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 divalentzinc, cadmium, mercury, copper and lead on the trans-root potentialand H⁺ efflux of excised roots.—J. exp. Bot. 38: 800–817. The action of Zn²⁺, Cd²⁺, Hg²⁺, Cu²⁺ and Pb²⁺ ions on the trans-rootpotential and H⁺ efflux of young excised maize roots has beenstudied. Micro-electrode implantations into root epidermal cellsconfirmed the root outer membranes as the major contributorin the trans-root potential changes. The effects of these ionson H⁺ efflux were studied over a period of time in a continuousflow cell apparatus, adequate controls allowing for transientinterference due to divalent cations at the pH probe. The addition of Zn²⁺, 5 to 100 µmol dm^–3, to thesolution bathing the roots reduces H⁺ efflux and depolarizesthe trans-root potential. However, in the presence of Mg²⁺,0?1 or 1?0 mmol dm^–3, not only is this depolarizationinhibited, but hyperpolarization is observed instead. Cd²⁺ affectstrans-root potential and H⁺ efflux at a much slower rate thanZn²⁺, suggesting a lower membrane permeability. Without Mg²⁺,Cd²⁺ hyperpolarizes the trans-root potential, but this is bettersustained in its presence. Hyperpolarization did not occur withHg²⁺, Cu²⁺ or Pb²⁺ whether or not Mg²⁺ was present Hg²⁺ andto a lesser extent Cu²⁺ are potent depolarizers of the trans-rootpotential and strongly inhibit H⁺ efflux. The maximum rates of depolarization observed in the absenceof 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, suggestingconsiderable differences in mode of action and/or membrane permeability.The lower membrane permeability of the sulphydryl reagent PCMBSapparently prevents ready access to the site(s) of action availableto Hg²⁺. The reductions in trans-root potential and H⁺ gradients inducedby this range of cations would be detrimental to the acquisitionof nutrients using these gradients as an energy source. In contrast,Zn^2+, , in the presence of adequate Mg²⁺, could be beneficialto nutrient uptake by maintaining a higher membrane potentialthan would occur in its absence. Possible modes of action for the observed effects are discussed. Key words: Trans-root potentials, H⁺ efflux, heavy metal ions