The propagation of slow wave potentials in pea epicotyls

Slow wave potentials are considered to be electric long-distance signals specific for plants, although there are conflicting ideas about a chemical, electrical, or hydraulic mode of propagation. These ideas were tested by comparing the propagation of hydraulic and electric signals in epicotyls of pea (Pisum sativum L). A hydraulic signal in the form of a defined step increase in xylem pressure (P-x) was applied to the root of intact seedlings and propagated nearly instantly through the epicotyl axis while its amplitude decreased with distance from the pressure chamber. This decremental propagation was caused by a leaky xylem and created an axial P-x gradient in the epicotyl. Simultaneously along the epicotyl surface, depolarizations appeared with lag times that increased acropetally with distance from the pressure chamber from 5 s to 3 min. When measured at a constant distance, the lag times increased as the size of the applied pressure steps decreased. We conclude that the P-x gradient in the epicotyl caused local depolarizations with acropetally increasing lag times, which have the appearance of an electric signal propagating with a rate of 20 to 30 mm min(-1). This static description of the slow wave potentials challenges its traditional classification as a propagating electric signal.