Carbon Flow from Nodulated Roots to the Shoots of Soybean (Glycine maxL. Merr.) Plants: An Estimation of the Contribution of Current Photosynthate to Ureides in the Xylem Stream

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Abstract
Kouchi, H. and Higuchi, T. 1988. Carbon flow from nodulated roots to the shoots of soybean {Glycine max L. Merr.) plants: An estimation of the contribution of current photosynthate to ureides in the xylem stream.–J. exp. Bot. 39: 1015–1023. Well-nodulated, water-cultured soybean plants were allowed to assimilate 13CO2 at a constant specific activity for 10 h and the 13C-labelling of total carbon and ureides in xylem sap was investigated. Labelled carbon appeared very rapidly in the xylem stream. Percentage of labelled carbon (relative specific activity, RSA) in xylem sap was 18% at 2 h after the start of 13CO2 assimilation and reached 53% at the end of the 10 h assimilation. The amount of labelled carbon exported from nodulated roots to the shoots via the xylem during the 10 h labelling period accounted for 33% of total labelled carbon imported into the nodulated roots. Ureides (allantoin and allantoic acid) in xylem sap were strongly dependent on currently assimilated carbon. The RSA of ureides in xylem sap had reached 83% at the end of the assimilation period. Labelled carbon in ureides accounted for 51% of total labelled carbon returned from nodulated roots to the shoots via the xylem during the 10 h assimilation period. A treatment with 20 mol m−3 nitrate in the culture medium for 2 d decreased the ureide concentration in the xylem sap slightly, but greatly decreased the RSA of ureides. By comparing the data with the results of analysis of the xylem sap of nodule-detached plants, it was concluded that the majority of labelled carbon exported to the xylem stream from nodules was in ureide form. A considerable amount of carbon was also returned from roots to shoots via the xylem stream but it was more dependent on (non-labelled) carbon reserved in the root tissues.