Optimizing planting density and nitrogen application to mitigate yield loss and improve grain quality of late-sown wheat under rice–wheat rotation

Late sowing is a critical factor that hinders achieving high-yield, good-quality wheat under rice–wheat rotation. Understanding the physiological basis and regulatory pathways that lead to high yield and sound quality late-sown wheat is crucial for developing effective cultivation strategies. A 2-year field experiment was conducted to investigate the effects of sowing date, nitrogen (N) application rate, and planting density on wheat yield, grain quality, population characteristics, and the underlying physiological factors. The results revealed significant interactions among the sowing date, planting density, and N application in regulating both yield and quality. Late sowing reduced grain yield primarily by reducing the number of spikes and kernels. However, the latter was improved by increasing N application and the planting density, thus mitigating the yield losses caused by late sowing. Moreover, the grain protein content (GPC) and wet gluten content (WGC) increased with delayed sowing dates and higher N rates but decreased with increased planting densities. For wheat yields over 9,000 or 7,500 kg ha–1, the latest sowing date should not be later than Nov. 4 or 15, respectively. In addition, specific criteria should be met, including a maximum of 1.5 and 1.0 million stems and tillers ha–1, a maximum leaf area index of 6.7 and 5.5, and a dry matter accumulation (DMA) at anthesis of 14,000 and 12,000 kg ha–1, respectively. For high-yield, good-quality late-sown wheat, the optimal combination is a 25% increase in the N rate (300 kg N ha–1) and a planting density of 2.25 million (N300D225) or 3.75 million (N300D375) plants ha–1 for 10- or 20-day delays in sowing, respectively. These combinations result in a higher leaf net photosynthetic rate, higher activities of leaf nitrate reductase, glutamine synthetase, grain glutamic pyruvic transaminase, and a lower sugar-N ratio during post-anthesis.