JIA-2018-09

2028 Tamaki Masahiko et al. Journal of Integrative Agriculture 2018, 17(9): 2024–2030 that the contact between P . oryzae Cavara conidia and CO 2 MB increased due to the stabilization and the decrease of ascend rates in the presence of BF, resulting in decrease of germination ratios. In addition, the shrinkage of MB caused by gas dissolution increases the inner pressure and the surface area, and a logarithmic increase in the shrinkage rate has been observed. Hydroxyl radicals are generated due to the decomposition of water molecules, associated with the collapse of MB during shrinkage, and directly attack the DNA, enzymes and other components of microorganisms, leading to cell death (Takahashi 2005). Takahashi et al . (2007) also showed the decomposition of phenol due to hydroxyl radicals produced by collapsing AirMB. Mulakhudair et al . (2017) suggested that the inactivation of P . putida was caused by the free radicals generated from CO 2 MB. Similarly, Kukizaki and Baba (2008) indicated that the size of the MB generated in an anionic surfactant solution was smaller than that of MB generated in a non-ionic surfactant solution, and a strong electrostatic repulsion between MB was induced. Because anionic and non-ionic surfactants were contained in the BF solution used in the present experiment, it might be a possibility to increase hydroxyl radical production with increasing anionic charge during MB treatment. Based on our results, not only dissolved CO 2 concentration and the low pH but also the generation of hydroxyl radicals had an effect on the inhibition of appressorium formation of P . oryzae Cavara conidia. The destruction or modification of the cell wall by the decompression-type CO 2 MB is more severe than that caused by CO 2 MMB and the gas-water circulating-type CO 2 MB (Fig. 2-C, D and E); this can be explained by the smaller size of the decompression-type CO 2 MB and the greater generation of hydroxyl radicals compared to the gas-water circulating-type CO 2 MB (Ikeura et al. 2017). In a similar study, MB promoted the breaking of algal cells by microwave treatment (Krehbiel et al . 2014). Specifically, it is presumed that MB directly act on the cell wall and A D C E 1 μm 1 μm 1 μm 1 μm 1 μm 3.0 kv ×4 300 3.0 kv ×4 300 3.0 kv ×4 300 3.0 kv ×4 300 3.0 kv ×4 300 B Fig. 2 Scanning electron microscopy (SEM) images of Pyricularia oryzae Cavara conidia treated with Blasin ® Flowable (BF) solution containing carbon dioxide millibubbles (CO 2 MMB) and both CO 2 microbubbles (CO 2 MBs). A, nontreatment. B, BF solution only. C, BF solution containing CO 2 MMB. D, BF solution containing the gas-water circulating-type CO 2 MB. E, BF solution containing the decompression-type CO 2 MB. BF solution diluted to 5000-fold was used for 5 min.