Compartmental Analysis of the Partitioning of Photo-assimilated Carbon in Nodulated Soybean Plants during the Light Period

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Abstract
Kouchi, H., Yoneyama, T. and Akao, S. 1986. Compartmental analysis of the partitioning of photo-assimilated carbon in nodulated soybean plants during the light period.—J. exp. Bot. 37: 994–1005. Dynamics of the partitioning of photo-assimilated carbon in vegetative nodulated soybean (Glycine max L.) plants in the light period was investigated by compartmental analysis based on data from steady-state 13CO2 assimilation experiments. The model assumes a total of 18 compartments consisting of active and temporary storage pools for soluble materials, starch and structural materials in leaves, stems plus petioles, roots and nodules together with respired carbon from the roots and nodules. Carbon flow between compartments was described by 22 rate parameters. The rate parameters were evaluated by a non-linear least square search method to optimize the fitness of the simulated results with the experimental tracer distribution. The compartment model was well applicable to interpret the carbon partitioning in whole plants. The analysis showed that: (I) The largest carbon flux during the light period was to storage materials (starch and temporary storage soluble pools) in the above-ground parts. The total flux to storage pools was considerably larger than the transporting flux to below-ground parts. (2) The main carbon flux to the nodules was via direct phloem pathways from the shoot and not via the compartment of root soluble materials. This flux was 72% of the total carbon flux from the shoot to the nodulated root system. (3) A large amount of carbon was returned to the shoot from below-ground parts. The total return of carbon flux to the shoot (85% from nodules) was equivalent to 54% of the total influx of carbon to below-ground parts. Direct carbon transfers between roots and nodules were relatively small.