Ultrafine Bubble Priming Promotes Adzuki Bean (Vigna angularis) Germination Under Drought Stress via Modulation of Seed Traits and Reactive Oxygen Species Production

Ultrafine bubble (UFB) priming has recently emerged as a promising technique to enhance seed germination, yet its physiological mechanisms remain unclear. This study investigated the effects of UFB priming on adzuki bean (Vigna angularis) seeds under stress-free and drought conditions. Four treatments were tested: Control (non-primed), DW (primed with distilled water), UFB1 (4.56 × 107 particles/mL), and UFB2 (1.13 × 108 particles/mL). For priming, seeds were immersed in each solution at 25 ± 1 °C for 24 h. Seed traits, including water uptake, coat thickness, and total reactive oxygen species (ROS) levels in the priming solutions and seeds, were analyzed. UFB priming reduced seed coat thickness and slowed early water uptake without affecting final moisture content. Total ROS levels displayed a concentration-dependent pattern, with higher UFB increasing ROS in water but reducing them on the seed surface. Under stress-free conditions, all priming treatments accelerated germination, although higher UFB slightly prolonged T50. Under drought simulated by polyethylene glycol (PEG 10 and 15%), non-primed seeds were strongly inhibited, whereas primed seeds maintained high germination rates (>97%). Our results demonstrated that UFB priming improved water uptake, seed coat structure, and ROS signaling, enhancing seed performance of adzuki bean under favorable and drought conditions.