Respiratory Utilization of¹³C-Labelled Photosynthate in Nodulated Root Systems of Soybean Plants

Kouchi, H., Akao, S. and Yoneyama, T. 1986. Respiratory utilization of ¹³C-labelled photosynthate in nodulated root systems of soybean plants.—J. exp. Bot. 37: 985–993. An improved method for the measurement of respiratory utilization of current photosynthate in the nodulated root system of water-cultured soybean (Glycine max L.) plants was developed using a steady-state ¹³CO₂ labelling technique. Well-nodulated plants at the late vegetative stage were allowed to assimilate ¹³CO₂ for 10 h in continuous light at a constant CO₂ concentration with a constant ¹³C abundance. The respiratory evolution of ¹³CO₂ from roots and nodules was measured continuously throughout the period of ¹³CO₂ assimilation and during a subsequent 36 h chase period by using a differential infrared ¹³CO₂ analyser. The plants were grown with nitrogen-free or $NO3-$ (15 mmol dm⁻³)-containing culture solution for 3 d before ¹³CO₂ assimilation. In plants grown without $NO3-$, nodule respiration averaged 69% of the total respiration of the underground parts over the full experimental period and the CO₂ respired reached an apparent isotopic equilibrium at 80–85% labelling after initiating ¹³CO₂ assimilation. By contrast, the CO₂ respired from the roots did not reach an isotopic equilibrium and labelling was only 56% at the end of exposure to ¹³CO₂ These findings demonstrated that nodule respiration is strongly dependent on recently assimilated carbon compared with root respiration. Plants supplied with $NO3-$ in the culture solution showed a decreased rate of nodule respiration and a slightly increased rate of root respiration. The extents and time courses of labelling of respired CO₂ from both the roots and nodules were similar in the presence and absence of $NO3-$ except that the maximum level of labelling of CO₂ derived from nodule respiration in plants with $NO3-$ was significantly higher (about 91%) than for plants growing without $NO3-$.