Slow wave potentials in cucumber differ in form and growth effect from those in pea seedlings

A positive hydraulic signal in the form of a xylem pressure step was applied to the roots of intact seedlings of Cucumis sativus L. and Pisum sativum L. Surface electrodes at three positions along the epicotyl/hypocotyl recorded a propagating depolarization which appeared first in the basal, then the central and sometimes the apical electrode positions and fitted the characteristics of a slow wave potential (SWP). This depolarization differed between pea and cucumber. It was transient in cells of pea epicotyls but sustained in cucumber hypocotyls. It was not associated with a change in cell input resistance in pea epicotyls but preceded an increase in the input resistance of cucumber hypocotyl cells. With the increased xylem pressure the growth rate (GR) of cucumber hypocotyls and pea epicotyls underwent a transient increase peaking after 5 min. If the depolarization reached the growing upper region, it preceded a sustained decrease in the GR of cucumber hypocotyls but only a transient decrease in the GR of pea epicotyls. A temperature jump in the root medium (heat treatment) induced a steep pressure spike in the xylem of the cucumber hypocotyl which showed similar electric and growth effects as the previously applied, non-injurious pressure steps. We suggest that the observed differences in the electric and growth responses between the species were caused by the closure of ion channels in depolarized cells of cucumber but not pea seedlings.