Root isotopic signatures (δ15N, ‰) of NH
+-fed plants correlated with the plant NH
+ toxicity/tolerance indicator (plant biomass ratio NH
- for each N concentration). The following N concentrations were represented in this analysis: 0.5 mM (upward triangle), 1.5 mM (circle), 2.5 mM (upside down triangle), 3 mM (square), 5 mM (star) and 6 mM (diamond). δ15N data of the (NH4)2SO4 used in NH4+-fed plants were +0.029, +0.5 and +2.31 ‰, and all three values fall within the area indicated (upper part of graph). The plant species that were cultured hydroponically and used for this statistical analysis were lettuce, spinach, tomato, ryegrass, pea, lupin and white clover. The dataset displayed represents the average values ± SE (at least n = 3, depending on species; see Methods). Linear regression was performed at P ≤ 0.05.