Melatonin and titanium dioxide nanoparticles alleviate salt stress and enhance fruit quality and biochemical traits in strawberry

Salinity stress is a major constraint limiting strawberry (Fragaria × ananassa Duch.) productivity by disrupting physiological processes, membrane stability, and antioxidant metabolism. This study evaluated the interactive effects of three salinity (Sa) levels (15, 30, and 45 mM NaCl), three melatonin (Me) concentrations (50, 150, and 250 µM), and three titanium dioxide nanoparticle (TiO₂ NP) levels (20, 40, and 80 mg L⁻¹) on fruit quality, physiological traits, and biochemical responses of strawberry plants. The experiment was conducted as a split-plot arrangement in a completely randomized design with three replications. Salinity significantly increased electrolyte leakage (EL), reaching 84.2% under 45 mM NaCl, whereas combined melatonin and TiO₂ NP treatments reduced EL to 48.6%. Total chlorophyll content was markedly improved by the combined treatments, with the highest value (287 mg g⁻¹ FW) recorded under 15 mM salinity with 50 µM melatonin and 80 mg L⁻¹ TiO₂ NPs. Biochemical responses enhanced due to salinity and the treatments resulted in increases of proline (up to 39.4 mmol kg⁻¹), total phenolic compounds (7.5 mg g⁻¹) and flavonoids (31.2 mg g⁻¹), while peroxidase (75 U g⁻¹ FW) and ascorbate peroxidase activities (2.6 U g⁻¹ FW) were also increased. The multi-trait genotype-ideotype distance index (MGIDI) analysis showed the treatment combination of S30M50T80 to have the most favourable treatment and recorded the lowest MGIDI value (1.184) and the best overall performance compared to each of the evaluated traits. These results indicate that the use of melatonin and TiO₂ nanoparticles together can reduce salinity-induced oxidative injury, increase antioxidant defense mechanisms, maintain photosynthetic pigments levels, and improve fruit quality in strawberry.