Why lichens are bad biomonitors of ozone pollution?

The combined effects of water availability and ozone (O3) on chlorophyll a fluorescence (ChlaF) were studied in three epiphytic lichens selected for their different ecology. The samples were exposed in open top chambers (OTCs) under different watering regimes with O3 AOT40 in the range 0–50,000 ppb. Further samples were exposed in a nearby wood, as controls. ChlaF measurements were taken before exposure, after 3- and 6-week exposure and after a subsequent 2-day recovery period to verify the long-term effects of O3 exposure. All species tolerated the pollutant well. However, there was a strong influence associated with the mode of exposure: the ChlaF emission remained steady over time in the controls, whereas it varied significantly in chamber-exposed samples, with a strong decrease of Fv/Fm in non-watered and morning-watered samples, and a small decrease in evening-watered samples. ChlaF emission characteristics were also influenced by the weather conditions of the day preceding measurements, with some species-specific differences possibly related to species ecology. The ozone-tolerance of lichens is thoroughly discussed on the basis of the cellular mechanisms that allow these organisms to overcome the oxidative burst associated with the cycles of dehydration–rehydration typical of poikilohydrous organisms.