Membrane potentials and ion permeabilities in flexor cells of the laminar pulvini of Phaseolus coccineus L.

The internal potential of flexor cells in slices of the laminar pulvini of Phaseolus coccineus has been recorded by standard microelectrode techniques in 100 eq m(-3) external salt solutions of various ionic compositions. The measured values are between-15 and-60 mV depending on the external medium. Treating the results with the Goldman equation yields the following relative permeabilities: K(+), 1.00; Na(+), 0.24; Cl(-), 0.19; NO 3 (-) , 1.6. The membrane potential was only slightly sensitive to external pH and Ca(2+). Metabolic inhibitors (azide, cyanide and salicylhydroxamic acid, carbonyl cyanid m-chlorphenyl hydrazone) caused only slight depolarizations (ca. 4 mV), which differed from the ion-induced changes by their slow time courses. The results are consistent with the hypothesis that the relatively impermeable Cl(-) is actively transported and osmotically efficient, whereas the well-permeable K(+) passively follows Cl(-) to maintain electroneutrality and is osmotically of only minor significance.