Influence of planting distance and density on the yield and photosynthetic traits of sweet potato (Ipomoea balatas L.) under an intercropping system with walnut (Juglans regia) saplings

Sweet potato is a high-yield food crop with strong adaptation and is widely cultivated in southwestern China. Adjusting crop planting distance and planting density to improve the yield and productivity of intercropping systems has become an important management practice around the globe. However, little is known about the response of the sweet potato yield and photosynthetic characteristics to intercropping with walnut saplings. Field experiments were conducted in a split-plot arrangement, e.g., three plant densities (Dl, low, 4 plants/m²; Dm, moderate, 8 plants/m²; and Dh, high, 12 plants/m²) were randomly assigned (subplot) to three planting distances (P1, 0.25 n; P2, 0.5 n; and P3, n) (main plot). The results showed that with increasing plant distance, the yield of high-density sweet potatoes decreased significantly (P < 0.05). However, the highest sweet potato fresh root tuber yield (FRTY) appeared in the combination of P1Dh (plant distance: 0.25 n; plant density: 12 plants m²). Our data further suggested that under the planting distance of P1 (0.25 n) or P2 (0.5 n), soil moisture content increased significantly (approximately 3%) with planting density (P < 0.05). The net photosynthetic rate (P_n) of sweet potatoes was significantly improved in cases of high sweet potato density and a short distance between sweet potatoes and walnut saplings, which ultimately increased yield. For the walnut saplings and sweet potatoes, a more compact planting distance was associated with higher values of P_n, T_r and G_s. Based on yield performance, photosynthetic traits and walnut sapling growth, the combination of 12 plants/m² and P1 is the optimal scheme for walnut/sweet potato intercropping systems.