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ENVIRONMENTAL STRESS AND ADAPTATION

Inhibition of N₂ Fixation in Soybean Is Associated with Elevated Ureides and Amino Acids¹

Department of Crop, Soil, and Environmental Sciences, University of Arkansas, Fayetteville, Arkansas 72704

Decreased N₂ fixation in soybean (Glycine max) L. Merr. during water deficits has been associated with increases in ureides and free amino acids in plant tissues, indicating a potential feedback inhibition by these compounds in response to drought. We evaluated concentrations of ureides and amino acids in leaf and nodule tissue and the concurrent change in N₂ fixation in response to exogenous ureides and soil-water treatments for the cultivars Jackson and KS4895. Exogenous ureides applied to the soil and water-deficit treatments inhibited N₂ fixation by 85% to 90%. Mn fertilization increased the apparent catabolism of ureides in leaves and hastened the recovery of N₂ fixation following exogenous ureide application for both cultivars. Ureides and total free amino acids in leaves and nodules increased during water deficits and coincided with a decline in N₂ fixation for both cultivars. N₂ fixation recovered to 74% to 90% of control levels 2 d after rewatering drought-stressed plants, but leaf ureides and total nodule amino acids remained elevated in KS4895. Asparagine accounted for 82% of the increase in nodule amino acids relative to well-watered plants at 2 d after rewatering. These results indicate that leaf ureides and nodule asparagine do not feedback inhibit N₂ fixation. Compounds whose increase and decrease in concentration mirrored the decline and recovery of N₂ fixation included nodule ureides, nodule aspartate, and several amino acids in leaves, indicating that these are potential candidate molecules for feedback inhibition of N₂ fixation.

Article, publication date, and citation information can be found at www.plantphysiol.org/cgi/doi/10.1104/pp.104.056317.

^* Corresponding author; e-mail lpurcell{at}uark.edu; fax 479–575–3975.

Received November 10, 2004; returned for revision January 17, 2005; accepted January 22, 2005.

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