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.

Key messageA plant growth regulator, 5-aminolevulinic acid, enhanced the saline-alkali tolerance via photosynthetic, oxidative-reduction, and glutathione metabolism pathways in pepper seedlings.AbstractSaline-alkali stress is a prominent environmental problem, hindering growth and development of pepper. 5-Aminolevulinic acid (ALA) application effectively improves plant growth status under various abiotic stresses. Here, we evaluated morphological, physiological, and transcriptomic differences in saline-alkali-stressed pepper seedlings after ALA application to explore the impact of ALA on saline-alkali stress. The results indicated that saline-alkali stress inhibited plant growth, decreased biomass and photosynthesis, altered the osmolyte content and antioxidant system, and increased reactive oxygen species (ROS) accumulation and proline content in pepper seedlings. Conversely, the application of exogenous ALA alleviated this damage by increasing the photosynthetic rate, osmolyte content, antioxidant enzyme activity, and antioxidants, including superoxide dismutase, catalase, glutathione reductase, and glutathione peroxidase, and reducing glutathione to reduce ROS accumulation and malonaldehyde content. Moreover, the transcriptomic analysis revealed the differentially expressed genes were mainly associated with photosynthesis, oxidation-reduction process, and glutathione metabolism in saline-alkali stress + ALA treatment compared to saline-alkali treatment. Among them, the change in expression level in CaGST, CaGR, and CaGPX was close to the variation of corresponding enzyme activity. Collectively, our findings revealed the alleviating effect of ALA on saline-alkali stress in pepper seedlings, broadening the application of ALA and providing a feasible strategy for utilize saline-alkali soil.