Crop and weed competition studies rarely determine how plant-to-plantinteractions alter the structure and physiology of crop roots. Soybean hasthe ability to detect neighboring weeds and to alter growth patternsincluding the allocation of resources to root growth. In this study, wehypothesized that low red : far red light ratio (R : FR) reflected fromaboveground vegetative tissue of neighboring weeds would alter soybean rootmorphology and reduce root biomass and nodule number. All experiments wereconducted under controlled conditions in which resources of light, water,and nutrients were nonlimiting. Low R : FR reflected from abovegroundneighboring weeds reduced soybean seedling root length, surface area, andvolume, including the number of nodules per plant. An accumulation of H2O2, an increase in malondialdehyde (MDA) content,a reduction in flavonoid content, and a decrease in1,1-diphenyl-2-picrylhydrazyl (DPPH)–radicle scavenging activity wereobserved. The reduction in flavonoid content was accompanied by a decreasein the transcription of GmIFS and GmN93and an increase in transcript levels of several antioxidant genes. Thesemolecular and physiological changes may have a physiological cost to thesoybean plant, which may limit the plant's ability to respond to subsequentabiotic and biotic stresses that will occur under field conditions.