A growth experiment on agar medium and a hydroponics experiment were carried out to study the nitrogen(N)metabolism of a low-N tolerant mutant(lnt1)of Arabidopsis thaliana under different N levels as compared with the wildtype(WT)Arabidopsis.On the agar medium,no apparent growth differences were observed between the lnt1 and WT plants under a normal N level of 9 mmol L-1 NO3-.However,under a low N level of 0.18 mmol L-1 NO3-,the growth of the WT plants was greatly retarded,while the lnt1 plants were not affected by low-N stress and showed similar growth with those grown under a normal N level.In the hydroponics experiment,the lntl mutant had higher activities of glutamine synthetase(GS)and NADH-dependent glutamate synthase(NADH-GOGAT)in both leaves and roots under N-deficient conditions.Moreover,they accumulated less ammonium(NH4+)but more free amino acids in leaves compared with the WT plants.These observations suggest that better N assimilation might contribute to the low-N tolerant phenotype of the lnt1 mutant.