Co-Application of 6-Benzyladenine and Melatonin Mitigates Waterlogging Tolerance in Eggplant

Waterlogging (WL) is a major abiotic stress that restricts eggplant productivity by disrupting plant growth, photosynthesis, cellular homeostasis, and antioxidant defense systems. Although both 6-benzyladenine (BA) and melatonin (MT) have been reported to enhance stress tolerance in plants, their combined effects against WL stress in eggplant (Solanum melongena L.)remain poorly understood. This study investigated the individual and synergistic roles of exogenous BA and MT in mitigating WL-induced damage in two eggplant varieties contrasting in WL tolerance: the tolerant cultivar BHB−6 and the susceptible cultivar BB−8. WL markedly inhibited plant growth and photosynthetic pigment accumulation while intensifying oxidative damage, as evidenced by increased hydrogen peroxide (H2O2) and malondialdehyde (MDA) contents in both varieties. In contrast, combined application of BA and MT significantly alleviated these adverse effects, outperforming the individual treatments by restoring growth, preserving chlorophylls, and suppressing oxidative stress. Under BA + MT treatment, BHB−6 exhibited superior stress adaptation, maintaining higher relative leaf water content (+15%) and accumulating greater levels of soluble sugars (+44%) and free amino acids (+80%) than only WL−stressed plants. This enhanced tolerance was associated with a marked activation of antioxidant defenses, including increases in the activities of catalase (CAT), peroxidase (POD), and ascorbate peroxidase (APX). BB−8 showed comparatively lower recovery, but BA + MT substantially improved its tolerance through enhanced osmotic adjustment and antioxidant defense, compared to only WL −stressed plants. Multivariate analyses further confirmed that the combined BA + MT treatment consistently outperformed the individual applications across physiological and biochemical traits. Overall, these findings demonstrate that co-application of BA and MT effectively enhances WL tolerance by strengthening antioxidant capacity, improving osmotic regulation, and maintaining physiological integrity, with more pronounced benefits in the resistant variety while providing substantial protection to the susceptible variety.