Water- and temperature-dependence of DNA damage and repair in the fruticose lichen Cladonia arbuscula ssp. mitis exposed to UV-B radiation

The induction of cyclobutane pyrimidine dimers (CPDs) by ultraviolet‐B radiation (UV‐B, 280–315 nm) and repair mechanisms were studied in the lichen Cladonia arbuscula ssp. mitis exposed to different temperatures and water status conditions. In addition, the development and repair of CPDs were studied in relation to the different developmental stages of the lichen thallus podetial branches. Air‐dried lichen thalli exposed to UV‐B radiation combined with relatively high visible light (HL, 800 μmol m^?2 s^?1; 400–700 nm) for 7 days showed a progressive increase of CPDs with no substantial repair, although HL was present during and after irradiation with UV‐B. Fully hydrated lichen thalli, that had not been previously exposed to UV‐B radiation for 7 days, were given short‐term UV‐B radiation treatment at 25°C, and accumulated DNA lesions in the form of CPDs, with repair occurring when they were exposed to photoreactivating conditions (2 h of 300 μmol m^?2 s^?1, 400–700 nm). A different pattern was observed when fully hydrated thalli were exposed to short‐term UV‐B radiation at 2°C, in comparison with exposure at 25°C. High levels of CPDs were induced at 2°C under UV‐B irradiation, without significant repair under subsequent photoreactivating light. Likewise, when PAR (300 μmol m^?2 s^?1) and UV‐B radiation were given simultaneously, the CPD levels were not lowered. Throughout all experiments the youngest, less differentiated parts of the lichen thallus – namely ‘tips’, according to our arbitrary subdivision – were the parts showing the highest levels of CPD accumulation and the lowest levels of repair in comparison with the older thallus tissue (‘stems’). Thus the experiments showed that Cladonia arbuscula ssp. mitis is sensitive to UV‐B irradiation in the air‐dried state and is not able to completely repair the damage caused by the radiation. Furthermore, temperature plays a role in the DNA damage repairing capacity of this lichen, since even when fully hydrated, C. arbuscula ssp. mitis did not repair DNA damage at the low temperatures.