Dynamics of Carbon Photosynthetically Assimilated in Nodulated Soya Bean Plants under Steady-state Conditions 1. Development and Application of 13CO2 Assimilation System at a Constant 13C Abundance

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Abstract
A long-term, steady-state 13CO2 assimilation system at a constant CO2 concentration with a constant 13C abundance was designed and applied to quantitative investigations on the allocation of photoassimilated carbon in nodulated soya bean (Glycine max L.) plants. The CO2 concentration in the assimilation chamber and its 13C abundance were maintained constant with relative variances of less than ±0.5 per cent during an 8-h assimilation period. At the termination of 8-h 13CO2 assimilation by plants at early flowering stage, the currently assimilated carbon relative to total tissue carbon (measured by the degree of isotopic saturation) were for young leaves (including flower buds), 13.9 per cent; mature leaves, 15.7 per cent; stems+petioles, 5.9 per cent; roots, 5.4 per cent and nodules, 6.9 per cent, 48 h after the end of the 13CO2 assimilation period, they were 12.3, 7.5, 7.4, 6.8 and 6.1 per cent, respectively. The treatment with a high concentration of nitrate in the nutrient media significantly decreased the allocation of 13C into nodules. Experiments on 13CO2 assimilation by plants at the pod-filling stage were also conducted. Labelling by 13C was weaker than at the early flowering stage, but an intense accumulation of 13C into reproductive organs was observed.

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