Nitrate Reduction by Roots of Soybean (Glycine max [L.] Merr.) Seedlings

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Abstract
Studies were conducted with 9- to 12-day-old soybean (G. max [L.] Merr. cv. Williams) seedlings to determine the contribution of roots to whole plant NO3- reduction. Using an in vivo -NO3- nitrate reductase (NR) assay (no exogenous NO3- added to incubation medium) developed for roots, the roots accounted for .apprx. 30% of whole plant nitrate reductase activity (NRA) of plants grown on 15 mM NO3-. N analyses of xylem exudate showed that 53-66% of the total-N was as reduced-N, depending on the time of day of exudate collection. This supported enzyme data that suggested roots were contributing significantly to whole plant NO3- reduction. In short-term feeding studies using 15N-NO3- significant and increasing atom percent 15N excess was present in the reduced-N fraction of xylem exudate at 1.5 and 3 h after feeding, respectively, which verified that roots were capable of reducing NO3-. Estimated reduced-N accumulation by plants based on in vivo -NO3- NR assays of all plant parts substantially over-estimated actual reduced-N accumulation by the plants. Evidently, the in vivo NR assay cannot be used to accurately estimate reduced-N accumulation but still serves as a useful assay for relative differences in treatment conditions.