Impact of Melatonin on Antioxidant 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. This study evaluates the efficacy of exogenous melatonin in alleviating moderate salt–alkali stress (120 mM) in poplar (Populus davidiana × P. bolleana ‘Baicheng Shanxinyang No. 1’) seedlings, investigating both pre- and post-stress treatments across a concentration range of 0–1000 μM. 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, reflecting activated stress responses. Melatonin at concentrations of 200–400 μM was the most effective in mitigating stress, significantly enhancing antioxidant enzyme activities such as superoxide dismutase (SOD) and catalase (CAT), restoring chlorophyll content, and reducing oxidative damage markers such as malondialdehyde (MDA). It also regulated osmotic balance in leaves, indicating improved cellular stability under stress. Notably, post-stress application required slightly higher melatonin concentrations to achieve comparable recovery, highlighting the critical influence of application timing. These findings provide valuable insights for optimizing melatonin use to improve poplar growth in saline–alkali environments and support molecular breeding efforts aimed at developing salt–alkali-tolerant poplar varieties.