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

Abstract

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, Zn^2+, , 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.