Humic substances mitigate physiological and biochemical responses to waterlogging stress in snake cucumber (Cucumis...

Extreme rainfall events driven by climate change increasingly cause waterlogging stress in horticultural crops, leading to severe growth inhibition, nutrient imbalances, and oxidative damage. The hypothesis of this study was that humic (HA) and fulvic (FA) acids alleviate waterlogging-induced stress, as reflected in changes in oxidative stress indicators and nutrient dynamics in snake cucumber (Cucumis melo var. flexuosus).

Seedlings were evaluated after 10 days of waterlogging, and HA or FA was applied to the root zone at a rate equivalent to 10 kg ha^-1.

Waterlogging caused substantial losses in plant growth parameters, chlorophyll content, and macro- and micronutrient concentrations, and increased hydrogen peroxide (H₂O₂) levels, malondialdehyde (MDA) concentrations, proline accumulation, and activities of CAT, POD, and SOD. Waterlogging significantly reduced plant growth (up to 36%) and chlorophyll content (up to 52%) and decreased macro- and micronutrient concentrations (up to 80-84%), while increasing hydrogen peroxide (H₂O₂), malondialdehyde (MDA), and proline levels, as well as antioxidant enzyme activities. These effects were ameliorated by the application of HA and FA, as evidenced by increases in shoot and root biomass, chlorophyll content, and leaf nutrient uptake under stress conditions. In addition, greater reductions in H₂O₂ and MDA levels, associated with lower lipid peroxidation, were observed in the presence of humic substances, along with alterations in CAT, POD, and SOD activities, indicating improved redox homeostasis. Both treatments promoted shoot and root biomass, increased chlorophyll content, and reestablished nutrient homeostasis under waterlogging stress. The regulation of antioxidant enzyme activities and the controlled accumulation of proline suggested improved redox balance between ROS-generating and ROS-scavenging systems in treated plants.

The modulation of antioxidant enzyme activities suggests that HA and FA may play a role in maintaining redox homeostasis rather than acting solely as stress-response regulators. In parallel, the increase in macro- and micronutrient concentrations reflects enhanced root function in nutrient acquisition from a hypoxic soil solution. Overall, humic and fulvic acids enhanced waterlogging tolerance in Cucumis melo var. flexuosus, highlighting the potential of humic substance-derived biostimulants as a sustainable strategy to improve crop resilience under flooding conditions.