The Action of Divalent Zinc, Cadmium, Mercury, Copper and Lead on the Trans-Root Potential and H+, Efflux of Excised Roots

Kennedy, C. D. and Gonsalves, F. A. N. 1987. The action of divalent zinc, cadmium, mercury, copper and lead on the trans-root potential and H+ efflux of excised roots.—J. exp. Bot. 38: 800–817. The action of Zn2+, Cd2+, Hg2+, Cu2+ and Pb2+ ions on the trans-root potential and H+ efflux of young excised maize roots has been studied. Micro-electrode implantations into root epidermal cells confirmed the root outer membranes as the major contributor in the trans-root potential changes. The effects of these ions on H+ efflux were studied over a period of time in a continuous flow cell apparatus, adequate controls allowing for transient interference due to divalent cations at the pH probe. The addition of Zn2+, 5 to 100 μmol dm−3, to the solution bathing the roots reduces H+ efflux and depolarizes the trans-root potential. However, in the presence of Mg2+, 0·1 or 1·0 mmol dm−3, not only is this depolarization inhibited, but hyperpolarization is observed instead. Cd2+ affects trans-root potential and H+ efflux at a much slower rate than Zn2+, suggesting a lower membrane permeability. Without Mg2+, Cd2+ hyperpolarizes the trans-root potential, but this is better sustained in its presence. Hyperpolarization did not occur with Hg2+, Cu2+ or Pb2+ whether or not Mg2+ was present Hg2+ and to a lesser extent Cu2+ are potent depolarizers of the trans-root potential and strongly inhibit H+ efflux. The maximum rates of depolarization observed in the absence of Mg2+ increase in the order Cd ≈ PCMBS ≪.lt; Zn ≈ Cu < Hg. This is similar to the relative maximum rates of H+ inhibition, Pb ≈ Cd ≪.lt; Zn < Cu < Hg, suggesting considerable differences in mode of action and/or membrane permeability. The lower membrane permeability of the sulphydryl reagent PCMBS apparently prevents ready access to the site(s) of action available to Hg2+. The reductions in trans-root potential and H+ gradients induced by this range of cations would be detrimental to the acquisition of nutrients using these gradients as an energy source. In contrast, Zn2+, , in the presence of adequate Mg2+, could be beneficial to nutrient uptake by maintaining a higher membrane potential than would occur in its absence. Possible modes of action for the observed effects are discussed.