Ethephon Application Modulated Nitrogen Use Efficiency in Maize (Zea mays L.) through Regulating Plant Nitrogen Uptake and Post-Silking Nitrogen Remobilization in Response to Nitrogen Rate and Plant Density.

Plant density and nitrogen (N) are two main factors achieved high yield and N use efficiency (NUE) in maize production, while ethephon is used widely to regulate plant growth for modulating lodging resistance and grain yield. Little is available about the role of ethephon in maize responded to N rate under various plant densities. A three-year field study was conducted with two ethephon application (0 and 180 g ha^-1), four N rates (0, 75, 150, and 225 kg N ha^-1), and two plant densities (6.75 plants m^-2 and 7.5 plants m^-2), to investigate the physiological mechanisms of ethephon on regulating plant growth and NUE in maize. Plant biomass and grain yield were significantly influenced by ethephon, N rate and plant density, and ethephon markedly decreased kernel weight and grain yield. Moreover, ethephon repressed whole-plant biomass while increased harvest index. Compared to control, ethephon declined NO₃^- and NH₄⁺ fluxes in xylem bleeding sap and pre-silking and post-silking N uptake, led to lower plant N content at silking and harvesting stages. Meanwhile, ethephon increased plant N concentration at silking stage while repressed stover N concentration at harvesting, and promoted post-silking N remobilization, then did not affect grain N concentration compared to control. Consequently, ethephon decreased N agronomic efficiency, partial factor productivity of N and N recovery efficiency, but improved N internal efficiency and N harvest index. These results suggested that ethephon could regulate plant N uptake and post-silking N remobilization for modulating NUE indices at different plant densities under various N rates.