Exogenous Abscisic Acid Enhances Waterlogging Tolerance in Lindera megaphylla

The waterlogging tolerance of Lindera megaphylla, an evergreen species valued for ecological restoration and its role in landscapes, remains unclear, hindering its broader use in riparian green spaces and rain gardens. This study systematically assessed its physiological responses to simulated waterlogging stress (control/CK, mild/W1, moderate/W2, and severe/W3) and exogenous abscisic acid (ABA) applications (0, 1, and 3 μmol/L). The results showed that severe waterlogging (28 d) drastically reduced seedling survival to 30%, inhibited growth, induced significant reactive oxygen species (ROS) accumulation and membrane damage (malondialdehyde (MDA) +118.59%, relative conductivity (REC) +85.54%), and decreased photosynthetic pigments (Chla −41.60%, Chlb −40.02%, Car −34.33%). Exogenous ABA (3 μmol/L) substantially alleviated stress, increasing survival by 60.61% and enhancing tolerance through three integrated processes: (1) enhancing antioxidant defense (superoxide dismutase (SOD) +10.63%, peroxidase (POD) +9.33%) and reducing ROS; (2) stabilizing osmotic regulation (lower soluble sugars, proteins, and proline and increased leaf water content by +7.89%); (3) preserving photosynthetic integrity, evidenced by restored chlorophyll levels and significantly improved photosystem II and I efficiency. This study is the first comprehensive demonstration that ABA enhances L. megaphylla’s waterlogging tolerance by coordinating antioxidant, osmotic, and photosynthetic responses.