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

144 5 Penetration of Ionic Solutes
6. How is cuticular penetration across grey poplar CM affected when the molecular
volumes of Ca 2+ salts increase from 100 to 500cm 3 mol −1 ?
7. How does light affect cuticular penetration of Ca salts across leaf surfaces of
Vicia faba?
Solutions
1. The partition coefficient of polar compounds (e.g., sugars) between the external
aqueous phase and the lipophilic cutin and wax phase is very low. Consequently,
according to (2.18) permeance P is very low.
2. Ionic solutes strongly bind hydration water, and this renders them insoluble in
lipophilic phases like cutin and wax.
3. Trichomes, stomatal ledges and anticlinal cell walls are sites of the leaf surface
where aqueous pores are preferentially located.
4. For this problem, (5.1) must be solved for Mt/M0 = 0.5. The half-time of
penetration t 1/2 is 6,931 s or 1.92 h.
5. Humidity affects cuticular penetration of salts in two different ways. It interacts
both with the salt deposit on the cuticle surface and with the cuticular membrane.
(a) Humidity must be higher than the POD of the salt, otherwise the salt
crystallises and it becomes immobile. The salt dissolves only above the POD. (b)
With increasing humidity, more water is sorbed to polar functional groups in the
cutin matrix, and this increases the number of aqueous pores, which in turn leads
to increased rates of salt penetration.
6. Based on the regression equation shown in Fig. 5.12, rate constants decrease
from 0.30 to 0.011h −1 , which is a factor of 27.
7. Size selectivity is not affected (Table 5.1), but rate constants of penetration are
higher in light (0.32h −1 ) than in the dark (0.17h −1 ). This is attributed to an
increased number of aqueous pores in stomatal ledges in light when stomata are
open.