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

106 4 Water Permeability
Similar counter-diffusion experiments were conducted with CM isolated from
15 species (Table 4.8). Water permeability of 13 species significantly decreased; the
exceptions were Nerium oleander and Hedera helix. These data suggest that in most
species water moved in two parallel pathways, aqueous pores and a lipophilic pathway
composed of cutin and waxes (model III B). After precipitation of AgCl, aqueous
pores are blocked and most water transport observed after treatment occurred
in the lipophilic pathway. If AgCl precipitates completely blocked aqueous pores,
15% (Stephanotis) to 64% (Populus canescens) of the water moved along aqueous
pores and 85–36% diffused in the waxy path.
Water permeance of Citrus MX membranes as determined using the cup method
is 1.6 × 10 −7 ms −1 (Schönherr and Lendzian 1981). It was estimated that about half
of this penetrated in cutin and the other half in aqueous pores (Sect. 4.5.2). The waxy
pathway in Citrus CM had a permeance of 1.23 × 10 −10 ms −1 (Table 4.8), which
suggests that waxes associated with cutin reduced permeance by a factor of 1,300.
Permeance of the aqueous path in fully swollen Citrus MX is 1.3 × 10−7 ms−1 (Sect. 4.5.2), while in Citrus CM P aqueous
w is 0.64 × 10−10 ms−1 , which is smaller
than in the MX by a factor of 2,031. It appears that most of the aqueous pores
observed in the MX were eliminated or covered in the CM by waxes. From the original
4.54×10 14 aqueous pores (Table 4.5), only 2.23×10 11 survived incrustation of
the limiting skin with waxes. These surviving pores respond to partial vapour pressure
just like those in the MX, because Pw of Citrus CM increased with humidity.
These data are compatible with model III C.
Table 4.8 Permeances (ms−1 ) of cuticular membranes isolated from 15 different species measured
before Ptotal w and after P AgCl
w the formation of AgCl precipitates. Results are means of at least
12 CM. P aqueous
w is the difference between Ptotal w and P AgCl
w . (Data from Schreiber et al. 2006)
Species P total
w × 10 10 P AgCl
w × 10 10 P aqueous
w × 10
(ms −1 ) (ms −1 ) (ms −1 )
10 Paqueous w
Leaf CM
Nerium oleander 0.71 0.73 0 0
Hedera helix 1.49 1.46 0 0
Stephanotis floribunda 4.71 4.01 0.70 0.15
Forsythia intermedia 1.97 1.58 0.39 0.20
Ligustrum vulgare 4.03 3.19 0.84 0.21
Vinca major 1.20 0.95 0.25 0.21
Prunus laurocerasus 1.10 0.76 0.34 0.31
Citrus aurantium 1.87 1.23 0.64 0.34
Juglans regia 4.51 2.96 1.55 0.34
Syringia vulgaris 3.51 2.06 1.09 0.41
Pyrus communis 8.30 3.92 4.38 0.53
Populus canescens 26.8 9.64 17.16 0.64
Fruit CM
Malus domestica 3.20 2.17 1.03 0.32
Lycopersicon esculent.. 24.8 15.7 9.10 0.37
Prunus domestica 33.2 12.2 21.0 0.63
P total
w