FRACTIONATION OF NITROGEN ISOTOPES DURING THE UPTAKE AND ASSIMILATION OF AMMONIA BY PLANTS

Two varieties (Nihonbare and Koshihikari) of rice plants (Oryza sativa L.) were grown hydroponically with two levels (20 and 100 mg N liter-1) of ammonia. Variations in levels of natural abundance of N-15 (delta-N-15) were analyzed in the ammonia and organic nitrogen of shoots and roots, as well as in the ammonia in the culture solution. There was substantial fractionation of nitrogen isotopes during the uptake of ammonia. When plants absorbed a large proportion of ammonia from a solution with a low concentration, less negative delta-N-15 values in plants and high positive delta-N-15 values in the ammonia remaining in solution were observed. The reverse was found when a smaller fraction of ammonia was absorbed from a solution with a higher concentration of ammonia. The delta-N-15 values of ammonia in shoots and roots were higher than in the respective constituent organic nitrogen, suggesting the fractionation of nitrogen isotopes during the assimilation of ammonia. Wild-type and mutant cells of the cyanobacterium (blue-green alga) Synechococcus PCC 7942 were grown in nitrate- or ammonia-containing medium as the source of nitrogen. Fractionation of nitrogen isotopes during the uptake of nitrate was limited, whereas that during the uptake of ammonia was considerable.