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Articles

Relative Content of NO₃^– and Reduced N in Xylem Exudate as an Indicator of Root Reduction of Concurrently Absorbed ¹⁵NO₃^–1

Department of Soil Science, North Carolina State University, Raleigh, North Carolina 27650

It is unclear if the relative content of NO₃^– and reduced N in xylem exudate provides an accurate estimate of the percentage reduction of concurrently absorbed NO₃^– in the root. Experiments were conducted to determine whether NO₃^– and reduced N in xylem exudate of vegetative, nonnodulated soybean plants (Glycine max [L.] Merr., `Ransom') originated from exogenous recently absorbed ¹⁵NO₃^– or from endogenous ¹⁴N pools. Plants either were decapitated and exposed to ¹⁵NO₃^– solutions for 2 hours or were decapitated for the final 20 minutes of a 50-minute exposure to ¹⁵NO₃^– in the dark and in the light. Considerable amounts of ¹⁴NO₃^– and reduced ¹⁴N were transported into the xylem, but almost all of the ¹⁵N was present as ¹⁵NO₃^–. Dissimilar changes in transport of ¹⁴NO₃^–, reduced ¹⁴N and ¹⁵NO₃^– during the 2 hours of sap collection resulted in large variability over time in the percentage of total N in the exudate which was reduced N. Over a 20-minute period the rate of ¹⁵N transport into the xylem of decapitated plants was only 21 to 36% of the ¹⁵N delivered to the shoot of intact plants. Based on the proportion of total ¹⁵N which was found as reduced ¹⁵N in exudate and in intact plants in the dark, it was estimated that 5 to 17% of concurrently absorbed ¹⁵NO₃^– was reduced in the root. This was much less than the 38 to 59% which would have been predicted from the relative content of total NO₃^– and total reduced N in the xylem exudate.

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