Influence of nitrogen source and concentration on nitrogen isotopic discrimination in two barley genotypes (Hordeum vulgareL.)

The occurrence of nitrogen isotope discrimination with absorption and assimilation of nitrate (NO3–) and ammonium (NH4+) was investigated using two genotypes of barley,Hordeum vulgareL. cv. Steptoe and Az12 : Az70, the latter of which lacks the characterized nitrate reductase isozymes. Plants were grown under two situations: a closed system with limited nitrogen or an open system with unlimited nitrogen, to elucidate the conditions and processes that influence discrimination. There was no discrimination observed for Az12 : Az70 when supplied with limited nitrogen. Discrimination was observed for Steptoe seedlings at high external NO3–concentrations, but not with low NO3–when assimilation is probably rapid and complete. The same pattern was observed for Steptoe when NH4+was supplied; indicating that for both nitrogen forms discrimination is dependent upon the presence of the assimilatory enzyme and the external concentration. The implications of this study are that both internal (assimilatory enzyme distribution) and external (source concentration) factors may have a larger impact on tissueδ 15N than the form of nitrogen utilized. This suggests that tissueδ 15N may not always be a reliable indicator of a plant's integrated nitrogen nutrition.