Mechanisms of plasma-activated water in inactivating melon bacterial fruit blotch pathogen Acidovorax citrulli: Regulation via nitrogen-to-oxygen ratio

Abstract Bacterial fruit blotch (BFB), caused by Acidovorax citrulli (abbreviated as A. citrulli), poses a severe threat to global melon production, necessitating efficient and eco-friendly control strategies. This study investigated the inactivation mechanisms of plasma-activated water (PAW) generated via a nitrogen-to-oxygen ratio-optimised plasma jet system, alongside its effects on seed germination. Experimental results demonstrated that increasing the O2 proportion shifted the discharge mode from homogeneous glow (N2-rich) to filamentary discharge (O2-rich), modulating reactive species at the gas-liquid interface. The emission spectral intensities of OH (A→X), N2 (C→B), and N2+ (B→X) decreased, while the O (3p→3s) spectral lines intensified. At N2:O2 = 4:1, PAW exhibited peak reactive nitrogen species (RNS) concentrations (NO3⁻: 130.51 mg/L; NO2⁻: 14.49 mg/L) and extreme acidity (pH 2.5), achieving a 7.53-log10 reduction in A. citrulli viability (p < 0.05). Scanning/transmission electron microscopy (SEM/TEM) and Fourier-transform infrared spectroscopy (FTIR) confirmed PAW-induced bacterial membrane perforation and protein denaturation. Comparative experiments revealed that the synthetic reactive oxygen and nitrogen species (RONS) solution achieved only 6.2% of the log-reduction efficacy of PAW, highlighting the synergistic role of plasma-induced acidity and RONS. Moreover, PAW-treated contaminated melon seeds exhibited a 20% increase in germination rate. By establishing quantitative correlations between plasma parameters, liquid-phase chemistry, and biological outcomes, this work provides theoretical and practical foundations for acidic PAW as a sustainable BFB control strategy under ambient conditions.