Improving nitrogen utilization efficiency is not only beneficial for increasing maize yield, but can also mitigate the environmental impact of excessive nitrogen fertilizer use. Numerous studies have evaluated the impact of plant growth retardants and plant density on plant lodging resistance and nitrogen uptake. However, the influence of plant growth retardants on nitrogen utilization efficiency under varying plant densities has been rarely reported. A field experiment was conducted in 2020-2021, which involved spraying EC (an ethephon and cycocel compound) at the 7th-leaf stage of maize with dosages of 0 (CK), 450, and 900 mL ha−1 at plant densities of 4.5, 6.0, 7.5, and 9.0 plants m−2. Compared to CK, application of EC (especially high dosage) significantly decreased plant height and dry matter, while increased stem diameter, plant horizontal-vertical ratio (PHVR, a new index, which we defined as the ratio of stem diameter of the basal first internode above ground to the plant height), and the number and area of vascular bundle. PHVR and vascular bundle morphology had significantly positive correlation with individual plant dry matter remobilization amount and its contribution to grain yield. Therefore, despite reduced dry matter weight was observed in EC treatment, the increased dry matter remobilization enhanced harvest index (HI). However, nitrogen uptake efficiency was not improved with the enhancement of PHVR and vascular bundle morphology, due to a decrease in dry matter accumulation. Inversely, improved PHVR and vascular bundle were beneficial to accelerate nitrogen translocation, thus increasing nitrogen utilization efficiency (NUtE) significantly by 4.3–31.1% compared with CK across densities. Increasing density simultaneously improve nitrogen uptake and utilization efficiency. Consequently, high dosage of EC application under high density not only could significantly enhance lodging resistance, but also improving NUtE and HI significantly through promoting the transport of dry matter and nitrogen.
