Ultrafine Bubble Irrigation Enhances Early Growth and Antioxidant Activity of Rice Seedlings Under Nutrient-Free Conditions

Ultrafine bubble (UFB)-enriched water promotes plant growth when nutrients are present. A key question is whether it can still encourage growth in the absence of nutrients. Therefore, this study examines how different UFB concentrations affect the early growth and development of rice seedlings under nutrient-free conditions where external nutritional application was excluded. The results showed that the examined morphological and physicochemical parameters were directly affected by the bubble concentration in the irrigation water. Higher bubble concentrations resulted in a significant increase in the fresh and dry weights of roots, primary and secondary root lengths, and specific root length. Similarly, higher bubble concentrations were also associated with greater shoot height, fresh weight, and dry weight. However, pigment concentrations were not clearly affected, except for anthocyanin. Hydrogen peroxide concentration increased proportionally with bubble concentration. Among the antioxidant enzymes assessed, peroxidase activity increased significantly with bubble concentration, whereas the other antioxidant enzymes showed no significant variation. Moreover, UFB irrigation significantly affected carbon metabolism, increasing soluble sugar content while reducing storage starch levels. In conclusion, the findings suggest that UFB-enriched irrigation can promote plant growth under nutrient-free external conditions by modulating stress-related molecules, activating antioxidant enzymes, and altering carbon metabolism.