Phenotyping and metabolomics insights into the effect of melatonin in lettuce under non-stress and salinity conditions.

Melatonin (MLT) is an indole derivative that exhibits hormone-like activities in plants, regulating multiple aspects of growth and development. Due to its role in mitigating oxidative stress and facilitating osmoprotectant accumulation, MLT enhances abiotic stress tolerance, although the pathways and metabolic mechanisms involved remain unclear despite being studied in various crops. This work aimed to investigate the changes elicited by the exogenous MLT application at different concentrations (10, 50, 150 μM) and its role in mitigating the salinity stress in Lactuca sativa L. through metabolomics and phenotyping approaches. Our results clearly indicated that MLT increases photosynthetic efficiency at high dosage (150 μM) at either early or late salinity stress conditions (p < 0.01). Untargeted metabolomics provided insight into the significant effect of salinity and MLT (p < 0.01 in both cases, according to multivariate chemometrics), mediated by a broad reprogramming involving secondary metabolism, phytohormones, fatty acids and amino acids biosynthesis. In detail, 150 μM MLT induced an adjustment of the phytohormones profile to reduce the salinity-induced damages. Our findings support the well-known potential of melatonin in alleviating salinity stress. These findings address existing challenges in studying the molecular effects of MLT in mitigating abiotic stress, providing insights into the biochemical pathways that drive its effectiveness. In this sense, further research is acknowledged to provide a multidisciplinary high throughput perspective leading to its exploitation in a wide range of crops of agricultural and economic importance.