Ozone tolerance in lichens: A possible explanation from biochemical to physiological level using Flavoparmelia caperata as test organism

Lichens are among the best biomonitors of airborne pollutants, but surprisingly they reveal high tolerance to ozone (O-3). It was recently suggested that this might be due to the high levels of natural defences against oxidative stress, related to their poikilohydric life strategy. The objective of this work is to give a thorough description of the biochemical and physiological mechanisms that are at the basis of the O-3-tolerance of lichens. Chlorophyll a fluorescence (Chl(a)F) emission, histochemical ROS localization in the lichen thallus, and biochemical markers [enzymes and antioxidants involved in the ascorbate/glutathione (AsA/GSH) cycle; hydrogen peroxide (H2O2) and superoxide anion (O-2(center dot-)] were used to characterize the response of the epiphytic lichen Flavoparmelia caperata (L.) Hale exposed to O-3 (250 ppb, 5 h d(-1), 2 weeks) at different watering regimes and air relative humidity (RH) in a fumigation chamber. After two-week exposure Chl(a)F was affected by the watering regime but not by O-3. The watering regime influenced also the superoxide dismutase activity and the production of ROS. By contrast O-3 strongly influenced the AsA/GSH biochemical pathway, decreasing the reduced ascorbate (AsA) content and increasing the enzymatic activity of ascorbate peroxidase (APX), dehydroascorbate reductase (DHAR) and glutathione reductase (GR) independently from the watering regime and the relative humidity applied. This study highlights that F. caperata can face the O-3-induced oxidative stress thanks to high levels of constitutive enzymatic and non-enzymatic defences against ROS formed naturally during the dehydration-rehydration cycles to which lichens are frequently exposed. (C) 2014 Elsevier GmbH. All rights reserved.