Green synthesis of zinc oxide nanoparticles using Cassia absus leaf extract and evaluation of their biofertilizer potential and in vitro antibacterial activity against phytopathogens

Conventional agriculture on chemical fertilizers and pesticides has serious environmental and health implications. This necessitates the development of more sustainable and eco-friendly alternatives. This study aimed to synthesize zinc oxide nanoparticles (ZnONPs) using Cassia absus leaf extract via a green synthesis approach and evaluate their dual role as biofertilizers and biopesticides in sustainable agriculture. The biosynthesized ZnONPs were characterized using UV-Vis spectroscopy, X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy-energy dispersive X-ray analysis (SEM-EDX), dynamic light scattering (DLS), and zeta potential analysis. A characteristic absorption peak at 355 nm confirmed nanoparticle (NP) formation, whereas XRD analysis revealed a hexagonal wurtzite structure with a crystallite size of 45-55 nm. SEM analysis showed spherical to quasi-spherical NPs with an average size of 75 nm, whereas DLS indicated a hydrodynamic diameter of 83.5 nm and a zeta potential of + 48.7 mV, confirming excellent colloidal stability. The in vitro antibacterial activity of ZnONPs was evaluated against the phytopathogens Ralstonia solanacearum, Xanthomonas euvesicatoria, and Clavibacter michiganensis subsp. michiganensis. Significant inhibition zones of 20 mm, 18 mm, and 27.5 mm, respectively, were observed, with minimum inhibitory concentrations (MICs) of 30, 40, and 20 µg/mL. In pot experiments using Capsicum annuum, ZnONPs significantly enhanced plant growth parameters, including plant height, leaf length, and leaf width, with up to a 37% improvement observed at moderate concentrations (5 µg/10 mL). These results demonstrate that ZnONPs offer an effective dual strategy for enhancing plant growth and controlling bacterial pathogens, making them promising candidates for sustainable agricultural applications.