CADMIUM UPTAKE AND NITROGEN-FIXING ABILITY IN HEAVY-METAL-RESISTANT LABORATORY AND FIELD STRAINS OF RHIZOBIUM-LEGUMINOSARUM BIOVAR TRIFOLII

Rhizobium leguminosarum bv. trifolii produced mucoid colonies on yeast mannitol agar, but non-mucoid colonies appeared with a frequency of 1 0(-7) on agar additionally containing 4 mM ZnCl2. Cells producing non- mucoid colonies were also obtained from root nodules of white clover f rom experimental field plots treated with heavy-metal-contaminated sew age sludge. Compared to mucoid strains, non-mucoid strains produced ap proximately half of the extracellular polysaccharide, two to three fol d higher levels of intracellular carbohydrate and possessed larger int racellular inclusions. They also showed up to two-fold times greater r esistance to Cd2+, Cu2+, Ni2+ and Zn2+. The total uptakes of Cd2+ were similar between the laboratory strains and amongst the field strains. In non-mucoid strains, Cd2+ was not removed by washing in 0.1 M HNO3, suggesting Cd2+ was bound intracellularly. All strains were effective in fixing nitrogen in white clover.