Figure 2.

Root isotopic signatures (δ¹⁵N, ‰) of NH₄⁺-fed plants correlated with the plant NH₄⁺ toxicity/tolerance indicator (plant biomass ratio NH₄⁺/NO₃^- 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.