Abstract
Salinity adversely affects plant growth and productivity, particularly during early developmental stages such as seedling establishment and heading. To mitigate salt stress, various natural and synthetic chemical regulators have been applied to soils and plants in crops such as wheat, rice, maize, and quinoa, as they are effective in alleviating multiple abiotic stresses. However, research on the role of melatonin in quinoa under salt stress remains limited. This study investigated the effects of exogenous melatonin on growth, osmotic regulators, inorganic ions, photosynthetic performance, and antioxidant capacity in quinoa (Chenopodium quinoa) under melatonin treatments alone (1, 10, and 100 µM), salt stress (200 mM NaCl), and combined treatments of melatonin (1µM + 200 mM, 10 µM + 200 mM, 100 µM + 200 mM). The results showed that salt stress significantly reduced the relative growth rate (RGR), fresh weight (FW), and leaf relative water content (RWC). In contrast, exogenous melatonin effectively alleviated these inhibitory effects, leading to improved growth performance under combined treatments. Melatonin application also increased photosynthetic pigment content (SPAD) across all leaf positions and significantly enhanced gas exchange parameters compared with salt stress alone. Furthermore, melatonin positively regulated osmotic adjustment by increasing soluble sugars and proline content in quinoa under salt stress. Notably, antioxidant defense was enhanced, with higher enzyme activities of SOD, POD, and CAT and a reduction in MDA content. Moreover, exogenous melatonin reduced Na(+), Cl(-), Na(+)/K(+), and H(2)PO(4) (-) levels in leaves, stems, and roots, while increasing K(+), NO(3) (-), and SO(4) (2-) levels, indicating improved ions homeostasis. This study highlights the protective role of exogenous melatonin in alleviating salt stress in quinoa, with 100 µM being an effective concentration under saline conditions. SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1007/s12298-026-01726-2.