Abstract: Optimizing planting density and water-fertilizer management is crucial for enhancing seed production performance of forage grasses. Based on an experiment with five-factor, five-level quadratic regression orthogonal rotary combinatorial design (1/2 implementation), we investigated the combined effects of planting density, water, and fertilizer on agronomic traits, seed yield and quality of Lespedeza potaninii, a native plant species in the desert steppe of Ningxia. The results showed that both planting density and irrigation quota significantly affected plant height, relative leaf chlorophyll content, and inflorescence number. Applications of N, P, and K fertilizers significantly influenced the number of reproductive branches and seeds per pod. Appropriate planting density and irrigation quota increased the relative chlorophyll content in leaves, and thus enhanced photosynthetic capacity. A balanced ratio of N, P, and K fertilizers coordinated vegetative and reproductive growth, maximizing production potential due to an adequate number of reproductive branches and ultimately achieving the highest seed yield. The maximum seed yield of 1525.48 kg·hm^-2 was obtained with a planting density of 8.5×10⁴ plants·hm^-2, an irrigation quota of 1475 m³·hm^-2, and the N, P, and K application rates of 15.0 kg·hm^-2, 27.5 kg·hm^-2, and 56.3 kg·hm^-2, respectively. Seed germination rate, germination potential, and germination index peaked at a P application rate of 110.0 kg·hm^-2, reaching 80.0%, 75.0%, and 29.8%, respectively, with a corresponding thousand-seed weight of 2.21 g. Based on the constructed mathematical model, the optimal combination for achieving high seed yield was a planting density of 8.05×10⁴-8.20×10⁴ individuals·hm^-2, an irrigation quota of 746.98-765.53 m³·hm^-2, an N application rate of 29.07-30.93 kg·hm^-2, a P application rate of 53.30-56.71 kg·hm^-2, and a K application rate of 36.33-38.67 kg·hm^-2. Our results demonstrated that both high yield and seed quality could be achieved through the integrated management of planting density, irrigation, and fertilization in desert steppe. These findings would provide a theoretical basis and technical support for seed production and ecological restoration in this region.

Key words: Lespedeza potaninii, water-fertilizer gradient, planting density, coupling effect, seed production performance