Abstract: To examine the effects of planting density and nitrogen application level on root tiller development and yield formation in extremely-late sowing wheat, a field experiment was conducted from November 2022 to June 2024 at the Hongbu National Experimental Base of the Wheat Research Institute, Shanxi Agricultural University. Using Jimai 22 as the test material, a split-plot design was employed, with the main plot representing three planting densities: 525×10⁴ plants·hm^-2 (D1), 600×10⁴ plants·hm^-2 (D2), and 675×10⁴ plants·hm^-2 (D3). Nitrogen fertilization regimes were applied as subplots, with base fertilizer, green manure, and jointing fertilizer applied at three different ratios: 5∶2∶3 (N1), 5∶4∶1 (N2), and 3∶4∶3 (N3). The effects of planting density and nitrogen application interaction on the number of secondary roots per plant, tiller number, leaf area index, aboveground dry weight, yield and yield components in extremely-late sowing wheat were examined. The results showed that at the same density, both the number of secondary roots per plant and leaf area index during the grain filling stage were highest in N3. The number of tillers was N3>N1>N2 in the D1 and D3 treatments and N3>N2>N1 in the D2 treatment during the 2022-2023 period. In the 2023-2024 period, the annual average followed the order N3>N2>N1. The aboveground dry weight of D2 and D3 treatments was N3>N2>N1. The yield exhibited patterns of N3>N1>N2 in 2022-2023 and N3>N2>N1 in 2023-2024, with the highest yield under the nitrogen fertilizer ratio of 3∶4∶3. During the grain filling stage, under the same nitrogen fertilizer ratio, the numbers of secondary roots, tillers, leaf area index, and aboveground dry weight per plant were the highest in the D3 treatment. Yield decreased and then increased with increasing density in both years, the highest yield was achieved at a density of 675×10⁴ plants per hectare and a nitrogen fertilizer ratio of 3∶4∶3. Among them, the yield increased by 2.76% to 17.79% in 2022-2023 compared with the other treatments, and by 8.24% to 31.86% in 2023-2024. Correlation analysis indicated that yield was significantly positively correlated with the number of stem tillers and leaf area index at the filling stage, with the number of panicles at the mature stage, and with the dry weight of the aboveground part, with a correlation coefficient of 0.91. Principal component analysis indicated that the overall score was highest when the sowing density was 675×10⁴ plants·hm^-2 and the nitrogen fertilizer ratio was 3∶4∶3. In conclusion, moderate density (675×10⁴ plants·hm^-2) and optimal nitrogen fertilizer management (nitrogen fertilizer ratio of 3∶4∶3) can increase the number of secondary roots, tillers per plant, leaf area index, aboveground dry weight, and yield. This combination can serve as an effective nitrogen management strategy for high-yield of efficient extremely-late sowing wheat cultivation in the southern Shanxi region.

Key words: extremely-late sowing, wheat, density and nitrogen interaction, root tiller development, yield