Physiological and Biochemical Measurements Reveal How Styrax japonica Seedlings Response to Flooding Stress

This study investigated the physiological and biochemical responses of Styrax japonica Sieb. et Zucc. seedlings to normal water and nutrient management (control group, CK), waterlogging (root submerged, T1), and partial submergence (partial stem submergence, T2) over a period of 25 days. Measurements of root activity, malondialdehyde (MDA) content, hydrogen peroxide (H2O2) content, antioxidant enzyme (SOD, POD, and CAT), and anaerobic respiratory enzyme (LDH, ADH, and PDC) activities were conducted every 5 days. The results showed the following: the seedlings of the control group maintained high root activity and low oxidative damage levels throughout the experiment; under T1 treatment, root activity initially increased but declined, while MDA and H2O2 content gradually increased; T2 seedlings showed little change initially, but root activity briefly increased at Day 20 before rapidly declining. Regarding the antioxidant system, the control group had the highest SOD activity, while seedlings under T1 and T2 treatments exhibited compensatory upregulation of CAT and POD activities (from Day 15 to 20). Additionally, under waterlogging stress, LDH and ADH activities significantly increased, reflecting the activation of anaerobic metabolic pathways, while PDC content continuously decreased, indicating that low-oxygen stress induced the accumulation of LDH and ADH but reduced ethanol fermentation. PCA revealed that the first two principal components explained 61.53% of the total variation, with PC1 (45.76%) reflecting the contrasting relationship between the activation of anaerobic metabolism (increased ADH and LDH activity) and decreased root activity under waterlogging stress, while PC2 (15.77%) primarily captured the responses of oxidative damage (increased MDA) and corresponding antioxidant defense (upregulated CAT and POD activities). Overall, S. japonica seedlings adapt to short-term waterlogging stress by regulating anaerobic respiration and antioxidant systems, but prolonged stress leads to a continued increase in H2O2 and a decline in antioxidant enzyme activities. This study provides experimental evidence and theoretical support for understanding the waterlogging tolerance mechanisms of S. japonica. This experiment provides important information on the adaptive mechanisms of plants under waterlogging stress.