within the bulb, and abrasion and wear are not likely to occur. Hence they have and
need only minimum amounts of wax.
These model calculations show convincingly that waxes which would form
mono- or bilayers on the surface of cutin, as is suggested by AFM investigations
(Koch et al. 2004), could be very effective as water barriers. These waxy barriers
can be very thin and still have a low permeance. A simple polymeric membrane
similar to polyethylene or parafilm would have to be much thicker, and energetically
this would be much more costly.
4.6.4.3 Estimation of Water Sorption in Wax and Thickness of the Waxy
Transpiration Barrier
In Sect. 4.6.3 a diffusion coefficient of about 10 −16 m 2 s −1 for water in cuticular
waxes isolated from the leaves of the species Ginko, Juglans and Prunus was
calculated (Table 4.12). This is significantly lower than Dw of water in synthetic
polymer membranes and MX membranes (Table 4.1), but it is similar to diffusion
coefficients for water calculated from extrapolated hold-up times obtained with CM
(Table 4.10). Can these diffusion coefficients be related to Pw measured for CM,
paraffin wax and aliphatic monolayers? The relationship of Pwv and Dw is given by
(2.18), assuming that the transport barrier of wax is homogeneous. The partition
coefficient Kww and the thickness of the wax barrier ℓ are needed to relate Dw to Pw.
In Table 4.12 Dw and Pw are listed, and the ratio Kww/ℓ can be calculated. With
wax coverages of 39, 51 and 83µgcm −2 for Ginko, Juglans and Prunus respectively,
ℓ can be calculated assuming that all the wax contributes to the formation of the barrier.
Using these values, wax/water partition coefficients for water can be calculated
Table 4.12 Diffusion coefficients in wax (Dw), permeances (Pw) of water across the CM, and
thickness ℓ of the wax layers in Ginkgo biloba, Juglans regia and Prunus laurocerasus. Wax/water
partition coefficients Kww are calculated by multiplying Kww/ℓ by ℓ. Thickness ℓ of the wax layer
was calculated dividing Kww by Kww/ℓ
Ginkgo Juglans Prunus
Dw (m 2 s −1 ) a 1.60 × 10 −16 1.06 × 10 −16 1.19 × 10 −16
Pw (ms −1 ) b 4.3 × 10 −10 6.3 × 10 −10 1.33 × 10 −10
ℓ (nm) calculated from wax amounts c 433 566 922
Pw/Dw = Kww/ℓ 2.69 × 10 6 5.94 × 10 6 1.11 × 10 6
Kww calculated d 1.16 3.36 1.03
ℓ (nm) calculated for Kww = 0.01 3.7 1.7 9.0
ℓ (nm) calculated for Kww = 0.04 14.9 6.7 36.1
a Calculated from equations listed in Table 6.10
b Data from Table 4.7
c Calculated from wax overages given in Table 4.7
d Kww calculated here have the units gram per volume (g water/cm 3 polymer)/(g water/cm 3 water)
and not gram per gram as given in Sect. 2.4