Nitrogen assimilation in soybean nodules

The ureides, allantoin and allantoic acid, have been found to exist in a wide variety of plant species usually in very small amounts, but in some species they play an important role in the storage and transport of nitrogen (19). In soybean plants, a very high content of allantoin has been demonstrated in the stems or shoots, only when the nodulation was well progressed (3, 8, 9, 11, 12, 18). This interesting phenomenon has been discussed in relation to various aspects of soybean physiology, especially the sites and pathways of allantoin synthesis (4, 5, 7, 10, 13–15, 18) and its roles in the vegetative and reproductive growth of soybean plants (6). Using 15N as a tracer, the present authors reported direct evidence that allantoin was vigorously synthesized in nodules, and that most of the allantoin in the shoots of well nodulated soybean plants was supplied from the nodules (14, 15, 18).