Impact of Melatonin on Oxidative Enzymes and Soluble Metabolites in Salt-Alkali Stressed Poplar (Populus spp.): A Comparative Study of Pretreatment and Post-Treatment Effects

Melatonin plays a crucial role in modulating plant stress responses; however, its potential for mitigating salt-alkali stress remains incompletely understood. The present study evaluates the efficacy of exogenous melatonin in alleviating moderate salt-alkali stress (120 µM) in poplar ( Populus spp .) seedlings, investigating both pre- and post-stress treatments (0–1000 mmol·L⁻¹). Physiological and morphological parameters including chlorophyll content, antioxidant enzyme activities, and osmolyte accumulation were analyzed to assess stress responses. Under salt-alkali stress, seedlings exhibited elevated stress markers and osmolyte levels, indicating activated stress responses. More importantly, melatonin at a concentration of 200 mmol·L⁻¹ was identified as the most effective in mitigating stress effects, significantly enhancing antioxidant enzyme activities such as superoxide dismutase (SOD) and catalase (CAT), while restoring chlorophyll content and reducing oxidative damage markers like malondialdehyde (MDA). Additionally, it contributed to the regulation of osmotic regulators in the leaves, indicating improved cellular stability under stress conditions. Notably, post-stress application of melatonin required slightly higher concentrations to achieve comparable levels of recovery compared to pre-treatment, underscoring the critical influence of application timing on its efficacy. These findings highlight the valuable insights into the strategic use of melatonin for stress mitigation and provide a foundation for molecular breeding efforts aimed at developing salt-alkali-tolerant poplar varieties.