Ozone and desiccation tolerance in chlorolichens are intimately connected: a case study based on two species with different ecology.

Tropospheric ozone (O3) causes severe damage to many vascular plants but not to lichens. It was recently suggested that this may be due to their high levels of natural defences against the oxidative bursts associated to their fluctuating water content. In this study, the combined effects of watering regime (with or without a daily spray of distilled water), air relative humidity (20 ± 5 vs. 80 ± 5% RH) and O3 (250 vs. 0 ppb, 5 h day-1 for 2 weeks) were monitored in two chlorolichens with different ecology, Parmotrema perlatum and Xanthoria parietina. Modulated chlorophyll a fluorescence (Chl a F), superoxide anion radical (O2•-) and hydrogen peroxide (H2O2) production, antioxidant content and enzyme activity of the ascorbate/glutathione cycle were measured after exposure and, for Chl a F, after 1 and 2 days of recovery. The species differed in the antioxidant profile (ascorbate was higher in X. parietina, glutathione in P. perlatum), and in the activity of ROS-scavenging enzymes, more intense in the hygrophilous P. perlatum than in the meso-xerophilous X. parietina. O3 slightly modified Chl a F parameters related to the controlled dissipation, with reduction of Fm, Fv/Fm (both species) and ETR (in P. perlatum), and increase in NPQ and qN (in X. parietina). It also influenced, particularly in P. perlatum, the content of H2O2, glutathione (GSH) and oxidized glutathione (GSSG) (but not that of O2•- and AsA + DHA) and the activity of superoxide dismutase, ascorbate peroxidase and dehydroascorbate reductase. These parameters, however, were more heavily affected by water availability. The hypothesis that lichens are O3-tolerant thanks to the constitutive antioxidant systems, intimately related to their poikilohydric life-style, is thus confirmed.