Dynamics of carbon photosynthetically assimilated in nodulated soya bean plants under steady-state conditions. 3. Time-course study on 13C incorporation into soluble metabolites and respiratory evolution of 13CO2 from roots and nodules

Well-nodulated soya bean (Glycine max L.) plants were allowed to assimilate 13CO2 for 10 h in the light, under steady-state conditions in which CO2 concentration and 13C abundance were both strictly controlled at constant levels. The respiratory evolution of 13CO2 from roots and nodules and 13C incorporation into various metabolic fractions were measured during the 13CO2 feeding and subsequent 48 h chase period. CO2 respired from nodules was much more rapidly labelled with 13C than that from roots. The level of labelling (percentage of carbon currently assimilated during the 13COM2 feeding period) of CO2 respired from nodules reached a maximum of about 87 per cent after 4 h of steady-state l3CO2 assimilation and thereafter remained fairly constant. The absolute amount of labelled carbon evolved by the respiration of the nodules during the 10 h 13CO2 feeding period was 1·5-fold that of root respiration. These results demonstrated that the currently assimilated (labelled) carbon was preferentially used to support nodule respiration, while root respiration relied considerably on earlier (non-labelled) carbon reserved in the roots. Sucrose pools were mostly composed of currently assimilated carbon in all tissues of the plants, since the levels of labelling accounted for 86–91 per cent at the end of the 13CO2 feeding. In the nodules, the kinetics and levels of sucrose labelling were in fairly good agreement with those of respired CO2, while in the roots, the level of labelling of respired CO2 was significantly lower than that of sucrose. Succinate and malate were highly labelled in both roots and nodules but they were labelled much more slowly than sucrose and respired CO2. The kinetics and levels of labelling of these Krebs cycle intermediates resembled those of major amino acids which are derived directly from Krebs cycle intermediates. It is suggested that large fractions of organic acids in nodules were physically separate from the respiration site.