Split Nitrogen Application Improves Wheat Canopy Structure, Stem Mechanical Traits, and Grain Yield

A large amount of evidence shows that reasonable nitrogen (N) fertilizer application can effectively promote wheat growth and development and improve the population’s production potential. It is unclear whether this growth promotion depends on optimizing population structure and function, thereby increasing wheat stem quality and grain yield. We compared four N topdressing treatments in a two-year field experiment on two varieties contrasting in lodging resistance Shannong 23 (SN23) and Shannong 16 (SN16). All treatments received 80 kg N ha−1 before sowing, and the remaining 160 kg N ha−1 was topdressed according to four schedules: rising (GS30) and booting stage (GS45) 1:1 (T1), all remaining N at jointing stage (GS32) (T2), jointing (GS32) and anthesis stage (GS65) 1:1 (T3), and all remaining N at booting stage (GS45) (T4). The results show that advancing the GS32 fertilizer to GS30 effectively ensured the number of spikes per unit area, while delayed application at GS45 significantly increased plant relative chlorophyll content (8.44%) and net photosynthetic rate (8.27%). In addition, this N fertilization strategy improved plant structure and increased cumulative daily radiation in the basal canopy. It was also associated with greater stem cellulose (17.91%) and lignin (16.36%) concentrations and increased stem breaking resistance. In summary, the GS30 + GS45 split schedule was associated with improved basal canopy light distribution, greater stem mechanical traits, and consistent grain yield under the conditions studied.