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

6.2 Steady State Penetration 157
on average values of P and D measured or calculated for individual MX membranes.
Accounting for considerable variability among individual membranes, the agreement
between corrected Kcalc and with Kdet is good to fair. Good agreement obtained
with MX from fruits and leaves of Citrus and Olea indicates that 2,4-D diffused in
cutin and cutin is fairly homogeneous throughout the MX membranes. With Clivia,
Ficus and Nerium MX, corrected Kcalc are still considerably lower than Kdet. Cuticles
of these three species have two different types of cutins, i.e., ester cutin near the
epidermal cell wall and cutan at the outer surfaces of the CM (cf. Sects. 1.2 and 1.4).
These types of cutin differ in polarity and structure, and it appears that D in cutan is
lower than in ester cutin. This could account for low Kcalc seen in Table 6.4. Hence
these three species still have a heterogeneous MX, but heterogeneity is much less
than in CM.
Having established that permeance of 2,4-D varies widely among species, we
now turn to the role of partition coefficients as determinants of P. Equation (6.6)
states that P is proportional to the partition coefficient, and Kerler and Schönherr
(1988b) measured permeance of Citrus CM using a selection of important
agricultural and environmental chemicals (Fig. 6.3). Solutes 1–5 are weak electrolytes,
and donor solutions were appropriately buffered to assure high and constant
log Permeance (m/s)
0.020
−5
−6
−7
−8
−9
−10
8
1 2
3
3
4
log K/V x (mol/cm 3 )
0.022 0.024 0.026 0.028 0.030 0.032
2
1
4
−11
1 2 3 4 5 6 7 8
log K
5
6
log P = 192 log K/V x - 13.2 (r 2 = 0.98)
Fig. 6.3 Logarithms of permeance P of Citrus aurantium CM measured at 25 ◦ C as a function
of log Kcw of solutes (red circles). Log P plotted vs log Kcw/Vx is shown as green squares.
Numbers refer to the test compounds used: 4-nitrophenol (1), 2,4-D (2), atrazine (3), 2,4,5trichlorophenoxyacetic
acid (4), pentachlorophenol (5), hexachlorobenzene (6), perylene (7),
diethylhexyl phthalate (8). The linear regression equation applies to the squares, with the exception
of compounds 6 and 8 (grey squares). Data were replotted or recalculated from data given by
Kerler and Schönherr (1988a, b). Vx is the equivalent molar volume of solutes in cm 3 mol −1
5
7
7
6
8