Cation currents in protoplasts from the roots of a Na⁺ hyperaccumulating mutant of Capsicum annuum

A wilty mutant (scabrous diminutive, sd) of Capsicum annuum L. hyperaccumulates Na⁺ in all tissues and has a lower K⁺ content in the roots. This has been shown to be due to a greater efflux of ⁸⁶Rb⁺ (K⁺) and influx of ²²Na ⁺ in the mutant. In this study, the transporters responsible for these fluxes were investigated by applying patch clamp techniques to protoplasts derived from root cortical cells. Inwardly rectifying K⁺ currents were comparable in the two genotypes, but a characteristically bigger outward K⁺ current was observed in protoplasts from mutant roots, correlating with a bigger efflux of ⁸⁶Rb⁺ from mutant plants. Whole-cell currents due to the movement of Na⁺ have also been studied in both genotypes. The magnitude of the time-independent inward currents that conduct Na⁺ at hyperpolarizing voltages were comparable in both genotypes. However, microelectrode measurements of membrane potentials in cortical cells of roots in high Na⁺ conditions revealed that the membrane potentials of the root cells in the mutants were approximately 60 mV more negative than in wild-type root cells. Quantitatively, this hyperpolarization is calculated to be sufficient to account for the increased Na⁺ influx in the mutants.