Abstract: This paper analyzed the differences in the responses of Arabidopsis thaliana’s two ammoniumhypersensitive mutants, amosd and vtc1, to exogenous ammonium at concentrations 0, 1, 5, 10, and 20 mmol·L^-1. The two mutants all showed ammonium-hypersensitive, but the sensitivity degree differed. With the increasing concentration of exogenous ammonium, the vtc1 showed ammonium toxicity earlier than amosd. However, when the exogenous ammonium concentration increased up to 20 mmol·L^-1, the amosd showed heavier toxicity symptom than vtc1, suggesting the vital roles of AMOSD in plant tolerance to high ammonium stress. When the two mutants suffered from the stress of exogenous ammonium, the plant tissues where the highest toxicity degree occurred had somewhat difference. As the amosd suffered from the stress, the development of true leaves was arrested most; when the vtc1suffered from the stress, root elongation was repressed most. Partial ammonium supply experiment showed that the amosd had ammonium- hypersensitivity when the shoots were supplied with ammonium, while the vtc1 did not have the same phenotype unless the roots were supplied with ammonium. These results clearly indicated that vtc1 was a root-supply ammonium hypersensitive mutant, while amosd was a leaf-supply ammonium hypersensitive one. With the development of foliar spraying and the increasing deposition of ammonium from atmosphere, plants are facing higher and higher ammonium stress, and thus, it is of importance to increase the plant shoot tolerance to ammonium stress. This study indicated that, due to its unique characteristics of hypersensitive to shoot-supply ammonium, mutant -amosd could be a suitable genetic material to study the mechanisms of plant shoot tolerance to ammonium stress, and useful to more systemically understand the mechanisms of plant ammonium toxicity and improve plant ammonium tolerance.

Key words: climate change, flax, development, yield.