Melatonin Modulates Photosynthetic Pigments, Ion Homeostasis, Antioxidants and Terpenes Profile to Mitigate Salinity Stress in Sweet Basil Plants

Abstract

Melatonin (MT) has been extensively studied for its ability to en-hance plant resistance to abiotic stresses. However, its impact on secondary metabolites, such as terpenes in salt-stressed sweet basil, remains unclear. This research examined the foliar application of MT (0, 50, and 100 μM) on sweet basil plants exposed to 75 and 150 mM NaCl. Salinity gradually hin-dered plant growth, affected photosynthetic pigments, and disrupted ionic balance by increasing Na+ absorption and decreasing K+ and Ca2+. Addition-ally, salt-stressed plants exhibited significant increases in H2O2 and malondialdehyde, resulting in reduced electrolyte leakage (EL%) and com-promised cell membrane stability. Conversely, applying MT alleviated these effects by boosting levels of ascorbic acid, carotenoids, total soluble phe-nols, superoxide dismutase, polyphenol oxidase, and phenylalanine ammo-nia-lyase. Notably, GC-MS analysis revealed a strong connection between MT application and terpene production. Out of 46 detected terpenes, β-lin-alool was the most abundant. The highest β-linalool ratio was observed at 100 μM MT under moderate salinity stress (75 mM NaCl). Under severe salinity conditions, this terpene was more suppressed compared to the con-trol. These findings suggest that, depending on salinity levels, MT may en-hance sweet basil's tolerance to salt stress by modulating the phenylpro-panoid and mevalonic acid pathways.