Harnessing foliar-applied melatonin to improve yield and stress tolerance in tomato (Solanum lycopersicum L.) under...

Drought stress severely constrains global crop production, limiting growth, yield, and fruit quality. This study evaluated the efficacy of foliar-applied melatonin (MT) in enhancing drought tolerance in tomato (Solanum lycopersicum L.) by modulating physiological and biochemical responses.

A two-season field experiment employed a split-plot design with three replications, testing two irrigation regimes [full irrigation (FI; 100% crop evapotranspiration, ETc) and deficit irrigation (DI; 60% ETc)], combined with five foliar MT concentrations (0, 50, 100, 150, and 200 µM) applied as foliar sprays.

DI alone significantly (P ≤ 0.05) reduced plant height (26.9%), total yield (44.0%), soil plant analysis development (SPAD) chlorophyll value (41.7%), and relative water content (RWC; 28.6%) compared to well-watered controls. Exogenous MT at 100 µM significantly (P ≤ 0.05) alleviated these effects under DI, increasing plant height by 32.9%, total yield by 51.8%, SPAD value by 51.1%, and RWC by 31.0% relative to untreated stressed plants. MT application enhanced photosynthetic efficiency and preserved leaf integrity, reflected in reduced electrolyte leakage and malondialdehyde content. These improvements stemmed from effective reactive oxygen species (H₂O₂ and O₂ ^•-) detoxification, accompanied by a significant (P ≤ 0.05) upregulation of enzymatic antioxidants [superoxide dismutase (SOD), peroxidase (POD), ascorbate peroxidase (APX), catalase (CAT), and glutathione reductase (GR)] and accumulation of non-enzymatic osmoprotectants including proline, ascorbate, α-tocopherol, glutathione, and total soluble sugars. The FI + 100 μM MT treatment yielded the highest fruit production (5.31 kg plant^-1).

These findings establish foliar MT application at 100 μM as an effective biostimulant strategy for sustaining tomato productivity under water-limited conditions, operating through coordinated key physiological and biochemical defense mechanisms. This approach offers a practical pathway toward more resilient crop production in water-scarce environments.