Physiological Mechanisms Of Tolerance To Ammonium Toxicity In Different Ecotypes Of Arabidopsis Thaliana

Ammonium is the soil of main nitrogen source because of the large application of nitrogen fertilizer and under the condition of waterlogging and low pH,but excessive ammonium can cause toxicity to plants.The effects of 2 mM NH₄⁺on the population of Arabidopsis thaliana were studied by using 95 ecotype Arabidopsis materials collected in this paper.The aim was to explain the mechanism of ammonium tolerance in different ecotypes at physiological level.1.2mM ammonium significantly inhibited the growth of shoot in Arabidopsis thaliana natural accessions,and ammonium accumulated in the shoot.The content of endogenous ammonium was negatively correlated with the shoot growth of Arabidopsis thaliana under 2mM nitrate and 2 mM ammonium treatments.Therefore,we used Ammonium content of tissues as the main factor,screened ammonium-dependent and non-ammonium-dependent Arabidopsis ecotypes.Among the ammonium-dependent ecotypes,Or-1,Ta-0 and HSM are ammonium-tolerant,Rak-2,Lpv-18,and Hi-0 are ammonium-sensitive.Among the non-ammonium dependent ecotypes,ammonium-tolerant are Bor-4,Mzk-0 and udull-34,ammonium-sensitive are MIB-22,NW-0 and Kno-18.Low ammonium stress significantly reduced the physiological nitrogen use efficiency of Arabidopsis thaliana population,and inhibited the absorption of K⁺,Ca²⁺,Na⁺,Mg²⁺,Mn²⁺,Zn²⁺,but promoted the absorption of Fe²⁺,had no effect on the absorption of Cu²⁺.2.ammonium-sensitive of ammonium-dependent ecotypes Arabidopsis thaliana under low ammonium stress showed that the decline of biomass was significantly lower than that in tolerant Arabidopsis ecotypes under nitrate.The expression levels of AMT1;1 and AMT1;2 is stronger in the root of tolerant than sensitive.The results of total nitrogen and ¹⁵N tracer analysis results indicated that ammonium uptake rate was higher than sensitive.Moreover the tolerant Arabidopsis thaliana of glutamine synthase?GS?is more higher than sensitive,which indicates the ammonium assimilation capacity of tolerant is stronger than sensitive Arabidopsis ecotypes.3.Non-ammonium-dependent ecotype Arabidopsis thaliana has ammonium-tolerant and ammonium-sensitive ecotypes at the same level of ammonium content.There was no difference in ammonium content between ammonium-tolerant and ammonium-sensitive ecotypes,but the fresh weight of ammonium-tolerant ecotypes was significantly higher than that of ammonium-sensitive ecotypes.The content of malondialdehyde?MDA?content in ammonium-tolerant ecotype of Arabidopsis thaliana was significantly lower than that in ammonium-sensitive ecotype,suggesting that ammonium-tolerant ecotype of Arabidopsis thaliana had lower lipid peroxidation and lower damage to cell membrane under 2 mM ammonium stress,compared with ammonium-sensitive ecotype.The antioxidant system of ammonium-tolerant ecotype Arabidopsis thaliana was significantly stronger than that of ammonium-sensitive ecotype,and the activities of catalase and peroxidase were significantly higher than those of ammonium-sensitive ecotype after 8 days of ammonium treatment.The results showed that ammonium-dependent ammonium-tolerant Arabidopsis thaliana could reduce ammonium toxicity by assimilating a large amount of free ammonium into plants through higher activity of glutamine synthase?GS?,rather than by reducing ammonium nitrogen uptake.Non-ammonium-dependent ammonium-tolerant ecotype Arabidopsis thaliana has a strong antioxidant system,and is more sensitive to ammonium,thus resisting ammonium stress.The results showed that ammonium-dependent and non-ammonium-dependent Arabidopsis thaliana resisted ammonium toxicity by absorbing more ammonium nitrogen under higher GS.In the follow-up research work,we found the tolerance genes closely related to ammonium metabolism,which is very important for the study of ammonium tolerance of rape under rice-oil rotation.