Physiological effects of the Mexico city atmosphere on lichen transplants on oaks

Two lichen species, Usnea ceratina Ach. and Everniastrum neocirrharum (Hale M. Wirth) Hale ex Sipman, were transplanted for 54 d into an oak forest (Q uercus rugosa) in the vicinity of Mexico City to assess their ability to su rvive near a highly polluted urban environment. Net photosynthesis based on dry weight and chlorophyll ll b decreased respectively ca. 30 and 25% comp ared with control samples in a less polluted site, ca. 100 km north of Mexi co City. There was no interspecific difference in the response of carbon fi xation, but E. neocirrhatum was more sensitive to chlorophyll b degradation near Mexico City than U. ceratina. Chlorophll a was also degradated (ca. 1 5%) near Mexico City, Changes in total carotenes were mostly dependent on s pecies and time rather than on location. High concurrent levels of ozone an d sulfur dioxide in the air are discussed as possible causes of the decline in the lichen photosynthesis and chlorophyll content.