Nitrogen transport in xylem of soybean plant supplied with15NO3-

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Abstract
Recently, the ureides have been recognized as the main transport form of fixed N from soybean nodules. On the other hand, it was suggested that a part of NO3 - absorbed from soybean roots was translocated to the shoots without undergoing any modification, or was exported to the shoots in the form of Asn and other amino acids after reduction and assimilation in the roots (McClure and Israel 1979; Ohyama and Kumazawa 1979; Ohyama 1984). However, information about the processes of NO3 - transport, storage, and assimilation in soybean roots is limited. Two distinct processes have been proposed for the NO3 - transport from soybean roots. When 15NO3 - was administered to the intact soybean plants, it was observed that some part of NO3 - was very rapidly assimilated or transported after absorption, whereas, another part was temporarily stored in the roots and slowly released to the shoots when the plants were transferred to the non-labeled conditions (Ohyama 1984). This result may suggest that a part of NO3 - was directly transported after absorption, and the other was once incorporated into a large storage pool of NO3 - (probably vacuoles) and subsequently NO3 - was released to the xylem at a constant rate. The Asn assimilation and transport processes in soybean roots seem to be more complicated. In previous studies, it was observed that the Asn concentration was relatively low in the nodulated soybean roots cultivated in N-free medium, but that the level was markedly increased at 24 h after NO3 - administration (Ohyama et al. 1989a). This may indicate a large temporary storage pool of Asn in the roots is formed along with the NO3 - absorption. On the other hand, some part of NO3 - was found to be once incorporated into protein in the roots. It is suggested that the Asn formed by protein degradation is transported to the shoots (Ohyama 1983, 1984). In this report, the transport of N originating from the absorbed NO3 - was studied by analyzing the xylem exudate collected from decapitated soybean plants at the time when 15NO3 - was supplied in the medium.