Exogenous melatonin protects alfalfa (Medicago sativa L.) seedlings from drought-induced damage by modulating reactive oxygen species metabolism, mineral balance and photosynthetic efficiency

Drought stress is a severe global environmental concern for the growth and development of alfalfa (Medicago sativa L.), which is crucial for agricultural sustainability owing to its high feed value and economic prospects. Melatonin (MT) has emerged as a multifaceted biomolecule that plays a fundamental role in regulating plants' multiple environmental stresses. However, the significant effect of exogenous MT on drought stress tolerance in the alfalfa forage crops remains inexpressible. We investigated the effects of foliar-sprayed melatonin at different doses (50, 100, 200 mu M) on alfalfa seedlings' growth and physiological responses under drought stress. Our results showed that drought stress reduced morphological growth attributes (by 12 to 54%), gas exchange parameters (by 37 to 88%), photosynthetic pigments (Chl a, and Chl b contents by 29%, and 40%), mineral contents, and increased lipid peroxidation (by 69%) and accumulation of reactive oxygen species (ROS) in alfalfa plants. In contrast, exogenous foliar application of MT significantly revived growth characteristics, boosted antioxidant enzyme activities, improved photosynthetic performance, decreased ROS-induced oxidative damage by increasing plant mineral elements, and lowered lipid peroxidation in plants under drought stress. In addition, MT also stimulated the accumulation of total soluble sugar and proline content by 48% and by 75% respectively, which were positively associated with improved leaf water status in drought-stressed plants. Meanwhile, our findings revealed that the highest efficiency was achieved at 100 mu M melatonin. In conclusion, these findings bring a new perspective on melatonin-mediated drought stress tolerance and provide a foundational understanding of alfalfa's future cultivation in drought-prone regions globally.