Foram feitos dois experimentos em casa de vegetação, empregando vasos de Leonard esterilizados, com quatro variedades de soja (Glycine max (L.) Merril) inoculadas com uma estirpe de Rhizobium japonicum considerada normal (SM1b) e outra de eficiência excepcional (R54a) para se estudar a correlação entre o teor de leg-hemoglobina e de molibdênio dos nódulos e a fixação simbiótica de nitrogênio. Enquanto a fixação de nitrogênio por grama de nódulos foi aproximadamente o dôbro com a estirpe excepcional, o teor de leg-hemoglobina dos nódulos por ela formados foi em média 27% maior, sendo, mesmo assim, altamente significativa a diferença entre as duas estirpes, nos dois experimentos. Entretanto, nas plantas inoculadas com a mesma estirpe, não houve correlação entre o teor de leg-hemoglobina e a eficiência nodular. O teor de molibdênio dos nódulos formados pela estirpe excepcional R54a, foi, aproximadamente, duas vêzes maior que o dos nódulos formados pela estirpe normal SM1b, correspondendo, assim, à diferença na eficiência nodular entre as duas estirpes.Two greenhouse experiments were carried out in sterilized Leonard jars with four soybean varieties inoculated with a Rhizobium strain considered normal (SM1b) and another with exceptional efficiency (R54a), to study the correlation of leghemoglobin and molybdenum content of the nodules and nitrogen fixation. Nitrogen fixation per unit nodule weight as evaluated by the regression of total plant nitrogen with nodule weight (Fig. 1) was about twice as high with the exceptional strain. The increase in molybdenum content was correspondent to the higher nodule efficiency being twice as high in nodules formed by the exceptional strain. It was thus suggested these nodules having doubled nitrogenase content. The increase in leghemoglobin content, however, was not correspondent to the increase in nodule efficiency. An increase of 136% in average of the nodule efficiency was accompanied by an increase of only 27% of the leghemoglobin content (Table 1, Fig. 2). Within strains there was no correlation of either molybdenum or leghemoglobin content with nodule efficiency. It was therefore suggested that for each group of strains nitrogenase and hemoglobin content in nodules and nodule efficiency are theoretically constant, variations being due to the experimental error. The difference between normal and exceptional strains of Rhizobium japonicum would then be due to a difference in the nitrogen fixing system or in the mechanism of incorporation of NH3
