Mechanisms of plasma-activated water in inactivating melon bacterial fruit blotch pathogen <i>Acidovorax citrulli</i>: 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 O 2 proportion shifted the discharge mode from homogeneous glow (N 2 -rich) to filamentary discharge (O 2 -rich), modulating reactive species at the gas–liquid interface. The emission spectral intensities of OH (A → X), N 2 (C → B), and N 2 + (B → X) decreased, while the O (3 p → 3 s ) spectral lines intensified. At N 2 :O 2 = 4:1, PAW exhibited peak reactive nitrogen species concentrations (NO 3 − : 130.51 mg l −1 ; NO 2 − : 14.49 mg l −1 ) and extreme acidity (pH 2.5), achieving a 7.53-log 10 reduction in A. citrulli viability ( p &lt; 0.05). Scanning/transmission electron microscopy and Fourier-transform infrared spectroscopy 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.