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 Na Cl and CaCl2 solutions for a wide range of concentrations (10 (-3) < C(M) < 10 (-1)). From membrane potential results the fixed charge concentration, X, and the transport number of the ions in the cuticular membrane were obta ined.:Taking into account the X value, an estimation of the: Donnan potenti al contribution to the membrane potential was carried out; it was found tha t the Donnan exclusion of co-ions only occurs at low concentrations, while at concentrations higher than 10 (-2) M the membrane potential is mainly du e to the different mobility of the ions in the cuticular membrane (diffusio n potential). Concentration dependence for both fixed charge concentration and transport numbers was also considered. The influence of chemical and th ermal treatments on membrane potential and electrical resistance was also s tudied. A decrease in the experimental values of both parameters as a conse quence of chemical treatment was found; results obtained with an annealed p epper membrane also show a reduction in the electrical resistance and in th e contribution of the Donnan potential to the measured membrane potential. These facts indicate a more open structure for the pepper cuticles due to b oth different treatments. (C) 1999 Elsevier Science B.V. All rights reserve d.