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

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Abstract
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.