Humic Substances Enhance Waterlogging Tolerance in Eggplant Through Differential Regulation of Redox Homeostasis and Nutrient Acquisition

Eggplant (Solanum melongena L.), the world’s fifth most important vegetable crop, is highly sensitive to waterlogging (WL), which limits its productivity under climate-driven flooding events. This study evaluated whether humic acid (HA) and fulvic acid (FA) alleviate WL-induced stress by improving nutrient acquisition and antioxidant defense. Eggplant seedlings were subjected to WL stress for 10 days with and without bio-stimulant treatments, under controlled conditions using a simple, completely randomized design. Waterlogging deteriorated soil properties, reducing pH from 7.7 to 5.4, organic matter by ~75%, soil nutrients 73%, resulting in a decline in plant growth (61%), nutrients (23–74%), chlorophyll concentrations (25–45%), and increased oxidative stress indicators (33–314%). Both HA and FA significantly mitigated these stress effects, restoring plant growth by 11–43%, increasing chlorophyll by ~42%, and enhancing nutrients by 10–17-fold. However, HA and FA differentially modulated stress responses. HA reduced H2O2 and MDA by 40–75% and normalized antioxidant enzyme activities, indicating redox homeostasis, while FA maintained relatively higher H2O2 levels (133.7 mmol kg−1) while improving growth and chlorophyll (~32–67%), suggesting a stress priming rather than stress suppression mechanism. Overall, HA conferred WL tolerance by limiting oxidative damage, whereas FA promoted growth through reactive oxygen species signaling, supporting the potential of humic substances as sustainable bio-stimulants for climate-resilient crop production.