Nitrogen and carbon flows estimated by N-15 and C-13 pulse-chase labeling during regrowth of alfalfa

The flow of N-15 and C-13 from storage compounds in organs remaining after defoliation (sources) to regrowing tissue (sinks), and C-13 losses through root or shoot respiration were assessed by pulse-chase labeling during regrowth of alfalfa (Medicago sativa L.) following shoot removal. A total of 73% of labeled C and 34% of labeled N were mobilized in source organs within 30 d. Although all of the N-15 from source organs was recovered in the regrowing tissue, much of the C-13 was lost, mainly as CO2 respired from the root (61%) or shoot (8%), and was found to a lesser extent in sink tissue (5%). After 3, 10, or 30 d of regrowth, 87, 66, and 52% of shoot N, respectively, was derived from source tissue storage compounds; the rest resulted from translocation of fixed N-2. Overall results suggest that most shoot C was linked to photosynthetic activity rather than being derived from mobilization of stored C in source organs. Furthermore, isotopic analysis of different chemical fractions of plant tissue suggests that between 14 and 58% of the shoot C derived from source tissues was linked to the mobilization of N compounds, not carbohydrates.