Nitrogen isotope composition of tomato (<i>Lycopersicon esculentum</i>Mill. cv. T‐5) grown under ammonium or nitrate nutrition

Abstract

ABSTRACT Studies that quantify plant δ 15 N often assume that fractionation during nitrogen uptake and intra‐plant variation in δ 15 N are minimal. We tested both assumptions by growing tomato ( Lycopersicon esculetum Mill. cv. T‐5) at NH 4 + or NO − 3 concentrations typical of those found in the soil. Fractionation did not occur with uptake; whole‐plant δ 15 N was not significantly different from source δ 15 N for plants grown on either nitrogen form. No intra‐plant variation in δ 15 N was observed for plants grown with NH + 4 . In contrast. δ 15 N 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 + 4 is assimilated immediately in the root, so organic nitrogen in the shoot and root is the product of a single assimilation event. NO − 3 assimilation can occur in shoots and roots. Fractionation during assimilation caused the δ 15 N of NO − 3 to become enriched relative to organic nitrogen; the δ 15 N of NO − 3 was 11.1 and 12.9% greater than the δ 15 N of organic nitrogen in leaves and roots, respectively. Leaf δ 15 N may therefore be greater than that of roots because the NO − 3 available for assimilation in leaves originates from a NO − 3 pool that was previously exposed to nitrate assimilation in the root.