Optimal water, nitrogen, and density management increased wheat yield by improving population uniformity

The ideal population type is the basis of high-yield and high-efficiency cultivation of wheat. Population uniformity is an important index to evaluate the population ideotype. Therefore, it is necessary to analyze the yield difference of winter wheat at different spike layers between different populations because spike layer affects the production function of population. Here, two 2-year field experiments were conducted to investigate the effects of irrigation times, nitrogen application rate, and planting density on wheat yield, population traits, sugar and dry matter accumulation, and photosynthetic parameters at different spike layers. The results indicated that optimal planting density (SD3), nitrogen (N2) and irrigation (W1 or W2) deceased the ineffective tillers number at flowering stage and improved the spike number at upper and middle spike layers, which leading to lower coefficient of variation (CV) and higher population uniformity. Increasing planting density, nitrogen, and irrigation promoted high grain yield and population-scale biomass accumulation mainly due to the increment of spike number and yield at upper and middle spike layers. But, the single-stem biomass and grain dry weight reduced with increased planting density whereas improved with an increase of nitrogen and irrigation. Increasing planting density, nitrogen, and irrigation improved the leaf area index (LAI) and light interception at the upper and middle canopy, but decreased it at the lower canopy. Furthermore, the chlorophyll content at flag leaf and penultimate leaf was higher than that of top third leaf. Thus, the single-stem and each organ biomass accumulation, and sugar content gradually decreased from the upper to the lower layers, leading to decreased grains number per spike and average grain weight. Increasing planting density decreased spike length, total soluble sugar content and dry matter accumulation of spike at different spike layers but improved these indicators in stem, which leading to decreases in grain number per spike; whereas these indicators improved with increased irrigation. Overall, these findings provided theoretical and practical basis for building ideal crop population, and breeding and cultivation of winter wheat with high yield. • Optimal density, nitrogen, irrigation improved spikes number at upper and middle layers. • Optimal density, nitrogen, irrigation decreased CV, improved population uniformity. • Optimal density, nitrogen, irrigation improved population-scale yield and biomass. • Optimal density, nitrogen, irrigation improved photosynthesis at upper and middle canopy. • Increasing planting density suppressed spike development, but irrigation promoted it.