Biogenic synthesis of zinc oxide nanoparticles as sustainable seed biostimulants for enhancing germination, enzymatic activity, and early seedling growth in wheat

Biogenic zinc oxide nanoparticles (ZnO NPs) have emerged as a sustainable approach to enhance early plant growth and micronutrient use efficiency in crops. In the present study, ZnO NPs were synthesized via two eco-friendly routes using Sauropus androgynus leaf extract (plant-mediated) and Pseudomonas isolate AUDP209 (microbial-mediated), and their differential effects on germination physiology and early seedling growth were evaluated in wheat. The synthesized nanoparticles exhibited nanoscale dimensions, with mean particle sizes of 42.3 nm and 28.5 nm for the microbial and plant-mediated systems, respectively, indicating route-dependent variation in physicochemical properties. Seed nano-priming with ZnO NPs significantly enhanced germination, seedling growth, and vigor compared to unprimed controls. Microbial-derived ZnO NPs at 500 ppm for 4 h achieved maximum germination (100% in UAS 428 and 99.5% in UAS 3020), along with the highest root length (19.4 cm), shoot length (14.85 cm), and seedling vigor index II (~34.8) in UAS 428. Plant-derived ZnO NPs at 500 ppm for 2 h also improved germination (~99%) and growth. Biochemical responses revealed enhanced metabolic activation, including increased dehydrogenase activity (up to 0.930 A480) and α-amylase activity (up to 0.133 A540), along with reduced electrolyte leakage (~26.4 µS cm-1), indicating improved membrane stability. The comparatively greater performance of microbial-derived ZnO NPs may be associated with differences in surface chemistry and biomolecular capping, which could influence nanoparticle-seed interactions and Zn availability during seed germination and early seedling growth. This study demonstrates the potential of biogenic ZnO nanoparticles as nano-biostimulants for enhancing germination and early seedling growth in wheat. These findings highlight the potential of biogenic ZnO nanoparticles as sustainable seed biostimulants for improving crop establishment and micronutrient use efficiency, supporting their application in environmentally sustainable agriculture.