Relative Content of NO(3) and Reduced N in Xylem Exudate as an Indicator of Root Reduction of Concurrently Absorbed NO(3).

It is unclear if the relative content of NO(3) (-) and reduced N in xylem exudate provides an accurate estimate of the percentage reduction of concurrently absorbed NO(3) (-) in the root. Experiments were conducted to determine whether NO(3) (-) and reduced N in xylem exudate of vegetative, nonnodulated soybean plants (Glycine max [L.] Merr., ;Ransom') originated from exogenous recently absorbed (15)NO(3) (-) or from endogenous (14)N pools. Plants either were decapitated and exposed to (15)NO(3) (-) solutions for 2 hours or were decapitated for the final 20 minutes of a 50-minute exposure to (15)NO(3) (-) in the dark and in the light. Considerable amounts of (14)NO(3) (-) and reduced (14)N were transported into the xylem, but almost all of the (15)N was present as (15)NO(3) (-). Dissimilar changes in transport of (14)NO(3) (-), reduced (14)N and (15)NO(3) (-) 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 (15)N transport into the xylem of decapitated plants was only 21 to 36% of the (15)N delivered to the shoot of intact plants. Based on the proportion of total (15)N which was found as reduced (15)N in exudate and in intact plants in the dark, it was estimated that 5 to 17% of concurrently absorbed (15)NO(3) (-) 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 NO(3) (-) and total reduced N in the xylem exudate.