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

80 4 Water Permeability
Accounting for the diffusional component present in Pviscous, Nevis (1958) wrote the
above equation as
�
Pviscous − Pdiffusion
rpore = 0.36
, (4.13)
Pdiffusion
which gives the average or equivalent pore radius in nm. In deriving (4.13), the bulk
quantities for the diffusion coefficient (Dw) and viscosity (η) of water are used.
These equations have been employed to estimate equivalent pore radii in the
polymer matrix from Citrus leaves (Schönherr 1976a) and onion bulb scales
(Schönherr 1976b). Beyer et al. (2005) published some data from which pore radii
of pepper and tomato fruit CM can be calculated. Data for other species are not
available.
Diffusion of water across polymer matrix membranes from Citrus leaves was
measured using tritiated water. When both buffer solutions contained 0.1 N, NaCl
permeance (Pdiffusion) was higher above pH 4 than in presence of 0.1 N CaCl2. The
effects of pH and type of counter ions demonstrate the involvement of three different
weakly acidic groups in the MX, as was already discussed (Sect. 4.3). Above pH 9,
phenolic hydroxyl groups are responsible for the effect of pH on permeance of MX
membranes in Na + form. Data for MX membranes in Ca 2+ form above pH 8 are not
available. Under natural conditions pH in the MX will certainly be