Abstract: This study investigated the alleviating effects and underlying mechanisms of exogenous melatonin (MT) on the physiological characteristics and photosynthesis of peony(Paeonia ostii ‘Feng Dan’) seedlings under hexavalent chromium stress. A hydroponic experiment was conducted with the following treatments: control (CK), chromium stress (Cr, 200 μmol/L K₂Cr₂O₇), and chromium stress combined with different concentrations of melatonin (50, 100, 200 μmol/L). The results showed that chromium stress severely inhibited seedling growth, with root dry weight, shoot dry weight, and total dry weight significantly decreasing by 42.3%, 38.9%, and 40.1%, respectively, compared to the control. Photosynthetic pigment synthesis was impeded, and the net photosynthetic rate (P_n) declined by 64.3%. The accumulation of reactive oxygen species (ROS) intensified, and the content of malondialdehyde(MDA), a product of membrane lipid peroxidation, increased by 167.0%. Exogenous application of melatonin, particularly at a concentration of 100 μmol/L, significantly alleviated these detrimental effects. This treatment restored total dry weight to 86.5% and P_n to 82.0% of the control levels. Concurrently, the activities of superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT) were significantly enhanced by 68.4%, 74.2%, and 59.7%, respectively, compared to the Cr-only treatment, while the MDA content was reduced by 52.8%. Furthermore, melatonin application significantly decreased chromium accumulation in both roots and shoots and improved the homeostasis of potassium and calcium ions. The study indicated that exogenous melatonin effectively enhanced the tolerance of peony seedlings to chromium stress through multiple synergistic mechanisms, including bolstering the antioxidant defense system, protecting the photosynthetic apparatus, and regulating ion balance.

Key words: peony(Paeonia suffruticosa), melatonin, chromium stress, reactive oxygen species, photosynthesis, antioxidant enzymes, ion homeostasis