Four Decades of Breeding and Increasing Plant Density Management Reshaped Maize Root System Architecture

Understanding how crop root system architecture (RSA) is influenced by genetics and management is vital for root-based improvement. We examined the impact of four decades of maize breeding on RSA using a panel of maize hybrids planted at different densities across six site-years in the Midwest. Root crowns were shovel-excavated and imaged via X-ray tomography, generating dozens of fine-grained 3D traits. Modern root systems were significantly smaller with fewer, thinner roots at higher density compared to older hybrids. However, the total amount of roots in the topsoil extrapolated across an acre was indistinguishable, revealing an equilibrium between crown size and density. Surprisingly, there was a 20% increase in the soil volume explored by individual modern hybrids, indicating increased overlap of root systems. We estimated modern hybrids share 43-46% of the topsoil with neighbors, compared to 14% in the oldest. Furthermore, when the oldest hybrids were grown at modern densities, their root crowns became more elliptical than modern hybrids, highlighting potential differences in avoidance response. Root systems of modern hybrids may be more intertwined and less competitive, revealing an adaptation to increased density stress in maize agriculture with implications for trait-based approaches to engineer more productive, efficient systems. HIGHLIGHTThis study uses a new panel of maize hybrids to explore how decades of yield-focused breeding and advancements in planting density have indirectly impacted maize root systems.