Green synthesised zinc oxide nanoparticles mitigate salinity-induced oxidative stress in Vigna radiata L

The productivity of the important dietary legume mungbean (Vigna radiata L.) is severely constrained by abiotic stress, specifically soil salinisation driven by climate variability. To address this challenge, this study evaluated the potential of a novel nanotechnology formulation based on the cyanobacterium Chroococcus minor for the green synthesis and capping of biogenic zinc oxide nanoparticles (CmZnONPs) to alleviate salt stress. The biomolecules of C. minor successfully stabilised and capped well-dispersed zinc oxide nanoparticles: UV-vis spectroscopy confirmed a characteristic peak at 340 nm with an optical band gap of 3.51 eV; FTIR identified functional capping groups with prominent ZnO stretching vibrations below 900 cm^-1; XRD indicated wurtzite crystallinity (average crystallite size 25.26 nm); TEM demonstrated ultrafine nanocores (5.86 nm); FESEM established aggregated structures (33 nm); EDAX confirmed high purity with zinc, oxygen, carbon, and nitrogen matrices; and DLS alongside ZP verified a hydrodynamic diameter of 33.96 nm with a robust surface charge of -37.43 mV. Employing a ten-group experimental design, unmitigated 100 mM NaCl stress caused acute physiological collapse, including severe root meristem arrest, a 51.19% depletion of endogenous auxin, a 54.36% reduction in total chlorophyll, and an over eightfold drop in the K⁺/Na⁺ ratio. Exogenous CmZnONPs at 50 mg L^-1 concentration significantly outperformed raw extract and ionic zinc treatments, effectively restoring shoot length, tissue hydration, photosynthetic pigments, and the K⁺/Na⁺ ratio. Mechanistically, CmZnONPs enhanced osmoregulation by upregulating total soluble proteins, soluble sugars, and antioxidant enzymes (superoxide dismutase at 1.88 U mg^-1 protein and catalase at 30.1 U mg^-1 protein), while activating non-enzymatic defences, phenolics, flavonoids, and alkaloids and maintaining balanced nitric oxide signalling. PCA, Pearson correlation, and hierarchical clustering confirmed that biogenic CmZnONPs can mitigate salinity-induced oxidative stress, providing an efficient, eco-friendly pathway to enhance legume resilience and productivity under salinity.