Abstract: The effects of nitrogen application rate, transplanting density and soil chemical properties on total plant alkaloids of upper leaves of flue-cured tobacco were studied in a field experiment with the flue-cured tobacco plants planted in different soil fertility areas in Enshi. The results showed that, ① compared with transplanting density, nitrogen application rate had a greater effect on total alkaloid content, but increasing planting density was helpful to decrease total alkaloid content under medium and low nitrogen application rates;② covariance analysis showed that the effect of nitrogen application rate on total alkaloids was significant, but the interaction between nitrogen application rate and transplanting density was not significant;③ the total alkaloid was negatively correlated with soil available phosphorus, and the available phosphorus entered the stepwise regression model in 2018 and 2017, and controlled the total alkaloid;④ the path analysis method of structural equation model （SEM） showed that with the increase of available phosphorus, the total alkaloid content decreased significantly, and the indirect path coefficient of available phosphorus was significant;⑤ The indirect path coefficient of available P and available K was larger, accounting for about 80% of the intermediate path coefficient,the total alkaloid could be affected indirectly by available potassium. Proper control of the amount of nitrogen fertilizer and high-density planting measures were adopted in some areas where the total alkaloids were easy to be high. Soil available phosphorus could be one of the main controlling factors ofthe total alkaloid.With the gradual enrichment of soil phosphorus, it would be beneficial to reducethe total alkaloid , but the effects of other quality factors of tobacco leaves needs further investigation.

Key words: flue-cured tobacco, upper leaf, total plant alkaloid, nitrogen application rate, planting density