Cysteine derivatives coordinate photosynthetic performance and osmotic adjustment to enhance drought tolerance in vegetable soybean

BACKGROUND: Drought stress severely constrains photosynthetic efficiency in vegetable soybean. However, the role of cysteine-based biostimulants in coordinating photosynthetic regulation and osmotic adjustment remains poorly understood. This study evaluated how N-acetylcysteine (NAC) and N-acetylcysteine trisulfide (NAC-TS) modulate photosynthetic function, photoprotection, and osmotic adjustment to enhance growth and reproductive performance under drought in two vegetable soybean (Glycine max L. Merr.) cultivars with different yielding capabilities and drought sensitivities (UVE17 and AGS429). RESULTS: Both cysteine derivatives slightly increased stomatal conductance under drought. N-acetylcysteine enhanced photosystem II (PSII) functionality in UVE17, as evidenced by increased performance index (PIabs), increased energy absorption and trapping per reaction centre, reduced energy dissipation, and improved pigment stability. In contrast, NAC-TS elicited stronger photochemical and energy flux responses in AGS429, while promoting higher chlorophyll and carotenoid accumulation in UVE17. These responses were closely associated with cultivar-dependent osmotic adjustment, where NAC increased relative water content and soluble sugars in UVE17, whereas NAC-TS enhanced proline and soluble sugars in AGS429. The coordinated improvement in water status, pigment stability, and osmotic balance helped sustain photosynthetic activity and protect the photosynthetic apparatus under drought stress. Consequently, these physiological adjustments translated into improved growth and reproductive performance, which were also strongly cultivar and treatment-dependent. NAC enhanced shoot biomass, leaf area, and shoot length (notably in UVE17), while NAC-TS increased shoot length and leaf area in both cultivars. At the reproductive level, NAC-TS showed improved reproductive development in AGS429 by increasing pod number. CONCLUSIONS: NAC and NAC-TS enhance drought resilience in vegetable soybean through coordinated regulation of photosynthesis, photoprotection, osmotic adjustment, and water relations, ultimately improving growth and reproductive performance. Their effects are strongly cultivar- and trait-dependent, highlighting the importance of genotype-specific optimisation of cysteine derivative applications for effective drought mitigation.