Differential UV-B stress reconfigures antioxidant pathways and growth dynamics through coordinated transcriptional and enzymatic regulation in Raphanus sativus L

• UV-B elicits intensity-dependent responses in Raphanus sativus • Low UV-B enhances antioxidant capacity and physiological performance • High UV-B increases ROS accumulation and lipid peroxidation • Antioxidant–growth balance shifts under elevated UV-B • Clear transition from acclimation to oxidative stress identified Ultraviolet-B (UV-B) radiation is a key abiotic stressor that alters plant growth and induces oxidative imbalance. However, the interactive effects of UV-B intensity and exposure duration on antioxidant regulation and morphophysiological performance in Raphanus sativus L. remain mechanistically unresolved in an intensity-dependent context. This study investigated how two UV-B intensities (0.2 and 10 µW cm⁻²) and exposure durations (6 and 12 h) modulate antioxidant systems, gene expression, and physiological performance in R. sativus L. Low UV-B induced moderate increases in antioxidant enzyme activities and gene expression with only minor reductions in photosynthetic parameters, indicating an acclimatory response. In contrast, high UV-B triggered strong upregulation of SOD, POD, CAT, GPX, and GST activities, accompanied by significant oxidative stress, reflected by elevated H₂O₂ and MDA accumulation. Under high UV-B (12 h), chlorophyll content, net photosynthesis, and transpiration rate declined by approximately 75%, 63%, and 69%, respectively, compared with control plants. These physiological impairments were associated with reduced growth traits, suggesting a shift from redox signaling to oxidative damage and defense–growth trade-offs. Multivariate correlation analysis further revealed intensity-dependent restructuring of antioxidant–growth coordination, distinguishing adaptive redox signaling at low UV-B from growth-inhibitory oxidative pressure at high UV-B. Collectively, the results identify a physiological threshold separating adaptive acclimation from stress-induced dysfunction and highlight inducible antioxidant capacity as a key determinant of UV-B resilience in radish.