Growth, membrane potential and endogenous ion currents of willow (Salix viminalis) roots are all affected by abscisic acid and spermine

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Abstract
Growth measurements of hormone‐treated roots from willow cuttings were combined with electrophysiological recordings to study hormone‐induced changes in membrane potential and in endogenous ion currents. The mean growth rate of roots was 10 ± 2 μm min−1 in regular nutrient solution. It increased to 13 ± 2 μm min+1 after application of spermine and decreased to 0.07 ± 0.01 μm min−1 after treatment with abscisic acid (ABA). Transient depolarizations were elicited in root cortex cells by spermine, while ABA caused a transient hyperpolarization. All changes in membrane potential were accompanied by transient responses of the endogenous current. These responses suggest that first anions, then cations leave the root during spermine‐induced depolarizations. 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 growth regulators ABA and spermine, long‐lasting extracellular vibrating‐probe measurements were performed. Control roots showed an inward current of about 1.5 μA cm−2 at the apical elongation zone and an outward current with a maximum density of 1.3 μA cm−2 at the central and basal elongation zone. The addition of ABA and spermine (final concentration 0.1 mM) to the bathing medium affected the endogenous current in opposite ways: ABA caused a reduction of inward and outward current, while spermine stimulated both. Since protons are a major component of the endogenous current, and sucrose can be taken up by root cells from the apoplast via symport with H+, a role of the endogenous current in growth regulation is indicated.