Effects Of Nitrogen Nutrition On Antioxidant Contents And Antioxidative Activities In Chinese Cabbage (Brassica Chinensis L.)

Public awareness of the purported health benefits of dietary antioxidants.such as ascorbic acid(AsA),vitamin E,phenolics,carotinoids,flavonoids and glutathione(GSH)has increased the demand for vegetable products with recognized and improved antioxidants quality.Therefore,the pathways that improve the synthesis of these compounds may yield benefits as widespread as improved plant stress tolerance and improved human physical and mental health. As we known,besides the genetic factor,the contents of the phytochemicals above were also affected by many environmental factors,e.g.light,temperature,water, fertilization and cooking processes.Hereinto,nitrogen nutrition has remarkable effects on the accumulation of AsA,phenolics,flavonoids and GSH in plants. Previous researches were conducted on the effects of nitrogen nutrition supply levels on the content of antioxidant substances in plants,however,these results were different and were limited to the content of phytochemicals,while its effect on antioxidative activities was not analyzed.In our reaseach,two cultivars of Chinese cabbage(Brassica Chinensis L.)were employed to investigate the effects of nitrogen supply levels and different NO₃^-:NH₄⁺ ratios on the contents of AsA, total phenolics,flavonoids,GSH,FRAP(Ferric reducing antioxidant power)value and DPPH(1,1-diphenyl-2-picrylhydrazyl)radical scavenging rate.1.A hydroponic experiment was conducted to investigate the effects of different nitrogen levels(2,10,18 and 26 mmol.L^-1)on the antioxidants contents and antioxidative activities in two cultivars(SZQ and DF)of Chinese cabbage (Brassica Chinensis L.).Results showed that appropriate nitrogen supply level(10 mmol.L^-1)facilitated to increase the plants biomass,but superfluous nitrogen supply decreased them.The nitrate content in plants increased obviously along with the increase of nitrogen supply.Highest contents of ascorbic acid,total phenolics,flavonoid,and DPPH radical scavenging rate and FRAP value were recorded when nitrogen deficient(2 mmol.L^-1).The contents of ascorbic acid,total phenolics and flavonoids and antioxidative activities were markedly decreased by the increases of nitrogen level,while GSH was not affected.However,the highest antioxidants accumulation were founded at the nitrogen level of 10 mmol.L^-1. Therefore,the nitrogen level of 10 mmol.L^-1might be the optimal nitrogen level for production of both high-yield and high-quality vegetable under this experimental condition2.A hydroponic experiment was conducted to investigate the effects of different NO₃^-:NH₄⁺ ratios(100:0,75:25,50:50 and 25:75)on the antioxdant contents and antioxidative activities in two cultivars(SZQ and DF)of Chinese cabbage(Brassica Chinensis L.).As the results shown,the fresh biomass and dry weight of Chinese cabbage were significantly increased when NO₃^-:NH₄⁺ ratio 100:0 was replaced by the treatment of NO₃^-:NH₄⁺ ratio 75:25,while keep increase the scale of ammonium in the nutrient solution inhibited them. Furthermore,the content of ascorbic acid,GSH,flavonoids and total phenolics of Chinese cabbage were enhanced with the increase of ammonium ratio,while nitrate content was decreased.In a word,the lowest conent of nitrate and highest biomass and antioxidant accumulation were founded at NO₃^-:NH₄⁺ ratio of 75:25, hence,it should be optimal for the aim of yield and quality.3.A pot experiment was carried out to study the effects of different nitrogen application rates(0,150 and 300 mg N·kg^-1soil)on the antioxidant contents and antioxidative activities in two cultivars(SZQ and DF)of Chinese cabbage (Brassica Chinensis L.).Results showed that fresh and dry weights of shoots were markedly increased by by increasing nitrogen rate from 0 mg N·kg^-1to 150 mg N·kg^-1,and were siginicantly decreased with further increasing nitrogen rate. Nitrate contents in leaf and petiole increased with increasing N rate.Higher nitrate contents were observed in cultivar of SZQ than that of DF.Highest contents of ascorbic acid,total phenolics,flavonoid,and DPPH radical scaveging rate and FRAP value were recorded in the control treatment(0 mg N·kg^-1).The contents of ascorbic acid,total phenolics and flavonoids and antioxidative activities were markedly decreased by the increases of nitrogen supply,while GSH was not affected.However,the highest antioxidant accumulation were founded at the application rate of 150 mg N·kg^-1soil.Therefore,the rate of 150 mg N·kg^-1might be the optimal nitrogen application rate for production of both high-yield and high-quality vegetable under this experimental condition.