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

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Abstract
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 (86Rb+) was used as a tracer for K+. Plants were pretreated with Cu or Cd (0, 2, 5 or 25 μM) for 0–300 min and then transferred to radiolabeled nutrient solution (150 μM 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 μM Cu increased active K+ influx, whereas pretreatment with 25 μM Cu decreased active K+ influx and intermediate Cu concentrations (5 μM) 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 86Rb+ for 7 days before being exposed to Cu or Cd (0, 1, 3, 5 or 10 μM) in unlabeled nutrient solutions for 24 h. Net efflux of K+ was measured as 86Rb+-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.

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