Increasing plant density underpins greater root length density and root biomass of modern maize production

Context: Breeding maize hybrids for improved yield has resulted in numerous changes to shoot phenotypes. We know less about how maize roots have changed despite their pivotal role in crop productivity. Understanding crop root traits is important for developing hybrids that can continue to increase yields and provide ecosystem services such as soil C storage under a changing climate. Objective: Our goal was to measure changes in root traits that occurred as a consequence of long-term breeding for grain yield alongside elevated planting densities. Methods: We planted four era panel maize hybrids, two old hybrids released pre-1950 and two new hybrids released after 2010, in a three-year field experiment under historic (30,000 plants ha⁻¹) and modern plant populations (69,000 plants ha⁻¹) in Lexington, KY. We used minirhizotrons to measure root traits in-situ at the V7, R2, and R6 growth stages (approximately 60 cm deep) and collected soil cores to 90 cm after harvest to measure root biomass. Results and conclusions: New hybrids produced greater yields than old hybrids, with new hybrids generating 50% more yield than old hybrids when both were planted at the modern planting density, and 42% more yield when planted at the historic planting density. The modern planting density increased areal root length density (mm root mm⁻² minirhizotron viewing area) at maturity by 23% and root biomass (kg ha⁻¹) by 69% for both old and new hybrids averaged across depths. Root biomass and areal root length density were similar between new and old hybrids regardless of planting density. However, at the R6 growth stage, the root length density of fine roots < 1 mm in diameter was lower for new hybrids than old hybrids for most combinations of planting density and depth. Significance: Even though the root biomass, areal root length density, and stover biomass were not different between old and new hybrids, breeding has likely had an indirect positive effect on soil organic C stocks by increasing the optimum planting density over time, resulting in greater C inputs and root length delivered to the soil.