Characterization of manganese toxicity in two species of annual medics

There is interest in the development of cultivars of Medicago SPP. tolerant to soil acidity, where manganese (Mn) can be a major constraint to plant growth. We evaluated the response of several ecotypes of M. polymorpha and M. murex to a high level of Mn (50 mg Mn kg(-1) soil), and its impact on biological N-2 fixation and mineral composition. Plant growth in pots was decreased by Mn toxicity, whether plants were dependent on N-2 fixation or provided with fertilizer nitrogen (N). Plant growth was impaired by high levels of Mn to a greater extent than was N-2 fixation. Tolerance to high levels of Mn differed between the ecotypes, with accession 66 of M. polymorpha and 194 of M murex being the most tolerant, A 500 muM concentration of Mn in nutrient solution led to decreased concentration of zinc (Zn) in the whole plant, and to modifications in the partition of iron (Fe), potassium (K), and calcium (Ca) between roots and shoots. Tolerance to Mn was negatively correlated with the concentrations of Mn and Zn in the roots.