Abstract

The activity of nitrogenase and the concentration of ammonia and allantoin (+ allantoic acid) in root nodules were measured throughout the growth period of soybean plants. Nitrogenase activity measured by acetylene reduction increased with plant growth and reached a maximum level at the flowering period. The level of ammonia and allantoin concentration in nodules was parallel with increased nitrogenase activity. At the late reproductive stage (pod-forming period), nitrogenase activity showed a marked decrease, but the ammonia and allantoin in the nodules remained at a constant level. Detached nodules from 56 day-old soybean plants were exposed to 15N2 gas, and the distribution of 15N among nitrogen compounds was investigated. Enrichment of 15N in allantoin and allantoic acid reached a fairly high level after 90 min of nitrogen fixation; ca. 22% of 15N in acid-soluble nitrogen compounds was incorporated into allantoin + allantoic acid. In contrast, enrichment of 15N in amide nitrogen was relatively low. No...

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