Objectives The physiological mechanisms of tolerance to ammonium toxicity were researched in order to study tolerant genes and provide physiological basis and theoretical guidance in different ecotypes of Arabidopsis thaliana.

Methods A hydroponics experiment was used to culture 95 Arabidopsis materials in normal culture medium (2 mmol/L nitrate treatment) for 8 days after Arabidopsis seedlings transplanted, and then transferred to nutrient solution containing 1 mmol/L (NH₄)₂SO₄ (2 mmol/L ammonium treatment) for 8 days. After harvesting, total nitrogen content, free ammonium concentration and glutamine synthase (GS) activity in shoot were measured. After 3 days of cultivation, the expression levels of major ammonium transporter genes AMT1; 1 and AMT1; 2 in root by RT-PCR were analyzed. Arabidopsis seedlings were cultured in normal medium for 8 days after transferring to 1 mmol/L (¹⁵NH₄)₂SO₄ with 5% abundance, shoot was harvested at 3, 6 and 24 hours for isotope analysis.

Results 2 mmol/L ammonium significantly inhibited the growth of shoot in Arabidopsis thaliana population, and ammonium accumulated in the shoot. The ammonium concentration under ammonium application was more than 1.5 times of that under nitrate, and ecotype Si-0 was 19.17 μmol/g, FW under ammonium, which was 20 times of that of the control. The content of endogenous ammonium was negatively correlated with the growth of Arabidopsis thaliana shoot under 2 mmol/L nitrate and 2 mmol/L ammonium treatments. Therefore, we used ammonium content of tissues in 2 mmol/L ammonium treatments as the main factor, screened ammonium-tolerant Arabidopsis ecotypes Or-1, Ta-0, HSM and ammonium-sensitive Arabidopsis ecotypes Rak-2, Lpv-18, Hi-0. The results showed that ammonium concentration in sensitive ecotypes was 1.7 to 10 times higher than that in tolerant Arabidopsis ecotypes under nitrate. The expression levels of AMT1;1 and AMT1;2 in the root of tolerant ecotypes was higher than those in root of sensitive ecotypes. The results of total nitrogen and ¹⁵N tracer analysis showed that the ammonium uptake rate in tolerant ecotypes was higher than that of sensitive ecotypes. Moreover, the results of glutamine synthetase (GS) showed that the GS activity of tolerant ecotypes was 1.1 to 1.8 and 1.2 to1.6 times of that of sensitive ecotypes under nitrate cultivation and ammonium cultivation respectively, which indicated the ammonium assimilation capacity of tolerant ecotypes was stronger than that of sensitive Arabidopsis ecotypes.

Conclusions Ammonium-tolerant Arabidopsis thaliana has higher GS activity to alleviate ammonium toxicity by assimilating large amounts of free ammonium in plants, rather than by reducing ammonium uptake.