Biostimulant-Mediated Enhancement of Quinoa Tolerance to NaCl-Induced Salinity Stress Through Foliar Application of Ascorbic Acid, Saponins and Quinoa Seed Extract

Quinoa (Chenopodium quinoa Willd.) is widely recognized for its relatively high tolerance to salinity, but substantial genotypic variation exists, with some varieties remaining sensitive to severe salt stress. Valorizing quinoa biomass and processing-derived resources also offers opportunities for developing zero-waste value chains and producing value-added food and non-food products. This study evaluated foliar applications of saponins (SAP, 0.15%), quinoa seed extract (QSE, 3%), and ascorbic acid (AsA, 0.2 mM) to improve the physiological and biochemical performance of quinoa under salinity (300 mM NaCl). Salinity markedly reduced biomass, photosynthetic pigment content, photosystem I and photosystem II efficiency, and protein levels, while increasing oxidative stress, osmolytes, and secondary metabolites. Foliar treatments substantially alleviated these effects, although responses varied among biostimulants. AsA showed the strongest growth-promoting effect, increasing total dry weight by 115% compared with salt-stressed plants. SAP particularly improved tissue water status, reduced lipid peroxidation, and stimulated osmolyte and antioxidant accumulation, whereas QSE markedly enhanced photosystem II performance. SAP and QSE also promoted biochemical acclimation by increasing levels of proline, soluble sugars, phenolics, flavonoids, tannins, and saponins while reducing malondialdehyde accumulation. Overall, AsA, SAP, and QSE enhanced the resilience of salt-sensitive quinoa under severe salinity, highlighting a potential zero-waste strategy in which quinoa-derived resources, particularly saponins and quinoa seed extract, can be valorized as foliar biostimulants to support sustainable quinoa production in salt-affected environments.