Effect of cadmium, chromium and copper on symbiotic and free-livingRhizobium leguminosarumbiovartrifolii

Plasmid-minus derivatives of Rhizobium leguminosarum biovar trifolii have been isolated. Cured strains lacked symbiotic properties, however they showed increased heavy metal resistance. In the presence of 70 ppm chromium the parent strain, unlike cured derivatives, is unable to grow explanta but can nevertheless nodulate clover. We propose that rhizobia can circumvent exposure to the heavy metal by entering the plant roots. Acetylene reduction tests showed that nodulated plants, grown in the presence of 10 ppm of chromium, had an increased nitrogenase activity compared to the control plants.