Inactivation of Fusarium oxysporum f. sp. melonis and Pectobacterium carotovorum subsp. carotovorum in hydroponic nutrient solution by low-pressure carbon dioxide microbubbles
Introduction
Hydroponic culture is increasing as a practice in agriculture because it offers the prospect of high productivity in limited cultivation space. Nutrient solutions used for hydroponic culture are re-circulated and so offers environmental protection and an effective use of resources. However, cyclic use of nutrient solution causes rapid spread of disease by infection via roots throughout an entire cultivation facility (Vanachter et al., 1988; Van Os, 2010, Van Os et al., 2012), so inactivation of plant pathogens in hydroponic nutrient solutions is indispensible. Ultraviolet light, heat, chlorine, iodine, photocatalytic reaction, ozone, and other methods have been investigated as inactivating methods for hydroponic nutrient solutions (Runia, 1995, Ehret et al., 2001, Koohakan et al., 2003, Igura et al., 2004, Bando et al., 2008, Ogai et al., 2008). These technologies can be effective, although they may not be appropriate in all circumstances.
Microbubbles (MB), fine bubbles less than 50 μm in diameter, have recently been extensively studied in many fields because of their specific properties (Takahashi et al., 2003, Takahashi, 2005). Microbubbles rise more slowly in water than millibubbles generated by air pumps, and dissolve easily in water when compressed. In agriculture, air MB have been used for promoting the growth of lettuce in hydroponic cultures (Park and Kurata, 2009), and ozone MB have been used to reduce Fusarium oxysporum f. sp. melonis and Pectobacterium carotovorum subsp. carotovorum in hydroponic nutrient solution and remove residual pesticides from fruit and vegetables (Ikeura et al., 2011a, Ikeura et al., 2011b, Kobayashi et al., 2011).
In the area of food sterilization, non-thermal processes involving supercritical carbon dioxide (SC-CO2) have been widely studied (Spilimbergo and Bertucco, 2003, Damar and Balaban, 2006, Garcia-Gonzalez et al., 2007). A hypothetical inactivation mechanism of SC-CO2 was stated as follow: solubilization of pressurized CO2 in the external liquid phase, cell membrane modification, intracellular pH decrease, key enzyme inactivation and cellular metabolism inhibition due to intracellular pH lowering, direct effect of molecular CO2 and HCO3− on metabolism, disordering of the intracellular electrolyte balance, and removal of vital constituents from cells and cell membranes (Garcia-Gonzalez et al., 2007). However, there has been no practical application of SC-CO2 to food sterilization because of the expensive equipment required for maintaining the high pressure necessary for exerting the inactivating effect stably. We therefore developed a low-pressure CO2 microbubbles (MB-CO2) system in which inactivation is achieved at a pressure lower than the critical pressure, and reported that Escherichia coli, Saccharomyces cerevisiae, and Lactobacillus fructivorans could be inactivated by MB-CO2 (Kobayashi et al., 2009, Kobayashi et al., 2010, Kobayashi et al., 2012a, Kobayashi et al., 2012b). However, the inactivation of plant pathogens in hydroponic nutrient solution by MB-CO2 treatment was not still tested. In addition, it is necessary to confirm the effect of CO2 dissolved into hydroponic nutrient solution at high concentration on the cultivating plants.
In this study, to propose MB-CO2 as a practical technique for inactivating plant pathogens, which were plant pathogens infecting into plant roots, a hydroponic nutrient solution of F. oxysporum f. sp. melonis, which causes wilt, and P. carotovorum subsp. carotovorum, which causes soft rot, was inactivated by MB-CO2. The inorganic components in the MB-CO2-treated hydroponic nutrient solution were analyzed, and a growth test of leaf lettuces with the MB-CO2-treated hydroponic nutrient solution was performed.
Section snippets
Preparation of plant pathogenic fungal and bacterial suspensions
F. oxysporum f. sp. melonis NBRC6385 and P. carotovorum subsp. carotovorum NBRC12380 were purchased from the National Institute of Technology and Evaluation (Kisarazu, Japan), and suspensions were prepared as reported previously (Kobayashi et al., 2011). F. oxysporum f. sp. melonis was incubated in potato dextrose agar (PDA, Difco, USA) plates for 7 days at 30 °C. Ten milli liters of 0.05% Tween 80 (Wako Pure Chemical Industries, Ltd., Osaka, Japan) were poured onto 10 cultured PDA plates. The
Inactivation of F. oxysporum f. sp. melonis and P. carotovorum subsp. carotovorum by MB-CO2
Inactivation of F. oxysporum f. sp. melonis by MB-CO2 is shown in Fig. 2(a)–(c). No inactivation of F. oxysporum f. sp. melonis was induced by MB-CO2 at 35 °C and 40 °C in heating coil and 1.0 MPa in mixing vessel, although 3-log reductions were occurred at 45 °C for 20 min. The MB-CO2 efficiency on the inactivation of F. oxysporum f. sp. melonis increased along with the temperature of the heating coil, because 5-log reductions were achieved by MB-CO2 with 40 °C for 20 min and 45 °C for 5 min of the
Conclusion
MB-CO2 had a strong effect on the inactivation of F. oxysporum f. sp. melonis and P. carotovorum subsp. carotovorum in hydroponic nutrient solution, and did not adversely affect the inorganic components in the hydroponic nutrient solution. Furthermore, the growth of leaf lettuces was almost unaffected when they were grown in the MB-CO2-treated hydroponic nutrient solution. These results suggest that MB-CO2 may be a breakthrough technique for inactivating plant pathogens in hydroponic nutrient
Acknowledgement
This study was financially supported by the Kurita Water and Environment Foundation.
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