Abstract:

Small type of Chinese cabbage[Brassica pekinensis（Lour.）Olsson]was used as material. The hydroponic experiment was conducted to study effects of ammonium︰nitrate ratios（0︰10，10︰90，15︰85 and 25︰75；total nitrogen concentration 5 mmol · L^-1）on growth，osmotic regulation and antioxidant enzyme activities of Chinese cabbage seedling under normal（200 μmol · m^-2 · s^-1）and low（100 μmol · m^-2 · s^-1）light intensity. After 14 days treatment，plants fertilized with ammonium︰nitrate（15︰85）under normal light intensity were the best in growth potential. The leaf area，growth range，fresh weight and dry weight of plants improved 33.8%，30.5%，77.9% and 72.9% compared with ammonium︰nitrate（0︰100），respectively. The leaf area，growth range，fresh and dry weight of the plants in low light intensity were apparently lower than those of plants in normal light intensity，indicating that low light intensity suppressed the growth of seedlings. The malonaldehyde（MDA），superoxide anionand hydrogenperoxide（H₂O₂）contents of seedling leaves in low light intensity apparently rose up，resulting in higher membrane lipid peroxidationin plants. However，the leaf area，growth range，fresh weight and dry weight of plants fertilized with ammonium∶nitrate（10︰90）improved 20.4%，14.9%，55.9% and 36.5% compared with ammonium∶nitrate（0︰100），respectively. Furthermore，ammonium∶nitrate（10︰90）significantly improved the soluble sugar，soluble protein contents and superoxide dismutase（SOD），peroxidase（POD），catalase（CAT），ascorbate peroxidase（APX）activities in Chinese cabbage，decreased the reactive oxide species contents，and effectively suppressed membrane lipid peroxidation. In normal light intensity，the ammonium∶nitrate ratio（15︰85）was beneficial for plant growth while the suitable ratio was 10︰90 in low light treated plants. In conclusion，appropriate ammonium︰nitrate（10︰90）enhanced shading tolerance of Chinese cabbage seedling by decreasing the contents of membrane lipid peroxidation substances，and increasing theactivities of antioxidant enzymesand the contents of osmotic regulation substances.

Key words: Chinese cabbage, ammonium∶nitrate, light intensity, osmotic regulation, antioxidant system