Plant growth retardant increases nitrogen utilization efficiency and harvest index in maize by optimizing the plant horizontal-vertical ratio and vascular bundle morphology

Improving nitrogen utilization efficiency is not only beneficial for increasing maize yield, but it can also mitigate the environmental impact of excessive nitrogen fertilizer use. Numerous studies have evaluated the impacts 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 (NUtE) under varying plant densities has been rarely reported. A field experiment conducted in 2020–2021 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, the application of EC (especially the high dosage) significantly reduced plant height and dry matter, while increasing stem diameter, the plant horizontal-vertical ratio (PHVR, a new index which we define as the ratio of stem diameter of the basal first internode above the ground to plant height), and the number and area of vascular bundles. PHVR and vascular bundle morphology had significantly positive correlations with individual plant dry matter remobilization amount and its contribution to grain yield. Therefore, despite the reduced dry matter weight observed in the EC treatment, the greater dry matter remobilization enhanced the harvest index (HI). However, nitrogen uptake efficiency was not improved with the enhancement of PHVR and vascular bundle morphology, due to a reduction in dry matter accumulation. In contrast, the improved PHVR and vascular bundle were beneficial for accelerating nitrogen translocation, thus increasing NUtE significantly by 4.3–31.1% compared with CK across the plant densities. Increasing density simultaneously improved nitrogen uptake and utilization efficiency. Consequently, a high dosage application of EC under high density could not only significantly enhance lodging resistance but also improve NUtE and HI significantly by promoting the transport of dry matter and nitrogen.