INDUCTION AND IONIC BASIS OF SLOW-WAVE POTENTIALS IN SEEDLINGS OF PISUM-SATIVUM L

Slow wave potentials (SWPs) are transient depolarizations which propag ate substantial distances from their point of origin. They were induce d in the epidermal cells of pea epicotyls by injurious methods such as root excision and heat treatment, as well as by externally applied, d efined steps in xylem pressure (P-x) in the absence of wounding. The c ommon principle of induction was a rapid increase in P-x. Such a stimu lus appeared under natural conditions after (i) bending of the epicoty l, (ii) wounding of the epidermis, (iii) rewatering of dehydrated root s, and (iv) embolism. The induced depolarization was not associated wi th a change in cell input resistance. This result and the ineffectiven ess of ion channel blockers point to H+-pumps rather than ion channels as the ionic basis of the SWP. Stimuli such as excision, heat treatme nt and pressure steps, which generate SWPs, caused a transient increas e in the fluorescence intensity of epicotyls loaded with the pH-indica tor DM-NERF, a 2', 7'-dimethyl derivative of rhodol, but not of those loaded with the pH indicator -bis-(2-carboxyethyl)-5-(and-6)-carboxyfl uorescein (BCECF). Matching kinetics of depolarization and pH response identify a transient inactivation of proton pumps in the plasma membr ane as the causal mechanism of the SWP. Feeding pump inhibitors to the cut surface of excised epicotyls failed to chemically simulate a SWP; cyanide, azide and 2,4-dinitrophenol caused sustained, local depolari zations which did not propagate. Of all tested substances, only sodium cholate caused a transient and propagating depolarization whose arriv al in the growing region of the epicotyl coincided with a transient gr owth rate reduction.