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 NO(x) and O(3) 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 NO(x) and O(3) (B-D). Two groups (C, D) were daily watered half an hour before the daily peak of NO(x) (C), and O(3) (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 F(v)/F(m) and reduced glutathione, but increased ion leakage, whereas the watered groups C and D showed only decreased non-photosynthetic-quenching, possibly derived from NO(x) 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.