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

108 4 Water Permeability
Table 4.9 Water permeance (Pwv) and diffusion coefficients (Dw) of cuticular membranes at 25 ◦ C
Species te (s) ℓ(µm) Pwv(ms −1 ) Dw(m 2 s −1 ) Cw(gkg −1 ) Cw(gkg −1 ) a
Schefflera L. 930 2.96 8.19 × 10 −7 1.57 × 10 −15 33 –
Clivia L. 770 6.50 1.14 × 10 −6 9.14 × 10 −15 17 –
Hedera L. 933 4.33 2.66 × 10 −6 3.35 × 10 −15 73 –
Nerium L. 960 12.81 3.25 × 10 −6 2.84 × 10 −14 34 24
Ficus L. 512 5.68 4.25 × 10 −6 1.05 × 10 −14 48 30
Citrus L. 264 2.87 1.20 × 10 −5 5.20 × 10 −15 139 63
Pyrus L. 550 3.12 1.22 × 10 −5 2.95 × 10 −15 270 43
Solanum F. 640 6.47 2.23 × 10 −5 1.08 × 10 −14 244 –
Capsicum F. 361 8.03 9.28 × 10 −5 2.98 × 10 −14 523 38
Lycopersicon F. 215 8.00 1.42 × 10 −4 4.96 × 10 −14 477 43
D was calculated from the hold-up time and the total thickness (ℓ) of CM; Cw is sorption of water
at 25 ◦ C and 100% humidity calculated from P/D [(3.17) and (3.20)] using the data by Becker
et al. (1986). L is leaf CM and F is fruit CM. (Data from Becker et al. 1986)
a Data taken from Chamel et al. (1991) derived from water vapour sorption
Permeance varied by a factor of 173, and ranged from 8.19 × 10 −7 (Schefflera
leaf) to 1.42 × 10 −4 ms −1 (tomato fruit). Diffusion coefficients varied by a factor of
only 32, and ranged from 1.57 × 10 −15 (Schefflera) to 4.96 × 10 −14 m 2 s −1 (tomato
fruit). According to theory [Chap. 2; (2.18)] one might conclude that variations in
membrane thickness and partition coefficient caused these differences. The Dw values
are much lower than those for synthetic polymers, PVA being the only exception
(Table 4.1). With some CM (Nerium and Ficus), water sorption (Cw) derived from
the Pwv/Dw ratio is low and similar to sorption in CM when determined gravimetrically
(Table 4.9, last column). With the other CM, water concentration calculated
from Pwv/Dw is unreasonably high, and much larger than that determined by a
sorption experiment. This indicates structural heterogeneity of these membranes.
When calculating Dw from hold-up time, the thickness of the CM enters into consideration.
Becker et al. (1986) used the weight average thickness, while the waxy
limiting skin or the cuticle proper is the limiting barrier both in water and solute
diffusion (Sects. 4.6 and 6.5). The thickness of the limiting skin in CM of various
plant species is not known, but plausible estimates are 100–500 nm (Sect. 1.4). In
calculating Dw from (2.5), thickness of the limiting barrier enters as ℓ 2 , and using a
thickness less than total thickness of the CM would result in lower diffusion coefficients
than those in Table 4.9. In Sect. 6.5.2 we estimate the diffusion coefficient
of water in cuticular wax as 1.2 × 10 −16 m 2 s −1 . This is about a factor of 10 lower
than Dw values in leaf CM estimated from the hold-up time (Table 4.9). With Citrus
CM, a Dw of 1.2 × 10 −16 m 2 s −1 would have been obtained with ℓ equal to 0.44µm.
This amounts to 15% of the total thickness of the CM and appears plausible. This
neglects the fact that the waxy diffusion path of water is tortuous, but in calculating
Dw for synthetic polymers membrane thickness (ℓ) is used (Table 4.1) and tortuosity
is also disregarded.
We can further test these cuticles for homogeneity by plotting Pwv vs 1/ℓ and Dw
vs 1/6× hold-up time. In homogenous membranes, these plots should be linear. The