Effects of copper and cadmium on uptake and leakage of K(+) in birch (Betula pendula) roots

Uptake and leakage experiments were performed to study the effects of copper and cadmium on K(+) fluxes in birch (Betula pendula Roth) roots. Labeled rubidium ((86)Rb(+)) was used as a tracer for K(+). Plants were pretreated with Cu or Cd (0, 2, 5 or 25 microM) for 0-300 min and then transferred to radiolabeled nutrient solution (150 microM K(+)) with or without 2,4-dinitrophenol (DNP) to separate the effects of heavy metal on active and passive K(+) influxes. Passive K(+) influx was decreased by pretreatment with Cu but was only slightly affected by pretreatment with Cd. Pretreatment with 2 microM Cu increased active K(+) influx, whereas pretreatment with 25 microM Cu decreased active K(+) influx and intermediate Cu concentrations (5 microM) did not affect active K(+) influx. The pretreatment effects of Cu on active and passive K(+) influxes increased with increasing pretreatment time. During the first hour, pretreatment with Cd decreased active K(+) influx with increasing pretreatment time, whereafter recovery began. To measure K(+) efflux, birch plants were loaded with (86)Rb(+) for 7 days before being exposed to Cu or Cd (0, 1, 3, 5 or 10 microM) in unlabeled nutrient solutions for 24 h. Net efflux of K(+) was measured as (86)Rb(+)-activity in the nutrient solutions 24 h after a heavy metal had been introduced. Efflux of K(+) increased with increasing Cu or Cd concentration in the unlabeled nutrient solution.The data indicate that Cu and Cd affected K(+) influx differently and that recovery mechanism(s), which were induced shortly after heavy metal introduction, counteracted the heavy-metal induced inhibition of active K(+) influx. Efflux of K(+) from plant roots over a 24-h period indicated that Cu and Cd had similar effects on K(+) efflux. There was no evidence of a recovery mechanism counteracting the heavy-metal-induced inhibition of K(+) efflux.