The melatonin improves the growth indices, photosynthetic performance, and essential oil content of Thymus daenensis L. under salt stress

Melatonin plays a crucial role in modulating metabolism and enhancing plant tolerance to biotic and abiotic stresses. This study investigated the mitigating effects of foliar-applied melatonin on growth and physiological responses of Daenian thyme (Thymus daenensis) under varying salinity stress in greenhouse conditions. A factorial experiment was conducted with four NaCl salinity levels (0, 40, 80, and 120 mM) and melatonin treatments (0 and 100 μM). Salinity significantly reduced plant biomass (fresh and dry shoot weight) and photosynthetic pigments (chlorophyll a, b, total chlorophyll, and carotenoids). Conversely, melatonin application improved these parameters across all salinity levels. Specifically, melatonin increased shoot dry weight by 16%, 18.5%, 3%, and 11.1% at 0, 40, 80, and 120 mM NaCl, respectively, compared to non-treated controls. Melatonin also substantially ameliorated chlorophyll content, with total chlorophyll increased by up to 42% under non-saline conditions. Furthermore, melatonin enhanced essential oil content at all salinity levels, achieving a maximum of 2.87% v/w at 40 mM NaCl. Photosystem II efficiency (F_v/F_m) was highest in melatonin-treated plants under non-stress and lowest in untreated plants under severe salinity. Melatonin mitigated salinity-induced reductions in performance index (PI_ABC) and quantum yield of electron transport (ΦEo), while reducing energy dissipation parameters. Overall, foliar melatonin effectively alleviates salinity stress effects, promoting physiological resilience, growth, and essential oil production in Daenian thyme. These findings underscore the potential of melatonin as a practical strategy to enhance crop tolerance and productivity in saline environments.