Melatonin and 6-benzyladenine improve waterlogging stress recovery of tomato by modulating physio-biochemical traits and antioxidant regulation

Tomato ( Solanum lycopersicum ) is a vital nutritional crop but is highly susceptible to waterlogging stress. Exogenous bio-stimulants could potentially offer a promising strategy to mitigate stress damage and enhance post-stress recovery in tomato. This experiment was thus designed in a factorial fashion to examine the individual and combined alleviation effects of 6-benzyladenine and melatonin during one-week waterlogging exposure and post-stress recovery stage. Waterlogging caused pronounced reductions in shoot and root biomass, photosynthetic pigments, and photosystem II efficiency, accompanied by increased membrane damage, lipid peroxidation and hydrogen peroxide accumulation. Foliar application of 6-benzyladenine or melatonin significantly alleviated these adverse effects by improving antioxidant activity, stabilizing chloroplast and membrane integrity, and sustaining growth. Notably, their combined application exerted an additive effect, resulting in near-complete restoration of biomass, pigment content, and fluorescence attributes during the recovery phase. Enhanced enzymatic and non-enzymatic antioxidant defenses, together with improved redox homeostasis, underpinned this superior performance. Individually, melatonin showed superior performance in enzymatic antioxidant performance and 6-benzyladenine excelled in biochemical regulation of proline, polyphenols and flavonoids synthesis. Our results demonstrated that combined application of 6- benzyladenine and melatonin triggers coordinated protective responses that reinforce physiological resilience and accelerate post-waterlogging recovery in tomato. This strategy holds promise for sustaining tomato productivity under increasingly unpredictable climatic conditions.