Discrimination of nitrogen isotopes during absorption of ammonium and nitrate at different nitrogen concentrations by rice (Oryza sativa L.) plants


Correspondence: T. Yoneyama Department of Applied Biological Chemistry, Graduate School of Agricultural and Life Sciences, University of Tokyo, Bunkyo-ku, Tokyo 113–8657, Japan. Fax: + 81 3 5841 8032


Studies of uptake of ionic sources of N by two hydroponically grown rice (Oryza sativa L.) cultivars (paddy-field-adapted Koshihikari and dryland-adapted Kanto 168) showed that the magnitude of the nitrogen isotope fractionation (ɛ) for uptake of NH4+ depended on the concentrations of NH4+ and cultivar (averaging –6·1‰ for Koshihikari and –12·0‰ for Kanto 168 at concentrations from 40 to 200 mmol m3 and, respectively, –13·4 and –28·9‰ for the two cultivars at concentrations from 0·5 to 4 mol m3). In contrast, the ɛ for uptake of NO3 in similar experiments was almost insensitive to the N concentration, falling within a much narrower range (+3·2‰ to –0·9‰ for Koshihikari and –0·9‰ to –5·1‰ for Kanto 168 over NO3 concentrations from 0·04 to 2 mol m3). From longer term experiments in which Norin 8 and its nitrate-reductase deficient mutant M819 were grown with 2 or 8 mol m3 NO3 for 30 d, it was concluded that the small concentration-independent isotopic fractionation during absorption of this ion was not related to nitrate reductase activity.