Short- and long-term effects of cadmium on transmembrane electric potential (Em) in maize roots

In this study, the effects of Cd on root growth, respiration, and transmembrane electric potential (Em) of the outer cortical cells in maize roots treated with various Cd concentrations (from 1 micro M to 1 mM) for several hours to one week were studied. The Em values of root cells ranged between -120 and -140 mV and after addition of Cd they were depolarized immediately. The depolarization was concentration-dependent reaching the value of diffusion potential (Ed) when the Cd concentration exceeded 100 micro M. The values of ED ranged between -65 to -68 mV (-66 +/- 1.42 mV). The maximum depolarization of Em was registered approx. 2.5 h after addition of Cd to the perfusion solution and in some cases, partial (Cd > 100 micro M) or complete repolarization (Cd > 100 micro M) was observed within 8-10 h of Cd treatment. In the time-dependent experiments (0 to 168 h) shortly after the maximum repolarization of Em a continuous concentration-dependent decrease of Em followed at all Cd concentrations. Depolarization of Em was accompanied by both increased electrolyte leakage and inhibition of respiration, especially in the range of 50 micro M to 1 mM Cd, with the exception of root cells treated with 1 and 10 micro M Cd for 24 and 48 h. Time course analysis of Cd impact on root respiration revealed that at higher Cd concentrations (> 50 micro M) the respiration gradually declined (~ 6 h) and then remained at this lowest level for up to 24 h. All the Cd concentrations used in this experiment induced significant inhibition of root elongation and concentrations higher than 100 micro M stopped the root growth within the first day of Cd treatment. Our results suggest that Cd does not cause irreversible changes in the electrogenic plasma membrane H+ATPase because fusicoccin, an H+ATPase activator diminished the depolarizing effect of Cd on the Em. The depolarization of Em in the outer cortical cells of maize roots was the result of a cumulative effect of Cd on ATP supply, plasmalemma permeability, and activity of H+ATPase.