Abstract
Studies were conducted with 9 to 12 day-old soybean (Glycine 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 approximately 30% of whole plant nitrate reductase activity (NRA) of plants grown on 15 mm NO3−.
Nitrogen analyses of xylem exudate showed that 53 to 66% of the total-N was as reduced-N, depending on the time of day of exudate collection. These observations 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 found in the reduced-N fraction of xylem exudate at 1.5 and 3 hours 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. Thus, 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.
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Selected References
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