Fig. 6: Ammonium toxicity is closely linked with acidic stress.
a, b Effects of NH3 application on shoot FW and proton concentrations in water extracts of 5-d-old Col grown on media containing 10 mM ammonium or 10 mM nitrate (mean ± SD; n = 3). Thirty-seven shoots from one plate constituted a single biological replicate. The pH was adjusted to pH 5.7 with 1 N KOH; subsequently, a commercial 25% (v/v) ammonia solution was added to adjust the pH from 5.7 to 6.7. A representative photograph of 11-d-old shoots grown on 10 mM ammonium (12 plants per plate) is shown. c Effects of higher pH on the FW of shoots from 11-d-old Col, ami2, and gln2 grown on media containing 10 mM ammonium for 11 d (mean ± SD; n = 6). Three shoots from one plate constituted a single biological replicate. The pH was adjusted to pH 5.7 with 1 N KOH; subsequently, 1 N NaOH was used to adjust the pH from 5.7 to 6.7 to maintain the potassium concentration constant among all samples. A representative photograph of 11-d-old shoots is shown. d FW of shoots from 11-d-old Col, stop1-KO mutant (stop1-k), and the stop1 mutant (stop1-m) grown on media containing 10 mM ammonium (mean ± SD; n = 20) or 10 mM nitrate (mean ± SD; n = 5) for 11 d. Six shoots from one plate constituted a single biological replicate. *P < 0.05; **P < 0.01; ***P < 0.001 (Welch’s t-test). e FW of shoots from 17-d-old plants grafted between Col (C) and the NR-null mutant (nr) 7 d after transfer to media containing 10 mM ammonium (NH4+) or 2.5 mM nitrate and 10 mM ammonium (NH4+NO3−) conditions (mean ± SD; n = 3). One shoot from one plate constituted a single biological replicate. A representative photograph of 17-d-old shoots 7 d after transfer to media is shown. a–c, e Different letters denote significant differences at P < 0.05 (Tukey–Kramer’s multiple comparison test). The scale bar represents 10 mm.
