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 a pplied to the roots of intact seedlings of Cucumis sativus L. and Pisu m sativum L. Surface electrodes at three positions along the epicotyl/ hypocotyl recorded a propagating depolarization which appeared first i n the basal, then the central and sometimes the apical electrode posit ions and fitted the characteristics of a slow wave potential (SWP). Th is 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 epicotyl s but preceded an increase in the input resistance of cucumber hypocot yl cells. With the increased xylem pressure the growth rate (GR) of cu cumber hypocotyls and pea epicotyls underwent a transient increase, pe aking after 5 min. If the depolarization reached the growing upper reg ion, it preceded a sustained decrease in the GR of cucumber hypocotyls bur only a transient decrease in the GR of pea epicotyls. A temperatu re jump in the root medium (heat treatment) induced a steep pressure s pike in the xylem of the cucumber hypocotyl which showed similar elect ric and growth effects as the previously applied, non-injurious pressu re steps. We suggest that the observed differences in the electric and growth responses between the species were caused by the closure of io n channels in depolarized cells of cucumber but not pea seedlings.