Exogenous N-Acetyl-L-cysteine Improves Soybean Saline–Alkali Tolerance by Enhancing the hGSH Pathway to Alleviate Oxidative Damage

Soil salinization and alkalization severely threaten global agricultural sustainability. Although N-acetylcysteine (NAC) is well known as an antioxidant in animal research, its effects on plant salt–alkali tolerance and the underlying mechanisms remain unclear. This study investigated the regulatory effects of foliar-sprayed NAC on two soybean (Glycine max) cultivars—salt-tolerant HF50 and salt-sensitive HN95—under saline–alkali stress. Exogenous NAC alleviated seedling growth inhibition by supplying cysteine, which elevated glutamate–cysteine ligase (GCL) activity and promoted homoglutathione (hGSH) accumulation. It further strengthened the ascorbic acid (AsA)–glutathione/homoglutathione ((h)GSH) cycle and elevated antioxidant enzyme activities, thereby relieving oxidative injury, while also activating DNA repair pathways to preserve genomic stability and protecting chloroplast and mitochondrial integrity. Notably, distinct genotype- and tissue-specific responses to NAC were detected. HN95 relied primarily on root antioxidant defenses in a strongly concentration-dependent manner, whereas HF50 coordinated antioxidant metabolism and DNA repair more effectively, showing consistent responses to NAC concentrations. These findings elucidate the coordinated physiological and molecular mechanisms by which exogenous NAC mitigates saline–alkali damage in soybean, providing a theoretical foundation for using NAC to enhance crop stress resilience and develop sustainable cultivation techniques for saline–alkali soils.