Melatonin-mediated cross-stress protection in tomato under individual and combined salinity and drought stress

Tomato production is increasingly threatened by the co-occurrence of salinity and drought, but the protective mechanisms of melatonin under their combination are not fully understood. This study investigated whether foliar-applied melatonin could mitigate the individual and combined effects of salinity and drought in tomato (Solanum lycopersicum). Plants were exposed to 0 or 100 mM NaCl and maintained at 90 or 60% field capacity. Melatonin (100 μM) was sprayed three times at five-day intervals, beginning one day before stress application, and plants were sampled after three weeks. Salinity and drought separately reduced growth, leaf water status, photosynthetic pigments, photosystem II (PSII) efficiency, gas exchange, and K status, while their combination generally caused the greatest inhibition. Combined stress also produced the greatest Na/K imbalance and the highest accumulation of hydrogen peroxide (H₂O₂), methylglyoxal, and malondialdehyde (MDA). Melatonin improved growth and photosynthetic performance under all stress conditions; under combined stress, it increased total dry weight, relative water content, and K by approximately 37%, 18.6%, and 50.2%, respectively, and reduced Na and MDA by 39.8% and 39.9%, respectively, relative to untreated stressed plants. These improvements were associated with higher antioxidant and glyoxalase activities, recovery of ascorbate/dehydroascorbate and reduced/oxidized glutathione redox balance, and coordinated changes in anthocyanin biosynthesis, phenolic metabolism, proline, glycine betaine, free amino acids, and soluble sugars. The protective response was especially evident under combined stress, whereas melatonin had little effect under normal conditions. Overall, melatonin provided stress-dependent protection by linking ion homeostasis, ROS and methylglyoxal detoxification, redox recycling, and osmotic and secondary metabolism. These findings support foliar melatonin as a promising approach for improving tomato tolerance to co-occurring salinity and drought.