Abstract: Chinese cabbage line 03B9 with high sensitivity to Black sesame spot were used as material to investigate the effects of (NH4)2SO4，NH4Cl，NH4NO3，urea，Ca(NO3)2 and KNO3 on Black sesame spot in Chinese cabbage. 03B9 and P8-2 with high sensitivity，C24 and P8-1 with high resistance to Black sesame spot were selected as materials，in hydroponic culture to study the effects of different NO3- -N︰NH4+ -N ratios（3︰7，5︰5，7︰3）with a total nitrogen concentration of 25 mmol · L-1 on nitrogen metabolism and antioxidant system. The determination indicators included activities of nitrate reductase （NR），glutamine synthetase（GS），glutamate dehydrogenase（GDH），polyphenoloxidase（PPO），the electric conductivity，and contents of nitrate nitrogen，ammonium nitrogen，malondialdehyde（MDA）and polyphenol. The results showed that ammonium nitrogen affected the occurrence of Black sesame spots more seriously than nitrate nitrogen and different NO3- -N︰NH4+ -N ratios affected the number of Black sesame spots. With the elevation of ammonium nitrogen ratio，the number of Black sesame spots in lines 03B9 and P8-2 increased more seriously than that in lines C24 and P8-1. In lines 03B9 and P8-2，the activity of PPO，electric conductivity，content of ammonium nitrogen and MDA increased，while the activity of NR and content of phenolics decreased. In lines C24 and P8-1，the activity of GDH and content of phenolics increased，while the activity of PPO，electric conductivity and content of MDA did not change significantly. It was presumed that in the high sensitive lines of Chinese cabbage，the increasing of the proportion of ammonium nitrogen influenced the activities of enzymes related to nitrogen metabolism，and then excessive ammonium nitrogen accumulated in plants，thus caused the injury of cell membrane. When contacted with PPO in cytoplasm phenolic compounds in vacuole were oxidized to quinone substances，and caused browning on the surface of petiole showing symptom of Black sesame spots. This research laid the foundation for further study on the mechanism of Black sesame spot in Chinese cabbage.　　

Key words: Brassica campestris L. ssp. pekinensis（Lour.）Olsson, Black sesame spot, nitrogen