Membrane potential differences and viability of grapevine root cells treated with HgCl2

The effect of mercury (Hg) on the electrophysiological and permeability properties of grapevine adventitious root cells was examined. In short-term experiments, the apical segments of adventitious roots were treated with different concentrations of mercury (0.01 µmol/L to 2 µmol/L HgCl2), and trans-membrane electrical potential differences (EM) were monitored in root cortical cells, localized in distinct root zones. Based on Hg-induced decrease of EM, we can confirm that the depolarization of the membrane is an instant Hg-response and the extent of EM decrease is not only time- and concentration dependent, but also related to the developmental stage of cells and their localization on root axis. The sensitivity of root cells to Hg declined in the direction of cell division zone > cell elongation zone > absorption zone. Long-term treatment of roots with 2 µmol/L mercury showed that Hg-induced decrease of EM was accompanied by an increase of K+ efflux and a decrease of the diffusion potential (ED) values. Application of fusicoccin (H+-ATPase activator) to the root medium caused an immediate hyperpolarization of the membrane in control and Hg-treated cells. Laser scanning confocal microscopy analysis confirmed that Hg reduced cell viability, which was accompanied with the occurrence of cell death hallmarks, like condensation of protoplasts, nuclei fragmentation and deposition of granular material.