Mechanisms for decreasing nitrate and TSNA concentrations in tobacco by spraying regulating chemicals. Gfo: glufosinate, NR: nitrate reductase, NRA: nitrate reductase activity, NiR: nitrite reductase, GS: glutamine synthetase, GSA: glutamine synthetase activity, Gln: glutamine, Glu: glutamate, and OG: oxaloacetate. After spraying Mo on tobacco during the fast-growth period, nitrate significantly decreased while NRA and soluble protein content increased. These decreased the amount of nitrate storage and promoted tobacco development during the fast-growth period. After spraying Mo during the fast-growth stage and spraying Mo and Gfo at the stage of maturity, NRA increased and GSA decreased in tobacco, which can significantly reduce nitrate accumulation and TSNA formation by nitrogen loss due to ammonia volatilization.