Utilization and Metabolism of Photoassimilated13C IN Soybean Roots and Nodules

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Abstract
Utilization and metabolism of photoassimilated carbon in soybean (Glycine max L. var. Akisengoku) nodules and roots were investigated using a short-term, steady-state 13CO2 labelling technique. Respiratory evolution of 13CO2 from roots and nodules was separately measured during the 2.5 h period of 13CO2 feeding and subsequent 7.5 h period of chase and the incorporation of 13C into soluble metabolites was determined by gas chromatography-mass spectrometry. Labelled carbon in respired CO2 appeared very rapidly in the nodules and the maximum level of the labelling was considerably higher in the respiration of the nodules than in that of the roots. Sucrose was rapidly labelled with currently assimilated carbon in both the roots and nodules. In the nodules, the time-course pattern and the levels of labelling of sucrose were in close agreement with those of respired CO2, while in the roots, the percentages of currently assimilated carbon in sucrose were much higher than those in respired CO2. Organic acids in the TCA cycle were strongly labelled with the currently assimilated carbon, although the levels of labelling were considerably lower than those of sugars. Comparisons between labelling patterns of TCA cycle components and of respired CO2 of nodules indicated that large amounts of individual organic acids were physically remote from the major site of respiration. An extremely rapid turnover of succinate was observed in the nodules. Cyclitols, malonate and glutarate were found in large amounts both in the roots and nodules, but their labelling rates were very low throughout the chase period. The overall results demonstrate quantitatively the strong dependency of nodules on recently assimilated carbon to support their function.