Humic and Fulvic Acid Induce Distinct Growth Recovery and Stress Acclimation Responses for Flooding Tolerance in Lettuce

Increasing rainfall, driven by climate change, threatens vegetable production, especially flooding-sensitive lettuce. This research comparatively evaluated fulvic acid (FA) and humic acid (HA) for mitigating flooding stress (FS) effects on growth, photosynthesis, oxidative stress, antioxidant defense, and mineral nutrition. FS reduced growth by 15–33%, chlorophyll by 37.6–41.7%, and nutrient accumulation by 12.3–64.6%. Both bio-stimulants alleviated these effects but showed distinct responses. Under FS, HA increased plant height, diameter, and fresh weight by 39, 43, and 93%, respectively, whereas FA increased fresh weight by 116.6% and root dry weight by 46%. Both treatments increased leaf area by 30.5%, chlorophyll-a by 19%, and total chlorophyll by 17.9%. HA enhanced N, K, Mn, and Zn by 158, 110, 207, and 79%, respectively, while FA enhanced S, B, and Na by 112.5, 24.2, and 172.7%, respectively. FA showed the highest H2O2 accumulation but lower MDA than HA, while increasing proline (72%) and catalase (70%), suggesting enhanced oxidative stress management. Overall, HA favored shoot growth, biomass, photosynthetic maintenance, and nutrient recovery, whereas FA favored root recovery and stress acclimation. These complementary responses highlight humic substances as eco-friendly bio-stimulants for flooding tolerance in lettuce. Future research should validate their field and molecular responses.