Effects of Exogenous Melatonin on Growth and Physiological Characteristics of Petunia Under Salt Stress

Petunia × hybrida is a highly valued ornamental species worldwide, prized for its bright flower colors and long flowering period. Soil salinization is a major abiotic stress that negatively impacts both agriculture and ornamental plant cultivation. Its detrimental effects stem from osmotic stress, ionic toxicity, and oxidative stress induced by the accumulation of reactive oxygen species (ROS). Melatonin, a multifunctional signaling molecule, can enhance plant resistance under adverse conditions. In this study, Petunia × hybrida cv. ‘Mirage Rose’ was used to investigate these effects. The five treatment groups consisted of control (CK), salt stress alone (NaCl, 200 mmol·L−1), and salt stress combined with 50, 100, or 200 µmol·L−1 melatonin (NaCl + MT50, NaCl + MT100, and NaCl + MT200). Evaluations covered developmental morphology, physiological and biochemical parameters, stomatal density, and transcript levels of antioxidant enzymes. Results indicated that high salinity significantly inhibited vegetative growth and reduced stomatal density while increasing the accumulation of malondialdehyde (MDA), superoxide anions (O2−), and hydrogen peroxide (H2O2). Exogenous melatonin application significantly alleviated these adverse effects, with 100 µmol·L−1 being the most effective concentration among the tested doses. This treatment enhanced the activity and gene expression of antioxidant enzymes, reduced membrane lipid peroxidation, promoted the accumulation of compatible solutes for osmotic balance, and improved stomatal development. Overall, 100 µmol·L−1 melatonin effectively enhanced salt tolerance in Petunia by regulating redox homeostasis and modulating stomatal characteristics.