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 Pb²⁺ and Cd²⁺ are affected by the presence of K⁺, Ca²⁺ or Mg²⁺. K⁺ had a little effect on uptake or toxicity of Pb²⁺ and Cd²⁺. Ca²⁺ or Mg²⁺ blocked both Cd²⁺ transport into rice roots and Cd²⁺ toxicity on root growth, which suggested that their detoxification effect is directly related to their blocking of entry of the heavy metals. Similarly, Ca²⁺ blocked both Pb²⁺ transport into the root and Pb²⁺ toxicity on root growth. The protective effect of Ca²⁺ on Pb²⁺ toxicity may be related to its inhibition of the heavy metal accumulation in the root tip, a potential target site of Pb²⁺ toxicity. Mg²⁺ did not ameliorate the Pb²⁺ toxicity on root growth as much as Ca²⁺ did, although it decreased Pb²⁺ uptake into roots similarly as Ca²⁺ did. These results suggest that the protective effect of Ca²⁺ on Pb²⁺ toxicity may involve multiple mechanisms including competition at the entry level, and that Pb²⁺ and Cd²⁺ may compete with divalent cations for transport into roots of rice plants.