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[Objective] The effects of nitrogen application amount and base-dressing ratio on the content of carbon metabolites and key enzyme activities in flue-cured tobacco were investigated, so as to provide a theoretical basis for rational application of nitrogen fertilizer and improvement of tobacco leaf quality and yield. [Method] A two-factor field experiment with different nitrogen application rates (6, 8, 10 kg/667 m²) and base topdressing ratio (5∶5, 6∶4, 7∶3) was carried out. Using flue-cured tobacco Yunyan 87 as the test material, the reducing sugar, total sugar content and activity of key enzymes, including sucrose synthase (SS), sucrose phosphate synthase (SPS), ADP glucose pyrophosphorylase (AGP), sucrose invertase (INV), soluble starch synthase (SSS), fructose 1,6-bisphosptase (FBPase) and α-amylase, were determined at resettling growth stage, the prosperous stage, the budding stage, the topping stage, and the mature stage of the foot leaf. [Result] With the mature of tobacco leaves, the contents of total sugar and reducing sugar in tobacco leaves showed a trend of first decreasing, then increasing and decreasing at last. The activity of INV, SS, α-amylase, AGP and FBPase showed as a unimodal curve of first increasing and then decrease, while the activity of SPS showed as a bimodal curve, and the activity of SSS showed a trend of first decreasing and then increasing. With the increase of nitrogen application, the content of total sugar and reducing sugar in tobacco leaves showed a downward trend. The SS activity was consistently decreasing, while the activities of INV, SSS and FBPase showed a trend of first decreasing and then increasing, and the activity of α-amylase, SPS and AGP showed a trend of increasing first and then decreasing. With the same nitrogen application, the contents of total sugar and reducing sugar in tobacco leaves were higher as a whole. The content of total sugar and reducing sugar were higher in low nitrogen treatment, while that of total sugar and reducing sugar were lower in the high nitrogen treatment. Relative studies indicated that the SSS activity was significantly negatively correlated with total sugar at the clustering stage.[Conclusion] When the nitrogen application amount is 6 kg/667 m² and the base topdressing ratio is 6∶4, the content of carbon metabolites and key enzyme activities in each period are more suitable, which is conducive to the accumulation of tobacco carbon metabolism-related substances. [ABSTRACT FROM AUTHOR]

(Objective] The changing law of chemical composition and sensory quality of cured tobacco in different transplanting periods in Heishi area of Weining was investigated, so as to provide theoretical basis for determining the most suitable transplanting period and improving the quality and yield of tobacco in Heishi area of Weining. (Method) Taking Yunyan 87 as the test material, a randomised block design was adopted to study the effects of different transplanting periods (April 10th, April 20th, April 30th and May 10th) on the chemical composition content of reducing sugar, total sugar, starch, nicotine, total nitrogen, protein, lactic acid, malic acid, total organic acid, potassium and chlorine of the flue-cured tobacco after harvesting and roasting, and the sensory quality was evaluated. (Result) With the delay of transplanting period, the reducing sugar content of tobacco leaf showed a trend of rising at first and then falling, the total sugar content showed a trend of fallingrising-falling, and the starch content showed a gradually decreasing trend. The total nitrogen and protein content showed a trend rising-fallingrising, the nicotine content showed a trend of rising at first and then falling, the chlorine content showed a trend of falling at first and then rising, and the potassium content showed a trend of rising. The total organic acid content showed a trend of first rising and then falling. The score of sweetness showed a trend of first rising and then falling, the score of aroma showed a trend of decreasing, the score of smoke showed a trend of first rising and then falling, and the score of taste showed a trend of first decreasing and then rising. The content of reducing sugar, total nitrogen, protein and chlorine showed a significant positive correlation with sweetness, while the content of total organic acid showed a significant negative correlation with sweetness, and the sweetness was closely related to the translucency of the aroma characteristics and the finesse degree of the smoke characteristics. The main factors influencing the fineness degree were total nitrogen and reducing sugar, and the main factors influencing the permeability were nicotine and reducing sugar, Nicotine, protein and reducing sugar had a greater influence on the formation of sweetness. [Conclusion] The most suitable transplanting period in Heishi area of Weining is April 10th. Under this condition the chemical compositions of cured tobacco are more coordinated, which is conducive to the improvement of sensory quality. [ABSTRACT FROM AUTHOR]