Dissolved oxygen thresholds drive adaptive functional trait remodeling in mesophytes via anatomical, physiological, and metabolic synergies.

Climate-exacerbated urban waterlogging imposes severe hypoxic stress on mesophytes. However, whether mesophytes possess dissolved oxygen (DO)-driven mechanisms for adaptive structural and physiological remodeling remains poorly understood. We conducted a hydroponic experiment with four representative mesophyte species (Gardenia jasminoides, Murraya paniculata, Ixora chinensis, and Heptapleurum actinophyllum 'Variegata') across four DO gradients (0-2, 2-4, 4-6, and 6-8 mg/L). Plant growth, physiology, and root anatomy were measured, and structural equation modeling (SEM) was applied to disentangle the synergistic pathways underlying waterlogging tolerance. We identified a critical DO threshold: concentrations below 2 mg/L triggered irreversible root degeneration and substantial mortality (50% in I. chinensis), whereas 4-8 mg/L represented the optimal range for maintaining normal growth and 100% survival. Mechanistically, moderate-to-high DO (4-8 mg/L) induced a coordinated shift in functional traits. SEM revealed that DO directly promoted aerenchyma formation (β = 0.77), enhanced internal oxygen transport to mitigate ROS accumulation and significantly reduced membrane lipid peroxidation (MDA, β = -0.39). Concurrently, DO activated the antioxidant enzyme system (β = 0.73) and proline osmoregulation (β = 0.40) to scavenge residual ROS, while increased chlorophyll a content drove plant height (β = 0.90) and biomass accumulation. The four species exhibited distinct adaptive strategies: I. chinensis (structural adaptation), G. jasminoides (physiological compensation), M. paniculata (root reinforcement and osmoregulation), and H. actinophyllum 'Variegata' (synergistic aboveground-belowground growth). This study advances mechanistic understanding of mesophytic stress tolerance under oxygen fluctuations and identifies DO-driven structural-physiological pathways as a reference for plant waterlogging adaptation research.