Water availability modifies tolerance to photo-oxidative pollutants in transplants of the lichen Flavoparmelia caperata

The hypothesis that a daily water supply allows a lichen to endure the negative effects of environmental concentrations of NOx and O3 was tested with a transplant experiment. Five groups (0, A–D) of Flavoparmelia caperata samples derived from the same thalli were used for destructive, pre-exposure measurements (0), or exposed for 5 weeks in the rural collection site (A), and in a urban site with high levels of NOx and O3 (B–D). Two groups (C, D) were daily watered half an hour before the daily peak of NOx (C), and O3 (D). The comparison between pre- and post-exposure measurements of stress biomarkers revealed that the different thallus hydration regime modified the pollution tolerance as well as the physiology of the exposed samples. The non-watered group B suffered an evident decrease in Fv/Fm and reduced glutathione, but increased ion leakage, whereas the watered groups C and D showed only decreased non-photosynthetic-quenching, possibly derived from NOx exposure. Ozone, which was higher in the rural than in the urban site, did not significantly affect the lichen metabolism. Our results re-open the discussion on the so-called ‘‘drought hypothesis’’, which suggests that the lichen desert observed in urban areas of central and eastern Europe is more a matter of dry microclimate than of air pollution