Exogenous Melatonin Enhances Salt Tolerance in Beetroot Through Physiological Mechanisms Independently of Membrane Glycerolipid Remodeling

Melatonin (Mel) is regarded as an eco-friendly substance that enhances plants’ tolerance to abiotic stresses, including salt stress. Plant cell membranes are the primary perception site for the detection and initiation of expeditious responses to changing environmental factors such as salinity. It is important to understand how the plant cell membranes remodel lipids under salt stress conditions. The present study investigates comparative changes in basic physiological traits and membrane glycerolipid contents in the leaves of red and white beetroot (Beta vulgaris ‘Ruby Queen’ and ‘Rodeo’, respectively) cultivars grown hydroponically with Mel (50 µM) application and salt stress treatment (S, 250 mM NaCl), separately and in combination (S + Mel). The results show that Mel reduced salt stress-induced changes in pigment contents, chlorophyll fluorescence (Fv/Fm), electrolyte leakage, H₂O₂ levels, and stomatal conductance, but failed to lower malondialdehyde contents under salt stress. Moreover, Mel led to cultivar-specific salt stress-based changes in total polar lipids, glycolipids, and phospholipids and their molecular species. However, up-regulation of leaf expression of the lipid biosynthesis genes, DGD1, DGD2, MGD, MGD2 and SQD2, was observed under S + Mel compared to salt stress alone. The present results, including cultivar specificity of changes in lipid classes and molecular species, suggest that exogenous Mel application causes positive changes in beetroot physiology through a mechanism likely to be largely independent of lipid metabolism.