Fixed charge and transport numbers in isolated pepper fruit cuticles from membrane potential measurements: Donnan and diffusion potential contributions

Membrane potential of native isolated pepper cuticles were measured with NaCl and CaCl2 solutions for a wide range of concentrations (10BC(M)B10). From membrane potential results the fixed charge concentration, X, and the transport number of the ions in the cuticular membrane were obtained. Taking into account the X value, an estimation of the Donnan potential contribution to the membrane potential was carried out; it was found that the Donnan exclusion of co-ions only occurs at low concentrations, while at concentrations higher than 10 M the membrane potential is mainly due to the different mobility of the ions in the cuticular membrane (diffusion potential). Concentration dependence for both fixed charge concentration and transport numbers was also considered. The influence of chemical and thermal treatments on membrane potential and electrical resistance was also studied. A decrease in the experimental values of both parameters as a consequence of chemical treatment was found; results obtained with an annealed pepper membrane also show a reduction in the electrical resistance and in the contribution of the Donnan potential to the measured membrane potential. These facts indicate a more open structure for the pepper cuticles due to both different treatments. © 1999 Elsevier Science B.V. All rights reserved.