Pb and Cd uptake in rice roots

Pb and Cd are heavy metal pollutants that inhibit plant growth. Using a cultivated rice variety (Dongjin, Oryza sativa L.), we studied how the transport and toxicity of Pb 2+ and Cd 2+ are affected by the presence of K + , Ca 2+ or Mg 2+ . K + had a little effect on uptake or toxicity of Pb 2+ and Cd 2+ . Ca 2+ or Mg 2+ blocked both Cd 2+ transport into rice roots and Cd 2+ toxicity on root growth, which suggested that their detoxification effect is directly related to their blocking of entry of the heavy metals. Similarly, Ca 2+ blocked both Pb 2+ transport into the root and Pb 2+ toxicity on root growth. The protective effect of Ca 2+ on Pb 2+ toxicity may be related to its inhibition of the heavy metal accumulation in the root tip, a potential target site of Pb 2+ toxicity. Mg 2+ did not ameliorate the Pb 2+ toxicity on root growth as much as Ca 2+ did, although it decreased Pb 2+ uptake into roots similarly as Ca 2+ did. These results suggest that the protective effect of Ca 2+ on Pb 2+ toxicity may involve multiple mechanisms including competition at the entry level, and that Pb 2+ and Cd 2+ may compete with divalent cations for transport into roots of rice plants.