Plant responses to UV-A1 radiation are genotype and background irradiance dependent

A large fraction of solar UV radiation that reaches the Earth's surface is in the UV-A1 waveband (350–400 nm). Despite its prevalence, it is unknown how strongly plant genotypes differ in their response to UV-A1, and how their responses to UV-A1 depend on the photosynthetic photon flux density (PPFD). We grew several horticultural (tomato, cucumber, and two lettuce cultivars) and a model species (Arabidopsis thaliana) under low (LL; 150 μmol m⁻² s⁻¹) and high PPFD (HL; 550 μmol m⁻² s⁻¹), each of which was paired with UV-A1 (peaking at 365 nm) at irradiances of 0, 20, and 100 µmol m⁻² s⁻¹. Arabidopsis was the most strongly affected, as under LL, addition of UV-A1 resulted in early flowering, changes in leaf shape, and strong decreases in shoot dry weight (∼40%). In all species, UV-A1 exposure induced photoinhibition (low F_v/F_m) only under LL, but not under HL. Under HL, exposure to UV-A1 also induced strong decreases in the concentrations of UV-absorbing compounds and anthocyanins in arabidopsis (Columbia-0) and lettuce cv. Klee, but not in other genotypes. Altogether, we found that UV-A1 had only mild effects on the morphology of horticultural species (e.g., petiole angle, leaf shape and curvature), did not significantly alter biomass or leaf biochemical compound concentrations, and these effects were almost entirely unaffected by background PPFD. In contrast, these traits were strongly affected by UV-A1 in arabidopsis, highlighting the importance of not relying solely on model species when exploring environmental effects on plants. We conclude that plant responses to UV-A1 radiation depend on genotype and background irradiance.