Evans, R.D., A.J. Bloom, S.S. Sukrapanna, and J.R. Ehleringer. 1996. Nitrogen isotope composition of tomato (Lycopersicon esculentum Mill. cv T-5) grown under ammonium or nitrate nutrition. Plant Cell Environ. 19:1317-1323.

Studies that quantify plant d15N often assume that fractionation during nitrogen uptake and intra-plant variation in d15N are minimal. We tested both assumptions by growing tomato (Lycoperiscon esculentum Mill. cv. T-5) at NH4+ or NO3- concentrations typical of those found in the soil. Fractionation did not occur with uptake; whole-plant d15N was not significantly different from source d15N for plants grown on either nitrogen form. No intra-plant variation in d15N was observed for plants grown with NH4+. In contrast, d15N of leaves was as much as 5.8% greater than that of roots for plants grown with NO3-. The contrasting patters of intra-plant variation are probably caused by different assimilation patterns. NH4+ is assimilated immediately in the root, so organic nitrogen in the shoot and root is the product of a single assimilation event. NO3- assimilation can occur in shoots and roots. Fractionation during assimilation caused the d15N of NO3- to becomed enriched relative to organic nitrogen; the d15N of NO3- was 11·1 and 12·9% greater than the d15N of organic nitrogen in leaves and roots, respectively. Leaf d15N may therefore be greater than that of roots because the NO3- available for assimilation in leaves originates from a NO3- pool that was previously exposed to nitrate assimilation in the root.