Dynamics of Carbon Photosynthetically Assimilated in Nodulated Soya Bean Plants under Steady-state Conditions 2. The Incorporation of 13C into Carbohydrates, Organic Acids, Amino Acids and some Storage Compounds

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Abstract
Nodulated soya bean (Glycine max L.) plants at the early flowering stage were allowed to assimilate 13CO2 under steady-state conditions, with a constant 13C abundance, for 8 h in the light. The plants were either harvested immediately or 2 d after the end of the 13CO2 feeding, divided into young leaves (including flower buds), mature leaves, stems+petioles, roots and nodules; the 13C abundance in soluble carbohydrates, organic acids, amino acids, starch and poly-β-hydroxybutyric acid was determined with a gas chromatography-mass spectrometry. The rapid turnover of 13C in the sucrose pools observed in all organs of the plants showed that sucrose was the principal material in the translocation stream of primary products of photosynthesis. At the end of the 13CO2 exposure, sucrose in the mature leaves as the major source organs and in the stems+petioles was labelled with currently assimilated carbon to about 75 per cent, whereas a much higher labelling of sucrose was found in the roots and in the nodules. This suggests the existence of two or more compartmented pools of sucrose in mature leaves and also in stems+petioles. The relative labelling patterns of individual organic acids and amino acids were similar in various plant organs. However, the rapid turnover of succinate and glycine was characteristic of nodules. Treatment with a high concentration of nitrate in the nutrient media increased the turnover rate of amino acid carbon in shoot organs and roots, while it markedly decreased the labelling of amino acids in nodules. The cyclitols, except for D-pinitol, were significantly labelled with assimilated 13C in mature leaves, but in nodules, the labelling was very much less. In the nodules, which were actively fixing atmospheric nitrogen, a large proportion (80–90 per cent) of currently assimilated carbon was found as sucrose and starch at the end of the 13CO2 feeding. This was also true of the roots. On the other hand, in young growing leaves, the distribution of currently assimilated carbon into sucrose, starch and other soluble compounds was much less. This suggests that a large amount of carbon assimilated by and translocated to young leaves was used to make up structural materials, mainly protein and cell wall polymers synthesis, during the light period.