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.

Background With the progress of industrialization and urbanization, cadmium (Cd) pollution in farmland is increasingly severe, greatly affecting human health. Sunflowers possess high resistance to Cd stress and great potential for phytoremediation of Cd-contaminated soil. Previous studies have shown that humic acid (HA) effectively mitigates plant damage induced by Cd; however, its alleviating effects on sunflower plants under Cd stress remain largely unknown. Results We employed four different concentrations of HA (50, 100, 200, and 300 mg L-1) via foliar application to examine their ability to alleviate Cd stress on sunflower plants' growth, chlorophyll synthesis, and biochemical defense system. The results revealed that Cd stress not only reduced plant height, stem diameter, fresh and dry weight, and chlorophyll content in sunflower plants but also altered their chlorophyll fluorescence characteristics compared to the control group. After Cd stress, the photosynthetic structure was damaged and the number of PSII reactive centers per unit changed. Application of 200 mg L-1 HA promotes sunflower growth and increases chlorophyll content. HA significantly enhances antioxidant enzyme activities (SOD, POD, CAT, and APX) and reduces ROS content (O-2(-), H2O2 and -OH). Totally, Application of 200 mg L-1 HA had the best effect than other concentrations to alleviate the Cd-induced stress in sunflower plants. Conclusions The foliar application of certain HA concentration exhibited the most effective alleviation of Cd-induced stress on sunflower plants. It can enhance the light energy utilization and antioxidant enzyme activities, while reduce ROS contents in sunflower plants. These findings provide a theoretical basis for using HA to mitigate Cd stress in sunflowers.