Nitrogen isotope composition of tomato (Lycopersicon esculentumMill. cv. T‐5) grown under ammonium or nitrate nutrition

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Abstract
Studies that quantify plant δ15N often assume that fractionation during nitrogen uptake and intra‐plant variation in δ15N are minimal. We tested both assumptions by growing tomato (Lycopersicon esculetumMill. cv. T‐5) at NH4+or NO−3concentrations typical of those found in the soil. Fractionation did not occur with uptake; whole‐plant δ15N was not significantly different from source δ15N for plants grown on either nitrogen form. No intra‐plant variation in δ15N was observed for plants grown with NH+4. In contrast. δ15N of leaves was as much as 5.8% greater than that of roots for plants grown with NO−3. The contrasting patterns of intra‐plant variation are probably caused by different assimilation patterns. NH+4is assimilated immediately in the root, so organic nitrogen in the shoot and root is the product of a single assimilation event. NO−3assimilation can occur in shoots and roots. Fractionation during assimilation caused the δ15N of NO−3to become enriched relative to organic nitrogen; the δ15N of NO−3was 11.1 and 12.9% greater than the δ15N of organic nitrogen in leaves and roots, respectively. Leaf δ15N may therefore be greater than that of roots because the NO−3available for assimilation in leaves originates from a NO−3pool that was previously exposed to nitrate assimilation in the root.