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

5.3 Cuticular Penetration of Electrolytes 137
able. There are indications that frequency of aqueous pores in cuticles decreases
during leaf expansion and maturation. Permeability to an Fe-chelate of Vitis vinifera
and Prunus persica leaves decreased dramatically during leaf expansion (Schlegel
et al. 2006). The upper astomatous leaf cuticle of poplar leaves can be isolated only
from leaves which are just fully expanded. With these leaves, relatively high rate
constants were measured for CaCl2, while permeability of the upper cuticle of older
leaves is almost zero (unpublished data). These two facts indicate a considerable
dynamic in development of aqueous pores and their closure in older leaves.
5.3.4 Size Selectivity of Aqueous Pores
Citrus MX membranes were almost impermeable to raffinose (Fig. 4.9), having a
molecular radius of 0.65 nm. The pentahydrate of raffinose has a molecular weight
of 595gmol −1 . Estimates of pore radii of other species are not available. Cuticles
over anticlinal walls and in cuticular ledges are permeable to berberine sulphate,
which has a molecular weight of 769gmol −1 . In Sect. 4.5 we argued that diffusion
of solutes in aqueous pores is increasingly hindered as solute size approaches pore
size. Size selectivity of aqueous pores in grey poplar CM (Schönherr and Schreiber
2004b) and Vicia faba leaf disks (Schlegel et al. 2005) has been studied using
calcium salts ranging in molecular weights from 111 to 755gmol −1 .
Anhydrous molecular weight was used in the original literature, since the hydration
numbers of these salts were not known. Size selectivity of the lipophilic or
waxy pathway has been characterised using equivalent molar volumes (Vx) calculated
according to McGowan and Mellors (1986). When size selectivity of aqueous
pores and of the waxy pathway are to be compared, the same variable for size
should be used. Vx of calcium salts was estimated based on a plot Vx vs molecular
weight (Fig. 5.9). The training set was composed of aliphatic and aromatic
organic molecules containing C, O and H atoms. With this set, Vx was smaller than
the molecular weight by a factor of 0.895. This factor was used to estimate Vx of calcium
salts (squares). A direct calculation of Vx was not possible since the equivalent
molar volume of Ca 2+ was not available.
Rate constants of penetration decreased with increasing molecular weight of the
Ca 2+ salts (Fig. 5.10), showing again coupling of cation and anion fluxes. Each divalent
calcium ion must be accompanied by two monovalent anions. Aqueous pores in
all cuticles were large enough to accommodate Ca-lactobionate having a molecular
weight of 755gmol −1 . A good linear correlation was obtained when logk was plotted
against molecular weights (solid lines) or molar volumes (dashed lines). With
Populus CM, rate constants decreased from 0.199 to 0.0285h −1 when molecular
weight increased from 100 to 500gmol −1 . Rate constants measured with Vicia leaf
disks were higher in light than in the dark, but slopes were similar. Broad bean leaves
have stomata and glandular trichomes on both leaf surfaces, while poplar adaxial
CM are astomatous and at the time of isolation unicellular trichomes have already
all been shed. The cuticle over guard cells and glandular trichomes is probably