Effects of combined application of chemical fertilizers and plant growth retardants on yield and formation of secondary metabolites of Ophiopogon japonicus

To alleviate the current paclobutrazol overuse in Ophiopogon japonicus, a two-factor randomized block experiment was carried out in Santai county, Mianyang city to compare the effects of three kinds of plant growth retardants, namely paclobutrazol (PP₃₃₃, 2.5 g·L^–1), uniconazole (S₃₃₀₇, 2 g·L^–1), and mepiquat (DPC, 0.05 g·L^–1), along with minute adjustments in fertilizer formula, on the agronomic traits, yield, total saponins, and total polysaccharides of O. japonicus. The results showed that minute changes in the fertilizer formula had no significant effects on the growth of O. japonicus (P > 0.05), and application of a plant growth retardant, except DPC, could increase the yield of O. japonicus. The applications of PP₃₃₃ and S₃₃₀₇ increased the yield of O. japonicus by about 37.03% and 16.65%, respectively, compared with control. Application of S₃₃₀₇ significantly increased the total polysaccharide content of O. japonicus, by about 25.45% (P < 0.05), which was greater than the increase caused by PP₃₃₃ and DPC. The three plant growth retardants significantly reduced the total saponin content of O. japonicus. Correlation analysis revealed that PP₃₃₃ and S₃₃₀₇ mainly increased the yield of O. japonicus by reducing the accumulation of dry matter in the aerial parts and promoting root swelling. The best production measures for top dressing application was using nitrogen fertilizer at 1 382.4 kg·ha^–1, phosphate fertilizer at 6 004.8 kg·ha^–1, and potassium fertilizer at 1 252.8 kg·ha^–1, in combination with spraying S₃₃₀₇, according to the grey relation-DTOPSIS comprehensive evaluation analysis. Those results indicated that the application of S₃₃₀₇ in the production of O. japonicus can effectively reduce the harm caused by PP₃₃₃ application without affecting the interests of farmers.