Submitted to: Plant Physiology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 10/5/1998
Publication Date: N/A
Citation: Interpretive Summary: An advantage of soybean as a crop is that it symbiotically fixes atmospheric nitrogen, rather than depend on the application of fertilizer nitrogen. However, the nitrogen fixation process in soybean has been found to be very sensitive to soil drying, with decreases in activity occurring even at high levels of soil water content. It has been suggested that ureides, which are synthesized in the nitrogen fixation process, may accumulate in the plant and cause a feedback inhibition of nitrogen fixation under drying conditions. The research reported in the paper, which involved an ARS-USDA scientist, examined the nitrogen fixation of soybean when the plants were supplied externally with ureides. Indeed, feeding soybean plants with ureides resulted in a dramatic decrease in nitrogen fixation activity. The ureides, however, did not accumulate at the site of nitrogen fixation in the plant so that it was concluded that the ureides were not the direct signal that inhibited fixation. While it is clear that accumulation of ureides are important in the nitrogen fixation response to soil drying, it was concluded that ureides may stimulate another signal that results in the direct inhibition of nitrogen fixation activity.
Technical Abstract: Sensitivity of N2 fixation to drought stress in soybean (Glycine max Merr.) has been shown to be associated with high ureide accumulation in the shoots, which leads to the hypothesis of a feedback mechanism on N2 fixation during drought. The ureide-feedback hypothesis was tested directly by first measuring the effect of 10 mM ureides applied by stem infusion or in the nutrient solution of hydroponically grown plants on acetylene reduction activity (ARA). An almost complete inhibition of ARA was observed within 4 to 7 d after treatment, accompanied by an increase in ureide concentration in the shoot but not in the nodules. Other N compounds including nitrate, urea, and especially asparagine, also inhibited nodule activity. The possibility was investigated that the decrease in N2 fixation was a consequence of ureide-dependent decreases in nodule permeability to O2. Ureide treatment resulted in a continual decrease in nodule permeability to O2 (Po), simultaneous to the inhibition of nitrogenase activity over 5-d treatment period.