Humic Substances Enhance Waterlogging Tolerance in Cabbage Seedling via Antioxidant Activation and Hormonal Reprogramming

As climate change continues to alter rainfall patterns and precipitation regimes across the globe, waterlogging is emerging as a widespread and pressing issue that threatens agricultural productivity and food security. In this study, we investigated the potential of humic substances to mitigate waterlogging stress in cabbage (Brassica oleracea L.) seedlings. Specifically, humic acid and fulvic acid solutions were applied to the growth medium at weekly intervals both before and after a 10-day waterlogging period. The effects of humic acid and fulvic acid applications on waterlogging-induced stress were evaluated through various physiological and biochemical parameters, including shoot fresh weight, root fresh weight, shoot dry weight, root dry weight, plant height, stem diameter, chlorophyll a, chlorophyll b, total chlorophyll, proline, malondialdehyde, hydrogen peroxide, indole acetic acid, gibberellic acid, abscisic acid, and antioxidant enzyme activities including catalase, peroxidase, and superoxide dismutase. The results indicated that waterlogging stress significantly impaired plant growth parameters, but these adverse effects were mitigated by humic acid and fulvic acid applications. The humic substances contributed to stress tolerance by modulating key biochemical responses, including a shift in proline, hydrogen peroxide, malondialdehyde, abscisic acid, and antioxidant enzyme activity levels, which otherwise increased under stress conditions. Furthermore, the decline in indole acetic acid and gibberellic acid content due to waterlogging was alleviated by humic acid and fulvic acid treatments. Overall, the findings suggest that humic acid and fulvic acid can effectively reduce the detrimental effects of waterlogging stress in cabbage seedlings, demonstrating their potential as biostimulants with comparable protective effects.