Seed treatment with acidified water improving rice seedling emergence under flooding stress through oxidative damage mitigation and energy supply maintenance

• Acidified water seed treatment (optimal pH 3.25–3.5) significantly improves seedling emergence in direct-seeded rice under flooding conditions. • The treatment enhances flood tolerance by activating alcoholic fermentation (via increased PDC and ADH activity) to sustain energy supply under oxygen deprivation. • It mitigates oxidative stress through upregulation of key antioxidant enzymes (SOD, POD, and CAT), thereby maintaining cell membrane integrity. Flooding stress is the primary adverse condition during seedling establishment in direct-seeded rice systems. This study investigated seed treatment with acidified water as a strategy to enhance seedling emergence and flood tolerance under flooding stress, while elucidating the underlying physiological mechanisms. Using rice varieties Yangdao6 (YD6) and Yongyou1540 (YY1540), seeds were pretreated with three acids (AcOH, HCl, H 2 SO 4 ) at six pH gradients, and seedling emergence rate, seedling phenotype, and associated parameters related to sugar metabolism and antioxidant defense were analyzed. The results showed that flooding stress severely suppressed rice seedling emergence, while acid-treatment effectively mitigated this inhibition, with pH 3.25-3.5 proving most efficacious. Under optimal conditions, the seedling emergence rate of YD6 exceeded CK (tap water treatment) by 44.00%–53.33%. Analysis of the physiological mechanisms revealed that acid-treatment ensured adequate energy provision for seedling development by significantly enhanced pyruvate decarboxylase (PDC) and lactate dehydrogenase (LDH) activities in the alcohol fermentation pathway, simultaneously suppressing alcohol dehydrogenase (ADH) activity to reduce toxic lactic acid accumulation and associated cellular damage. Additionally, acid-treatment effectively scavenges excess reactive oxygen species (ROS) generated under flooding stress by significantly increasing superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT) activities, reducing lipid peroxidation damage, and preserving cellular integrity. In conclusion, seed treatment with acidified water at optimal pH 3.25-3.5 effectively enhances seedling emergence under flooding stress by coordinating anaerobic respiration metabolism and the antioxidant system, providing a practical approach for enhancing seedling establishment in direct-seeded rice production.