Photosynthetic performance of photobionts under intense visible and UV-A radiation in lichens with light-absorbing or light-reflecting fungal pigments.

This study investigated the effect of exposure to direct sunlight and its main components, i.e. visible (VIS) radiation and long-wavelength UV-A (UV-A1; 350-400 nm) radiation, on the photosynthetic performance of two mat-forming epigeic lichens Cetraria aculeata (melanic species with light-absorbing cortex) and Cladonia uncialis (usnic species with light-reflecting cortex). A short-term (one-hour) irradiation stress was applied to metabolically active thalli and parameters related to photosynthesis at various recovery periods after treatment was examined through complementary in vivo chlorophyll fluorescence techniques (OJIP transients and quenching analysis), 77 K fluorescence emission spectra and assimilation pigment measurements. In both species, considerable but reversible photoinhibition of photosystem II was observed following sunlight and VIS radiation exposure, as evidenced by a significant decrease in FV/FM and increased K and J steps of the OJIP curves; albeit photodamage resulting from sunlight was more pronounced. Although UV-A1 had negligible inhibitory effect, it stimulated pigment synthesis in C. aculeata and enhanced photochemical energy utilization in C. uncialis. The two species revealed different strategies to cope with excess light energy; namely, C. aculeata relied on enhanced non-photochemical quenching (NPQ) and redistribution of excitation energy toward PSI, whereas C. uncialis preserved a more efficient donor site of PSII (higher FV/F0). Overall, VIS radiation was identified as the primary factor impairing PSII, posing a substantially greater threat than UV-A1. These findings might be relevant for understanding lichen performance under variable and atypical weather events, such as sunshowers or scattered showers with sunny spells.