Underground Oxygen Deficiency Alters the Spatial Distribution Pattern of Rooting in Alternanthera philoxeroides

Terrestrial and amphibious plants frequently encounter severe oxygen deficiency in waterlogged environments. While the impacts of nutrient and water availability on root development are well documented, how underground oxygen status drives the spatial configuration of root systems remains poorly understood. In the present study, we used a custom hydroponic system to impose three dissolved oxygen treatments corresponding to 5%, 50%, and 100% air saturation and investigated the spatial rooting responses of Alternanthera philoxeroides. Our results demonstrated that low oxygen levels significantly restricted overall plant growth, reducing total root number, total length, and biomass allocation to the root system. However, under hypoxic conditions, plants actively altered their spatial rooting patterns by producing more roots at the top solution layers and fewer roots in the bottom layers, a spatial stratification that was more pronounced under 5% than 50% air saturation. Specifically, low dissolved oxygen availability was associated with an asymmetrical distribution of root traits, with greater root number, length, surface area, volume, and fork number on nodes closer to the solution surface than on deeper nodes. Conversely, well-oxygenated conditions (100% air saturation) promoted a more uniform root trait distribution across all vertical layers. These findings reveal a dissolved oxygen-related spatial rooting pattern, demonstrating that A. philoxeroides actively modulates its root architectural traits as a primary strategy to maximize oxygen acquisition in hypoxic environments, rather than relying solely on physiological tolerance.