Membrane Potentials and Resistances of Excitable Cells in the Petiole and Main Pulvinus of Mimosa pudica

Resting potentials of excitable cells in the petiole and main pulvinus of Mimosa pudica depended on the external concentrations of potassium (K+) and sodium (Na+), and peaks of the action potentials depended on the external concentration of chloride (Cl~) in the excitable protoxylem cells of the petiole and the upper-half cells of the main pulvinus, which suggests that the action potential in these cells is a Cl~-spike. In the lower-half cells of the main pulvinus, the peak of the action potential did not show a clear dependence on K+, Na+ or Cl~. This implies that there is a decrease in the ion selectivity of the membrane toward the peak of the action potential. Effective membrane resistances of the excitable protoxylem and phloem cells of the petiole, and the lower-half cells of the main pulvinus were 3.9±3.1 MQ (mean±SD, n = 7), 9.4±8.2 MSi (n = 5) and 5.0±2.9 MJ2 (n=15), respectively. The membrane resistance of the lower-half cells of the main pulvinus decreased on its sudden bending, but not always.