Effect of Nitrogen Source on Ureides in Soybean

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Abstract
In field-grown soybeans the percentage of N in the xylem as ureides increased with increasing N2 fixation. During a 9-wk collection period, the ureide content varied from 9.0-69.2% of the xylary N. Between 9 and 11 wk (early pod fill), there was a good correlation (r = 0.93) between C2H2 reduction and the percent N in xylem as ureides. The percent N as ureides, however, does not always indicate the reliance of the plant on symbiotic [Rhizobium japonicum] N2 fixation. This ureide content also depended on the level of NO3- available to the roots. Non-nodulated soybeans given from 0-200 kg N per hectare produced xylem sap which averaged from 31.8-9.0% N, respectively, in the xylem as ureides over the 9-wk period. Feeding of 15N2, 15NH4, or 15NO3 to greenhouse-grown soybeans indicated substantial differences in the initial distribution of N by the xylem stream, but the ultimate distribution of N between plant parts and grain did not vary with available N or percentage of xylary N as ureides. Amino acids, not ureides, were the major source of N in the phloem. The soybeans maintained a similar composition in phloem irrespective of the xylem sap constituents, with N derived from N2, NH4, or NO3 being equally accessible to the phloem stream.