Integrated physiological, transcriptomic and metabolomic analyses reveal the mechanism of UV-B radiation enhancement to alleviate drought stress damage in Tibetan hulless barley (qingke).

Drought (D) and ultraviolet-B (UV-B) radiation present severe, frequently co-occurring constraints on crop productivity, particularly in high-altitude environments such as the Qinghai-Xizang Plateau. Although drought severely impairs plant growth and photosynthetic capacity, UV-B radiation can counterintuitively mitigate drought-induced damage; however, the molecular and metabolic mechanisms underpinning this stress cross-talk remain poorly understood. Here, using integrated physiological, transcriptomic, and metabolomic profiling, we examine how Tibetan hulless barley (qingke) responds to individual and combined drought and UV-B exposure. While isolated drought stress markedly reduced plant biomass, grain yield, and photosynthetic efficiency, supplementary low (10 kJ m-2 d-1) or moderate (13 kJ m-2 d-1) UV-B doses substantially alleviated these adverse effects. Mechanistically, combined UV-B and drought (UVB + D) treatment attenuated cellular oxidative damage, evidenced by reduced malondialdehyde and reactive oxygen species levels by elevating antioxidant enzyme activities and promoting the accumulation of glutathione, flavonoids, and phenolics. Furthermore, UVB + D was associated with restoration of photosynthetic capacity by upregulating core components of photosystem II, photosynthetic electron transport, and light-harvesting complexes. At the systems level, UVB + D selectively reconfigured primary and secondary metabolism, redirecting flux toward amino acid biosynthesis and the phenylpropanoid pathway to enrich protective flavonoids, phenolics, indole derivatives, and amino acids. Crucially, hormonal profiling demonstrated that UV-B dampens drought-triggered abscisic acid and jasmonate signaling while restoring the synthesis and signaling of growth-promoting phytohormones (auxin, gibberellins, cytokinins, and brassinosteroids). Together, our findings elucidate the coordinated molecular and metabolic networks through which UV-B mitigates drought stress, offering promising genetic targets for breeding climate-resilient crops tailored to extreme environments.