Promotion Effect of Ultrafine Bubbles/Nanobubbles on Seed Germination

The venturi-type UFB-generating system was used to generate air and O2 UFBs in distilled water. The number concentrations of air UFBs were roughly controlled by adjusting the generation time and UFB waters ranging from 1.4 x 108 mL-1 to 1.0 x 109 mL-1 were prepared. Barley seeds were submerged in beakers filled with distilled water and UFB water in a ratio of 10 mL of water per seed. The experimental observation of seed germination clarified the role of UFB number concentration, that is, a higher number concentration induced earlier seed germination. In addition, the excessively high UFB number concentration caused the suppression of seed germination. A possible reason for the positive or negative effect of UFBs on seed germination is thought to be ROS generation (hydroxyl radicals and ∙OH, OH radicals) in UFB water. This was supported by the detection of ESR spectra of the CYPMPO-OH adduct in O2 UFB water. However, the question still remains: how can OH radicals be generated in O2 UFB water?