Optimizing root system architecture to improve root anchorage strength and nitrogen absorption capacity under high plant density in maize

Ideotype root system architecture is crucial for achieving high yields in maize by enhancing lodging resistance and nitrogen absorption, particularly under high planting densities. However, there is limited research on this topic in maize. The objectives of this study were to reveal the relative importance of different root system traits in root anchorage strength and nitrogen absorption, and to investigate their variations in response to increased plant density. To clarify this, a two-year field experiment was conducted in 2018 and 2019, involving four different lodging resistant maize genotypes and two plant densities. Root system traits including root crown architecture, root morphology per whorl, and root distribution in upper and lower soil layers were fully characterized. Root lodging resistance and nitrogen absorption capacity were quantified through artificial root lodging tests and 15N labeling, respectively, based on measuring root lodging rate and plant nitrogen content. In comparison to lodging susceptible genotypes XY335 and XD20, lodging resistant genotypes LS1 and FM985 exhibited stronger root anchorage strength and lower root lodging risk. These characteristics were primarily attributed to their wider root crown width, larger projected root area, larger root angle, thicker brace roots, and wider distribution of the root system in the upper soil layer. However, LS1 was unfavorable for nitrogen absorption due to the larger root skeleton increased metabolic costs of soil exploration, which led to reduced root elongation, shallower rooting depth, and thus limited nitrogen acquisition from the soil. FM985 demonstrated a comparable nitrogen absorption capacity to XY335 (slightly higher than LS1) mainly because of the larger growth angle of the outermost crown root and greater specific root length of embryonic roots. With increased plant density, LS1and FM985 had larger reductions in root system traits, but maintained a larger root crown architecture than that of XY335 and XD20, leading to a lower lodging risk. XY335 and XD20 exhibited a stronger capacity for nitrogen acquisition at high plant densities, attributed to the increased root surface area, root volume and root dry matter of embryonic roots and root dry matter in the subsoil. Taken together, maintaining an optimized root crown architecture, coupled with an increase in surface area, volume and dry matter of embryonic roots and root dry matter in the subsoil, appears to be a more feasible approach for reducing lodging incidents and enhancing nitrogen absorption at a dense population.