Abstract: ObjectiveEffects of different nitrogen sources(organic nitrogen fertilizer and inorganic nitrogen fertili-zer)on carbon and nitrogen metabolism and key gene expression in the upper leaves of flue-cured tobacco were investiga-ted to provide theoretical basis for further regulating the application of fertilizer and improving the quality of tobacco leaves.MethodOrganic nitrogen fertilization and inorganic nitrogen fertilization were set as two treatments on the basis of the same nitrogen amount.The tobacco seedlings were sampled 60,70,80 and 90 d after transplanting.The ultrastruc-ture of the cells in different treatments were observed,and contents of total sugar,total nitrogen and nicotine in tobacco leaves were determined,then expression of key genes affecting carbon and nitrogen metabolism were analyzed.ResultThe starch granules treated by organic nitrogen fertilizer increased in the tobaccoupper leaf cells,the content and volume of osmiophilic granules also increased with the prolongation of transplanting time.Total sugar content in the two nitrogen fertilizer treatments decreased first and then increased in the upper leaves at mature period,and the one treated with or-ganic nitrogen fertilizer was higher than the other. The total nitrogen content in the two treatments decreased with the prolongation of growth period. Compared with the inorganic nitrogen fertilizer treatment,total nitrogen content in the treatment of organic nitrogen fertilizer was significantly lower(P<0.05,the same below)at the mature period.However, the total nitrogen contents in the two treatments tended to be consistent 90 d after transplanting.The nicotine content in the two nitrogen treatments showed an upward trend at mature period.Compared with the inorganic nitrogen fertilizer treat-ment,the nicotine content treated by organic nitrogen fertilizer was significantly lower 80 d after transplanting,but the nicotine content at other sampling periods were all higher than the other treatment.The analysis for key gene expression in carbon metabolism pathway showed that sucrose synthase gene(SuS1),starch synthase gene(SS1),granule bound starch synthase gene(GBSSI),amylopectin synthase gene(SBE1)and alpha amylase gene(AMML)were highly expressed,and this was the main reason for the large amount of accumulation of total sugar in tobacco upper leaves.The analysis for key gene expression in nitrogen metabolism pathway showed that the high expression level of nitrate transporter gene(NRT1) and ornithine decarboxylase gene(ODC)under organic nitrogen source was the main reason for the increase of nicotine content in upper leaves.ConclusionThe application of organic nitrogen fertilizer is beneficial for the expression of key genes in carbon and nitrogen metabolism pathway of tobacco upper leaf at mature stage,which may increasethe content of total sugar and nicotine in tobacco upper leaf and decrease the total nitrogen content.