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Nanobubbles (NBs) have been reported to be effective at accelerating the metabolism of living organisms, but the mechanism is not yet well understood. In this study, the production of reactive oxygen species (ROS) by NBs and its effect on seed germinations were investigated. The fluorescence response of APF to NB water was measured. It changed depending on the NB number density which decreased with storage time. This indicated that NBs could produce ROS and the amount of ROS had positive correlation with the NB number density. The fluorescence intensity of APF increases linearly with the concentration of H 2 O 2 in the range of 0−1 mM. Just after the NB generation, the oxidative capacities represented by amount of ROS of oxygen NB water and gas-mixture (air + nitrogen) NB water were estimated to be equivalent to 0.5 and 0.3 mM H 2 O 2 respectively. The seed germination tests were performed in the NB water, distilled water and H 2 O 2 solutions. The germination rate at each observation times of seeds submerged in gas-mixture NB water and 0.3 mM H 2 O 2 solutions were both higher than those submerged in distilled water. The amounts of superoxide radicals in the seeds were detected using NBT staining. The results of absorbance data proved that the amounts of O 2•− in seeds submerged in gas-mixture NB water and in 0.3 mM H 2 O 2 solution were similar and significantly higher than those in the distilled water. These results indicated that moderate level of exogenous ROS produced by NB water played an important role in seed germination.

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The storage of ripe tomatoes in low-O(2) conditions with and without CO(2) promotes γ-aminobutyric acid (GABA) accumulation. The activities of glutamate decarboxylase (GAD) and α-ketoglutarate-dependent GABA transaminase (GABA-TK) were higher and lower, respectively, following storage under hypoxic (2.4 or 3.5% O(2)) or adjusted aerobic (11% O(2)) conditions compared to the activities in air for 7 days at 25 °C. GAD activity was consistent with the expression level of mRNA for GAD. The GABA concentration in tomatoes stored under hypoxic conditions and adjusted aerobic conditions was 60-90% higher than that when they are stored in air on the same day. These results demonstrate that upregulation of GAD activity and downregulation of GABA-TK activity cause GABA accumulation in tomatoes stored under low-O(2) conditions. Meanwhile, the effect of CO(2) on GABA accumulation is probably minimal.