Effect of nitrogen source on ureides in soybean [Glycine max, nitrogen partitioning, xylem and phloem constituents differences]

In field-grown soybeans (Glycine max L. Merr. cv Harosoy), the percentage of N in the xylem as ureides increased with increasing N2 fixation. During a 9-week collection period, the ureide content varied from 9.0 to 69.2% of the xylary N. Between 9 and 11 weeks (early pod fill), there was a good correlation (r = 0.93) between C2H2 reduction and the per cent N in xylem as ureides. The per cent N as ureides, however, does not always indicate the reliance of the plant on symbiotic N2 fixation. This ureide content also depended on the level of NO3 - available to the roots. Non-nodulated soybeans given from 0 to 200 kilogram N per hectare produced xylem sap which averaged from 31.8% to 9.0% N, respectively, in the xylem as ureides over the 9-week period. Feeding of ^{15}\text{N}{}_{2}$, 15NH4, or ^{15}\text{NO}{}_{3}$ 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.