Effect of ultrafine bubble water on germination of brown and unhulled rice seeds under soaking and priming conditions

Ultrafine bubbles (UFB) water has gained attention as a chemical-free method for seed treatment; however, its effects on rice seed germination remain unclear, particularly under drought stress. This study evaluated the effects of UFB on rice seeds (brown and unhulled) using two approaches: continuous soaking and priming followed by germination under osmotic stress. During continuous soaking, germination dynamics differed between seed types, while no significant differences were detected between UFB and distilled water within each type. During priming, UFB reduced water uptake in brown rice but enhanced later-stage hydration in unhulled rice, accompanied by a reduction in husk thickness. Total reactive oxygen species (ROS) levels in both the solution and seed surface also varied between seed types under UFB treatment. Under drought stress, final germination remained high (≥ 80%) across all treatments, although germination slowed with increasing PEG concentration. Priming markedly accelerated germination compared with no-priming, with unhulled rice showing a greater reduction in time to 50% germination (T50) than brown rice. Among priming treatments, UFB showed a stronger promotive effect than distilled water in brown rice, whereas its impact on unhulled rice was limited and depended on stress levels. Overall, these findings indicate that UFB influences germination through interactions with seed structure, hydration behaviour, and oxidative processes, highlighting its potential as a sustainable strategy for improving rice seed germination under drought conditions.