GROWTH, MEMBRANE-POTENTIAL AND ENDOGENOUS ION CURRENTS OF WILLOW (SALIX-VIMINALIS) ROOTS ARE ALL AFFECTED BY ABSCISIC-ACID AND SPERMINE

Growth measurements of hormone-treated roots from willow cuttings were combined with electrophysiological recordings to study hormone-induce d changes in membrane potential and in endogenous ion currents. The me an growth rate of roots was 10 +/- 2 mu m min(-1) in regular nutrient solution. It increased to 13 +/- 2 mu m min(-1) after application of s permine and decreased to 0.07 +/- 0.01 mu m min(-1) after treatment wi th abscisic acid (ABA). Transient depolarizations were elicited in roo t cortex cells by spermine, while ABA caused a transient hyperpolariza tion. All changes in membrane potential were accompanied by transient responses of the endogenous current. These responses suggest that firs t anions, then cations leave the root during spermine-induced depolari zations. From the changes of the endogenous current an apparent efflux of anions (presumably Cl-) and cations (presumably K+) of 200 to 700 pmol cm(-2) per depolarization was calculated. To further investigate a possible relation between endogenous ion currents, growth and the gr owth regulators ABA and spermine, long-lasting extracellular vibrating -probe measurements were performed. Control roots showed an inward cur rent of about 1.5 mu A cm(-2) at the apical elongation zone and an out ward current with a maximum density of 1.3 mu A cm(-2) at the central and basal elongation zone. The addition of ABA and spermine (final con centration 0.1 mM) to the bathing medium affected the endogenous curre nt in opposite ways: ABA caused a reduction of inward and outward curr ent, while spermine stimulated both. Since protons are a major compone nt of the endogenous current, and sucrose can be taken up by root cell s from the apoplast via symport with H+, a role of the endogenous curr ent in growth regulation is indicated.