Water permeability of isolated cuticular membranes: The effect of pH and cations on diffusion, hydrodynamic permeability and size of polar pores in the cutin matrix.

The upper astomatous cuticle of Citrus aurantium L. leaves was isolated enzymatically or chemically, extracted with lipid solvents and used for the determination of water diffusion (P d ) and osmotic water permeability (P f ). The water permeability was strongly dependent on the pH value and the cations of the buffer solutions. In presence of monovalent alkali metal ions P d increased almost five fold between pH 3 and 11. The shape of the plot P d vs. pH suggests the presence of 3 different dissociable groups fixed to the membrane matrix. They are tentatively identified as two carboxyl groups dissociating between pH 3 to 6 and 6 to 9, respectively, and as phenolic hydroxyl groups dissociating above pH 9. The carboxyl group dissociating between pH 6 and 9 discriminated between alkali metal ions according to their ionic radius. Water permeability was lowest in the Li(+) from and increased in the order Li(+)<Na(+)<K(+)<Rb(+). The water permeability of membranes in Ca(2+) form was only slightly higher than that of membranes in H(+) form and little dependent on pH. The energy of activation which amounted to 13 kcal mol(-1) was constant over the temperature range of 5 to 40°C and pH independent. Since P f was greater than P d it was concluded that the cutin matrix contained polar pores and that water transport caused by a chemical potential gradient was both by diffusion and by viscous flow. The porous nature of the membranes was also confirmed by the fact that they are permselective according to size of the permeating molecule. Using the empirical equations of Paganelli and Solomon (1957) and Nevis (1958) the equivalent radius of the pores was estimated to be 0.46 and 0.45 nm, respectively. This estimate is in good agreement with the observations that (a) [(14)C]urea (molecular radius r s =0.264 nm) and [(3)H]glucose (r s =0.444 nm) penetrated the membranes and (b) the reflection coefficient was equal to one for raffinose (r s =0.654 nm) and sucrose (r s =0.555 nm) but 0.95 for glucose and 0.78 for urea. Both, the reflection coefficient and the pore radius estimates were pH independent, hence the increase in water permeability with increasing pH was due to an increase in the number of pores per unit area (1 cm(2)) from 5x10(10) at pH 3 to 15.8x10(10) at pH 9.