Cobalt and Nitrogen Fixation in Lupinus Angustifolius L.

S ummary Bacteroids from cobalt‐deficient root nodules of Lupinus angustifolius L. cv. Unicrop were isolated and assayed for products of the cobalamin‐dependent enzymes methionine synthetase, ribonucleotide reductase and methylmalonyl‐CoA mutase. Since bacteroids from lupin nodules did not show propionate‐dependent O 2 uptake, it was not possible to assess whether in vivo action of methylmalonyl‐CoA mutase was affected, although it appears unlikely to be of major significance. Cobalt deficiency decreased the DNA content per bacteroid in crown nodules, but not in later‐formed lateral nodules. The percentage of amino‐acid nitrogen accounted for by methionine was also lowered in bacteroids from crown nodules but not lateral nodules.