Response of canopy light and nitrogen distribution and nitrogen accumulation characteristics in densely planted soybean to uniconazole treatment

To further increasing soybean yield and explore the role of uniconazole (UZ) in dense planting systems, a field experiment was conducted with five planting densities (15×10⁴, 18×10⁴, 21×10⁴, 24×10⁴, and 27×10⁴ plants∙hm^-2) and two UZ levels (60 mg∙L^-1 UZ and water as a control) in 2022−2023. The effects of UZ on canopy light and nitrogen distribution, nitrogen accumulation and allocation characteristics were examined with increasing planting densities. Results indicated that, as planting density increased, the fraction of photosynthetically active radiation (FIPAR) in the upper canopy increased, while it decreased in both the middle and lower canopy layers. Concurrently, nitrogen content of each canopy layer (NCC) in the upper canopy initially increased and then decreased, whereas NCC in both the middle and lower canopy layers decreased. UZ application reduced the FIPAR in the upper canopy but increased it in the middle canopy, with minimal effect on the lower canopy. Furthermore, UZ application significantly increased canopy NCC, with the largest effect observed in the middle canopy. As planting density increased, both the light extinction coefficient (K_L) and nitrogen reduction coefficient (K_b) increased, whereas UZ application reduced both K_L and K_b. Without UZ application, the matching coefficient of light and nitrogen (K_L/K_b) decreased as planting density increased, whereas UZ application narrowed the gap between K_L/K_b and the ideal value. Temporally, as planting density increased, the maximal accumulation (Y_m) and maximal speed of accumulation (V_m) of nitrogen initially increased and then decreased. The time reached maximal rate of accumulation (t_m) occurred earlier, and the duration of rapid accumulation (T) was shortened. UZ application delayed tm, extended T, and concurrently increased both Y_m and V_m. K_L and K_b were negatively correlated with the K_L/K_b and the eigenvalues associated with nitrogen rapid accumulation phase, yield was positively correlated with Y_m, t₁, t₂, V_m, t_m and T. Increasing planting density resulted in increased biomass and nitrogen accumulation in vegetative organs, but reduced the economic coefficients for biomass and nitrogen, as well as the nitrogen content in each organ. UZ application increased biomass and nitrogen accumulation in reproductive organs, as well as the economic coefficients for biomass and nitrogen. Consequently, the highest soybean yields were achieved at planting densities of 18×10⁴ plants∙hm^-2 for water-treated plants and 21×10⁴ plants∙hm^-2 for UZ-treated plants. In conclusion, the spatial distribution of canopy light and nitrogen influenced the temporal dynamics of nitrogen accumulation, distribution and yield formation. UZ application optimized nitrogen accumulation characteristics by coordinating canopy light and nitrogen distribution, and increased the optimal planting density from 18×10⁴ to 21×10⁴ plants∙hm^-2, resulting in a yield increase of 11.9%–23.2%.