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

4.6 Water Permeability of Isolated Astomatous Cuticular Membranes 103
Effect of humidity
2.4 a CM control
2.0
CM methylated
1.6
1.2
0.8
1.6
1.4
1.2
1.0
b
MX control
MX methylated
0 10 20 30 40 50 60 70 80 90 100
Relative humidity (%)
Fig. 4.18 Effect of relative humidity in the receiver on water permeance (Pw) of Prunus laurocerasus
CM and MX at 25 ◦ C. Permeance was measured using tritiated water in the donor and
isolated astomatous cuticular membranes (CM), or polymer matrix membranes (MX). Methylation
of membranes was obtained by diazomethane. Error bars are 95% confidence intervals. Data
were taken from Schreiber et al. (2001)
equal to 9.72 ×10 −10 ms −1 , Pw and PBA are numerically equal. If PBA is lower, then
Pw > PBA, and if PBA is larger than 9.72×10 −10 ms −1 , Pw < PBA. For example, when
PBA = 10 −8 ms −1 , Pw is 6.3 × 10 −9 ms −1 . With species having PBA of 10 −10 ms −1 ,
the calculated Pw is a little higher (1.2 × 10 −10 ms −1 ). This could be taken as evidence
that in CM from species having a low permeance for benzoic acid, some water
diffused in aqueous pores, while in CM having a high BA permeance, all or most
water diffuses in the waxy path, and the contribution of a parallel aqueous paths
goes unnoticed. In view of the large error bars seen in Fig. 4.17, this conclusion
may very well be wrong.
Both permeances are related to D, K and ℓ, and this can be written as
logPw
logPBA
= log(DwKww)/ℓ
= 0.86. (4.20)
log(DBAKBA)/ℓ
If water and benzoic acid use the same waxy pathway,ℓ is the same and cancels. Kww
for benzoic acid is around 20 for waxes of P. laurocerasus, G. biloba and J. regia.
The respective DBA values in waxes of these species are 3.5 × 10 −17 , 4.4 × 10 −17
and 5.6×10 −17 m 2 s −1 . This is the sequence seen on the regression line in Fig. 4.17.
Dw × Kww is not known for these three species, but their products should be smaller
by a factor of 7.2 (10 0.86 ). The wax–water partition coefficient for water is much
smaller than for benzoic acid, which implies that D for water must be considerably
larger than for benzoic acid.