Why chartreuse? The pigment vulpinic acid screens blue light in the lichen Letharia vulpina

Chlorophyll fluorescence, infrared gas exchange and photoinhibition data consistently show that vulpinic acid in L. vulpina functions as a strong blue light screening compound. The cortical lichen compounds, parietin, atranorin, usnic acid and melanins are known to screen photosynthetically active radiation (PAR), thereby protecting the underlying photobionts. The role of the toxic UV-/blue light-absorbing vulpinic acid in lichen cortices is poorly documented. By comparing controls with acetone-rinsed Letharia vulpina thalli (75% reduced vulpinic acid concentration), we aimed to test PAR screening by vulpinic acid. We exposed such thalli to blue, green and red irradiance, respectively, and recorded light quality-specific light saturation curves of CO₂ uptake, quantum yields of CO₂ uptake (QY_CO2) and effective quantum yields of PSII (Φ_PSII). We also quantified light quality-dependent photoinhibition after 4-h exposure to 400 µmol photons m^-2 s^-1. In controls, the greatest high light-induced reductions in CO₂ uptake and Φ_PSII, as well as the strongest photoinhibition [lowered maximal quantum yield of PSII (F_v/F_m)], occurred in red light, followed by green, and was low in blue light. Removal of vulpinic acid significantly exacerbated photoinhibition, reduced Φ_PSII, and increased QY_CO2 in blue light. By contrast, acetone rinsing had no or weak effects in green and red lights. Comparing control with acetone-rinsed thalli, blue light screening was estimated at 69% using Φ_PSII data and 49% using QY_CO2. To compensate for the 25% residual vulpinic acid left after rinsing, we repeated the screening estimation by comparing responses in blue and red lights. This resulted in 88% screening using Φ_PSII data and 77% using QY_CO2. The consistent responses in all photosynthetic parameters support the hypothesis that vulpinic acid functions as a blue light screen in L. vulpina.