Influence of calcium, magnesium, sodium, potassium and pH on copper toxicity to barley (Hordeum vulgare)

The extent to which Ca²⁺, Mg²⁺, Na⁺, K⁺ ions and pH independently influence copper toxicity to barley (Hordeum vulgare) was assessed by measuring root growth in nutrient solutions. Increased Ca²⁺ activity resulted in a sixfold decrease in EC50_Cu²⁺ values, while a positive relationship between the cation activity and the EC50 was expected. Increased Mg²⁺ activity resulted in a twofold increase in EC50_Cu²⁺ values. Na⁺, K⁺ and H⁺ activities did not significantly affect Cu²⁺ toxicity. The obtained results indicated that competition for binding sites between Cu²⁺ and cations such as Ca²⁺, Mg²⁺, Na⁺, K⁺ and H⁺ is not an important factor in determining Cu²⁺ toxicity to H. vulgare. However, the EC50s could, with one exception, be predicted within a factor three based on the free Cu²⁺ activity, indicating that the free Cu²⁺ activity cannot only be used to predict metal toxicity to aquatic, but also to terrestrial organisms.