Responding to cold stimuli and spatial propagation of electrical signals in Mimosa pudica

Mechanisms underlying electrical signaling in the petiole of Mimosa pudica were investigated, with a focus on the generation and propagation of action potentials in response to external cold stimuli. Cold water was applied to specific locations along the petiole to induce stimulation, and changes in the potential differences between four points along the petiole and a reference electrode inserted into the soil were recorded. A sharp negative peak (approximately –100 mV), corresponding to the generation of an action potential, was first observed near the stimulation site and subsequently propagated along the petiole toward both the leaves and the stem. Although the propagation rate of the action potential varied among individual plants, it remained nearly constant at 10 ± 4 mm s⁻¹. Immediately after the action potential reached the base of the petiole, known as the main pulvinus, a positive electrical signal propagated simultaneously throughout the entire region. At this time, the electrical resistance between the stem and the petiole via the main pulvinus decreased abruptly. Because it is well-known that an aqueous solution is released from the main pulvinus into the extracellular domain (mainly the petiole), the sudden decrease in electrical resistance and the positive shift in potential differences around the main pulvinus are thought to result from the release of this aqueous solution. In addition, the temperature-response mechanism of Mimosa pudica was elucidated by comparison with the temperature-dependent characteristics of the membrane potential between two aqueous phases separated by a cation-exchange membrane.