Metabolism of [13C]‐labelled photosynthate in plant cytosol and bacteroids of root nodules of Glycine max

Well‐nodulated soybean (Glycine max L. Merr. cv. Akisengoku) plants were allowed to assimilate 13CO2. Plant cytosol and bacteroid fractions were isolated from nodules, and the kinetics of [13C]‐labelling of soluble carbohydrates, organic acids and amino acids were investigated. The concentrations of all metabolites, with the exception of trehalose and 3‐hydroxy‐butyrate, were 10‐ to 1000‐fold higher in plant cell cytosol than in bacteroids. The major portion of trehalose was found in bacteroids and 3‐hydroxybutyrate only in bacteroids. Sucrose was most highly labelled with 13C in nodules, and the levels and time‐course of labelling of sucrose were in good agreement with those of respired CO2 from the nodules. The levels and time‐courses of labelling of sucrose were closely similar in cytosol and bacteroids. Glucose was less labelled than sucrose and the level of labelling was consistently higher in cytosol than in bacteroids. The levels of [13C]‐labelling of organic acids and amino acids in nodules were lower than those of sucrose and of respired CO2. Tricarboxylic acid cycle intermediates, particularly succinate, were considerably less labelled in bacteroids than in the cytosol. All amino acids detected were also much more rapidly labelled in the cytosol. The results are discussed in relation to the utilization and possible compartmentation of carbon substrates in nodule tissues.