Distribution of Externally Applied15NH4+or15NO3−in White Lupins at Different Developmental Stages

White lupins (Lupinus albus L., var. Kievsky mutant) were grown in Mitscherlich pots. 15NH4NO3 or NH4 15NO3 (10 or 95 at% 15N exc.) was applied at the late vegetative stage (14–16 leaves) or at the generative stage (end of bloom), respectively. Dry weight increase (ΔDW) of the plants as well as the 15N distribution in different organs and N fractions (NO3 −-N, NH2-N, protein N) were investigated after 5 days and 50 days (maturity). The following results were obtained: 1. In comparison with the 15N derived from NO3 −, the 15N derived from NH4 + was more strongly retained in the roots. When transported into the shoot, it was more strongly translocated into the tip. 2. More 15N derived from NH4 + was incorporated into proteins and reduced soluble N compounds than 15N derived from NO4 −. Even in the above-ground plant tissues, a considerable proportion of the latter was found as NO3 −. 3. In the long-term experiment (harvest at maturity 50 days after application), no difference in translocation and incorporation could be found between the two offered 15N sources.