Effects of Applying Nitrogen and Potassium on Lilium lancifolium Growth and Accumulation of Secondary Metabolites in Bulbs

Lilium lancifolium is a plant resource used as both medicine and food because it is enriched with polysaccharides, polyphenol compounds, and saponins. Increasing the quality of Lilium species is based largely on improvement using methods such as selective breeding and proper fertilization. In this study, we investigated the different responses of L. lancifolium bulbs to treatment with nitrogen (N) and potassium (K) in Hoagland solution. A pot experiment was conducted with four N rates and five K rates under a completely random design. The agronomic traits, N and K contents, and concentrations of active compounds were determined in bulbs, including total phenols, flavonoids, polysaccharides, and saponins. L. lancifolium treated with N and K exhibited increases in the plant height, leaf number, and chlorophyll content compared with the control (N0 + K0). The bulb circumference increased by 17.41% under N2 (609.80 mg L−1) + K2 (523.34 mg L−1) compared with N0 + K0. Individual or combined application of N and K increased the total phenol, flavonoid, and saponin contents, especially under N2 + K3, with the highest increases of 1.87–2.93 times compared with N0 + K0. However, the individual application of N decreased the polysaccharide contents by 2.78–42.04%. Hoagland solution containing 443.24–572.87 mg L−1 N and 573.61–759.16 mg L−1 K is recommended to improve the active contents of bulb components based on regression analysis. Our results demonstrate that the combined application of N and K is important for obtaining high-quality L. lancifolium bulbs.