Effects of Exogenous 5-AminolevuliniC Acid (5-ALA) on Alfalfa (Medicago sativa L.) under NaCl-induced Salinity Stress

5-Aminolevulinic acid (5-ALA) is a plant growth regulator, but its effect on alfalfa (Medicago sativa L.) tolerance to salinity stress is limited. The objective of this study was to investigate the impact of foliar application of 5-ALA on alleviating NaCl-induced salinity stress in alfalfa. Four seedlings’ treatments in soil culture, including control (CK), 0.1 mmol L−1 5-ALA, 150 mmol L−1 NaCl, and 150 mmol L−1 NaCl + 0.1 mmol L−1 5-ALA, were conducted for measurement using methods at morphological, physiological and ultrastructural levels. The results showed that salinity stress inhibited leaf size, leaf number, shoot height, and biomass. Similarly, salinity stress decreased photosynthesis by degrading pigments, reducing photosynthetic gas exchange parameters, increasing stomatal closure and damaging leaf ultrastructure. Additionally, salinity-induced disruptions in ion homeostasis, osmotic balance, and oxidative equilibrium exacerbated plant stress. However, foliar application of 5-ALA proved instrumental in mitigating these detrimental effects. Notably, 5-ALA treatment bolstered growth metrics, enhanced pigment biosynthesis, improved photosynthetic performance, facilitated stomatal regulation, and preserved leaf morphology. Moreover, 5-ALA treatment effectively modulated ion transport, osmotic regulation, and redox balance by attenuating Na+ accumulation, reactive oxygen species production, and lipid peroxidation, while bolstering cellular membrane integrity, osmoprotective mechanisms, and antioxidant defenses. Correlation and principal component analyses underscored the interplay and synergistic effects of these pathways. 5-ALA has a multifaceted role in mitigating salinity stress in alfalfa, and this study underscores the efficacy of 5-ALA as a proactive strategy for enhancing salinity tolerance, growth, and productivity in alfalfa cultivation.