Partitioning and metabolism of nitrate, asparagine, and allantoin in the soybean shoots at the grain-filling stage

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Abstract
The major N components in the xylem streams of soybeans are nitrate, amides (particularly asparagine) and ureides (allantoin and allantoic acid) (10). The proportions of these components in the xylem sap vary depending on the forms of N which soybean plants take up. In the plants depending on the symbiotically fixed N, ureides are predominant N forms in the xylem sap, whereas the fraction of nitrate and/or amides increases in association with the decrease of ureides when combined forms of nitrogen (as nitrate, ammonium, or urea) are supplied (3,10). Yoneyama and Ishlzuka (8) have also shown that the patterns of primary distribution of N in soybean plants differed depending on the forms of N taken by plants. The form of N transported in the xylem stream is considered to be an important factor which controls the primary distribution of N (9). The differences in the partitioning and metabolism of nitrate, amides, and allantoin in the soybean shoots at vegetative stages have been already discussed (7). In the experiment reported here, 15N-labeled nitrate, asparagine, or allantoin were fed via the cut stems of soybean shoots at the grain-filling stage, and the primary distribution and metabolism of these N compounds were investigated.