THE PROPAGATION OF SLOW-WAVE POTENTIALS IN PEA EPICOTYLS

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