Photoperiod modulates UV-B-induced acclimation to high light in Norway spruce and Scots pine seedlings

UV-B radiation, perceived by the photoreceptor UVR8, induces biochemical changes that shape plant growth, development, and stress acclimation. While UV-B and UVR8-mediated responses are well characterized in angiosperms, their role in conifers remains poorly understood. We investigated UV-B-induced acclimation to high light (HL) stress in Norway spruce (Picea abies) and Scots pine (Pinus sylvestris) seedlings grown under short (8 h) or long (12 h) photoperiods and pre-exposed to ambient (2.9 μmol m⁻² s⁻¹) or slightly elevated (4.9 μmol m⁻² s⁻¹) narrowband UV-B before HL exposure. UV-B-induced cross-tolerance to HL was strongly influenced by photoperiod in both species, with significant interactions between photoperiod, UV-B dose, exposure duration, and HL treatment. Under short photoperiod, UV-B pre-exposure did not provide protection against HL-induced photoinhibition. However, under long photoperiod, pre-treatment with elevated UV-B effectively sustained PSII efficiency (Fv/Fm) during subsequent HL stress in both Scots pine and Norway spruce. The underlying mechanisms, however, differed between species: Norway spruce relied on enhanced antioxidant capacity, while Scots pine utilized increased flavonoid accumulation and differential HY5 expression. We report full-length UVR8 coding sequences in both species, supporting functional conservation of UVR8 signaling in gymnosperms. Our findings advance understanding of UV-B-induced cross-tolerance in conifers and highlight the potential of UV-B to improve seedling resilience under changing light environments.